CPL Met Flashcards

Question

What is the ionosphere have importance to

Answer 1

The ionosphere is of importance to radio wave propagation in that it has the ability to reflect or absorb radio waves depending on their frequency

Answer 2

Temperatures within the thermosphere rise rather steadily, especially towards the higher levels, due to very high ultraviolet radiation

Answer 3

About 10(5) (100,000) pascals

Answer 4

The standard assumes the temperature to be 0°C (melting ice) while the force of gravity (g) is assumed to be 9.806m/sec2 at a latitude 45 at mean sea level

Answer 5

The Kew type

Answer 6

The location where the barometer is installed must have its 'g' established and incorporated in correction cards for complete accuracy to be obtained

Answer 7

Index error is associated with the behaviour of mercury and the manufacture of the instrument

Answer 8

An advantage of the aneroid barometer is its application in the barograph. Thus instrument not only only reads the atmospheric pressure at any time but also records it on a graph

Answer 9

Feet per hectopascal = 96T / P | T is absolute temperature, P is the ambient pressure in hPa and 96 is a mathematical factor

Answer 10

Expressed in degrees kelvin, avoids negative values in temperature. The relationship between it an Celsius is that the freezing point of pure water under standard conditions, 0°C, is equal to 273 kelvin. Since one degree kelvin equals 1°C, it follows that the boiling point of water under standard condition, 100°C, equals 373 kelvin

Answer 11

Is zero degrees kelvin; this temperature has never been reached yet although scientists have come close to it. It is said that absolute zeros, molecular movement stops

Answer 12

96 x (273 +/- temp) / hPa

Answer 13

The reduction to sea level should take into account the height of the station above sea level, the ambient pressure there and the mean temperature within the column down to sea level

Answer 14

Most countries use the station temperature as being representative of the column temperature. Although approximate, it is sufficiently accurate and acceptable for stations at elevations below 1,500ft ASL (about 500m). When stations are higher than this ISA values are used

Answer 15

Except in global areas where pressure systems stagnate. Highs and lows and their associated extensions, tend to migrate to the east especially so in mid latitudes of both hemispheres

Answer 16

One change, which is caused not by travelling weather systems but by temperature influences, is the semi diurnal variation of pressure. A diurnal cycle means a daily or 24 hour cycle

Answer 17

At approx 1600 hours, the pressure drop is at its maximum and a barograph would record this as the bottom of the semi diurnal pressure curve. Pressures increase beyond this time and peak around 2200 hours. A further decrease then begins cumulating in another bottom of the curve at around 0400 beyond which time a second peak is formed around 1000 hours

Answer 18

The term pressure gradients which is the change of pressure with horizontal distance measured from high to low

Answer 19

The pressure gradient always acts at right angles to the isobars

Answer 20

Air is dry and of constant composition at all altitudes

Answer 21

Is constant at 9.8m/sec

Answer 22

Is based on the same values as those in the ISA except that the JSA does not include a tropopause and assumes a temperature lapse rate of 2°C per 1,000ft (no decimal point)

Answer 23

Sea level pressure correctly set on the sub scale of an altimeter, whether the aircraft is airborne or on the ground, will produce an altimeter reading above sea level. This reading is known as the absolute altitude

Answer 24

This is the atmospheric pressure obtained from stations and reduced to mean sea level using ISA values. When QNH is set on the sub scale of an altimeter the instrument will read absolute altitude, more commonly referred to as just altitude

Answer 25

This is the ambient atmospheric pressure at a given datum. With the sub scale set to QFE, the altimeter will indicate vertical distance (height) above a datum

Answer 26

This is the altimeter sub scale setting of 1013.25 hPa. With this (ISA) values set, the altimeter will read pressure altitude which means the atmospheric pressure, expressed in terms of altitudes which corresponds to that pressure in the ISA atmosphere

Answer 27

This is the atmospheric pressure from stations, reduced to mean sea level using existing atmospheric values. QFF is used only for stations of elevation 1,500ft AMSL and lower

Answer 28

When the temperature is higher than ISA, QNH is higher than QFF and when the temperature is lower than ISA, QNH is lower than QFF

Answer 29

When the sub scale is changed to a value less than QNH I.e turn the sub scale down, the altimeter will read down - it reads lower than actual height above sea level

Answer 30

When the sub scale is changed to a value greater than QNH, I.e turn the sub scale up, the altimeter will read up - it reads higher than actual height above sea level

Answer 31

The three main units of measurement of temperature are Celsius (formerly referred to as centigrade), Fahrenheit and kelvin (also referred to as absolute)

Answer 32

This consists of a glass tube within which another tube containing mercury. This mercury will expand with warmer and contract with cooler temperatures. A small reservoir for the surplus mercury is at the bottom of the inside tube. The top of the mercury will indicate the temperature being measured

Answer 33

This type is most commonly used but it ceases to be totally satisfactory when temperatures are below -36°C, the approximate freezing point of mercury

Answer 34

A modified mercury thermometer is used to measure and retain, for the time being, the maximum temperature. The modification consists of a restriction within the mercury holding glass tube, just above the reservoir. When the highest temperature for the day has occurred and cooling begins, the mercury contracts but the restriction within the glass tube prevents it from returning to its reservoir and the thin column of mercury breaks at the restriction

Answer 35

The restriction traps the mercury above it and the highest day temperature can be read even hours after its occurrence

Answer 36

This type is normally used when low temperatures are involved because alcohol has a low freezing point

Answer 37

The grass minimum temperature is usually measured with this type of thermometer. Within the thin column of alcohol is an index, which moves down with the retreating alcohol when air temperature drops. The index is designed so that when temperature increases, the expanding alcohol passes it without affecting its position. Thus the minimum temperature for the night can be read at any time after daybreak

Answer 38

This type incorporates a two chamber design, one of which contains a special type of fluid. As temperature changes, the fluid expands or contracts, activating a small piston, which controls the indicating needle. This type of thermometer is often used in engines to measure and display operating temperatures

Answer 39

Metals of different make up contract or expand differently with temperature changes. By joining each end of two dissimilar metals, an EMF (electromotive force) is produced when temperatures changes. This EMF can then be used, provided it is suitably amplified, to activate the indicating needle

Answer 40

Two dissimilar metals are joined together but instead of using the EMF when temperatures change, the actual movement between the two metals is used to activate the indicating needle

Answer 41

This type of thermometer is normally used in radiosondes (balloon carried meteorological measuring equipment) and modern electronic temperature gauges. Electrical circuit resistance changes as the temperature alters when the radiosonde ascends and the changed radio signals produced are received and analysed by a station receiver. The information can be recorded so that a total picture of temperatures with height can be produced

Answer 42

When there is a requirement for a continuous reading of temperatures, a thermograph is used. In essence this is a similar device to the barograph, using a clockwork mechanism to rotate the graph drum. A pen, activated by a thermometer (often a ni-metallic type), traces the temperatures throughout a certain time period

Answer 43

Surface air temperature is measured at approximately 1.5 meters above the ground so that free flowing air can influence the thermometer reading. The thermometer is packed in a Stevenson screen, a louvered box arrangement, which ensures that sunlight does not fall on the thermometer

Answer 44

Radiation is the movement, passage or transmission of heat from one body to another without the use of intermediary matter such as air. Thus radiation can travel through space where there is no air. The transmission of radiation energy is in the form of electromagnetic waves

Answer 45

The solar energy produced by the sun falls within a wavelength range of 0.5u to 4.0u (u is a micrometer, also known as a micron). Because of the very high source temperatures involved, solar energy is shortwave radiation

Answer 46

Ultraviolet approx 9% Visible light 45% Infrared 46%

Answer 47

Ultraviolet radiation is largely absorbed by the ozone contained in the stratosphere while a large part of the visible radiation is absorbed by water vapour. Also, co2 absorbs a very small percentage of incoming solar radiaiotn

Answer 48

Because of the large distance from the sun, earth receives only about 0.0005% of the total solar radiation emitted into space. Of this minuscule amount, some radiation is reflected back into space by cloud tops while particles within the atmosphere prevent some solar radiation reaching the earth surface due to the effect of scattering. Although this scattering process expels some radiation into space again, some if redirected to earth, this is known as sky radiation

Answer 49

The total solar radiation received by the earth is the sum of sky radiation and direct radiation, called global solar radiation

Answer 50

The distance between sun and earth, the altitude of the sun and the length of day

Answer 51

Due to the tilted spin axis, the altitude where solar input is perpendicular shifts north and south. The vertical sun is at its most northern latitude (23 degrees north) in June and at its most southern latitude (23 degrees south) in December - these dates are known as solstices

Answer 52

When the angle of sun rays to the earth surface is perpendicular or nearly so, the solar energy pre square meter is more than when the angle to the surface is oblique I.e the radiation is then spread over a wider area

Answer 53

Hours of daylight and dark are equal all over the world when the perpendicular sun is exactly over the geographical equator. This occurs twice a year, on or about 22 March and 22 September

Answer 54

Pyranometers

Answer 55

Most modern pyranometers use a sensing element consisting of alternating black and white wedge shaped thin copper sectors. The white sectors have a coating of highly reflective white paints whereas the black sectors have a coating of highly absorbent black paint. When the unit is exposed to solar radiation, the temperature difference created between the white and black sectors is a function of radiation intensity and can be recorded

Answer 56

Compared to the temperature inherent in the sun, earth temperature is far colder. Due to this, the radiation given off by earth is in the long wave, low frequency band, between 4u and 80u. This radiation is called terrestrial radiation and involves infrared radiation

Answer 57

Water vapour, cloud droplets, carbon dioxide and ozone are strong absorbers of infrared radiation, which means that those substances become transmitters of energy themselves and through that, they will air warming of the atmosphere to some extent

Answer 58

Not all infrared radiation is absorbed by the three atmospheric constituents. Between 8.5u and 13.0u, terrestrial radiation readily escapes

Answer 59

The energy budget which essentially means that the atmospheric temperature, resulting from radiation, has a relatively steady annual global average of around 15°C

Answer 60

From below

Answer 61

Incoming solar radiation excites earth surface molecules, which as a consequence cause the surface to heat up. Thus a hot plate effect results, which warms the air above the surface

Answer 62

Although terrestrial radiation is a major factor in warming the atmosphere, its release from earth tends to cool the surface

Answer 63

Through conduction and convection

Answer 64

Conduction is the transfer of heat through inter molecular action. The excited (warmed) earth molecules mix with the air molecules that are in touch with the surface and through this process sensible heat is transferred from one to the other. By sensible heat is meant heat that can actually be felt as distinct from latent heat

Answer 65

When solar energy has excited and heated the surface and conduction has warmed a thin layer of air above the surface, convection now transfers this heat to layers of air molecules aloft

Answer 66

Around 2pm

Answer 67

Just before sunrise

Answer 68

Soil types containing amounts of water such as peat, can have considerably greater depth of warming. However, too much water will have the opposite effect

Answer 69

Any surface that heats quickly and cools just as quickly will involve a widely undulating diurnal variation of temperature curve whereas a surface that either resists or retains insolation will have a mildly or nil fluctuating curve

Answer 70

Provided the angle of insolation is large enough, solar radiation is readily accepted by oceans. However, at angles of 15 degrees and less (shallow insolation), reflection of solar radiation back into the atmosphere is 50% and possibly more. At angles of input 60 degrees and more (steep insolation) the reflection is barely 3%

Answer 71

Maritime climates rarely have extremes of temperature

Answer 72

This means the amount of thermal heat units (expressed in joules) required to raise the temperature of a unit mass by 1°C

Answer 73

Specific heat, mixing, evaporation and wind

Answer 74

Water is rarely in a static state. There is invariably some wave action and a certain amount of vertical current flow. Any solar energy received is spread through a far deeper layer of water than on land. Even a substantial solar input, which might have had a dramatic effect on land, will have relatively little effect in water. Mixing tends to slow slow down changes in water temperature

Answer 75

Solar input will try to evaporate some water, and this demands latent heat. Therefore, even if the air temperature over the ocean had heated up a little after sunrise it soon would cool again due to this demand for latent heat

Answer 76

Over the land the roughness of the terrain interfered with this wind flow and as a result the surface wind will be somewhat lighter than it might have been had the land been totally flat. Thus, over the oceans where frictional effects are less, winds are generally stronger

Answer 77

Considering solar radiation first, the amount reflected back into space depends on cloud types cloud cover (in oktas) and cloud thickness. Any cloud (depending on type) will cause less reception of insolation at the surface and, as a result, surface air temperatures will not alter greatly and the graph will tend to straighten out. Add to this the effect of cloud on absorption of outgoing terrestrial radiation, its own emission of radiation and its blanketing effect, and a barely fluctuating graph will result

Answer 78

Albedo can be measured by an albedometer, which consists of two pyranometers: one facing upwards and the other facing downwards. The former measures global radiation (direct and refracted) while the latter measures reflected solar radiation, their combined readings being a function of albedo

Answer 79

Condensation is the return of water vapour into liquid (or solid) water, the main method being cooling of the atmosphere. The most common methods is cooling through ascent when adiabatic processes produce the desired effect

Answer 80

The condensation process cannot take place unless water vapour can cling around small particles in the atmosphere such as soot, salt particles and so on (dust particles have shown to be rather ineffective). These are collectively called condensation nuclei

Answer 81

Sea salt is particularly effective in condensation formation since it has a great affinity for water

Answer 82

When air of great purity such as found in the Antarctic, the lack of condensation particles may prevent condensation at a temperature when it should have occurred

Answer 83

As evaporation takes place, water vapour adds to the partial pressure of air and a stage will be reached where water vapour content cannot increase any further. This is referred to as the saturation vapour pressure of moist air

Answer 84

It shows that over ice surfaces, the saturation value is less than had the surface been water, which means that water vapour condenses more readily over ice than over water. At temperatures approaching 40°C, it is almost impossible to saturate air

Answer 85

Latent heat is the energy required or given off to make the change - it does not affect the temperature of the substance that is changing

Answer 86

Latent heat of evaporation is absorbed or stored by the water vapour do that when the water vapour returns to the liquid state again (condensation) thus stored latent heat is released

Answer 87

Water has greatest density and therefore least volume at a temperature of +4°C. At temperatures either side of that, density decreases and volume increases

Answer 88

By far the greatest producer of precipitation however, is not the evaporation that may occur from local water sources, but the horizontal movement of moisture in the atmosphere, known as moisture advection. The average annual precipitation on a global scale is approximately 85cm (33 inches) but this amount varies greatly from place to place around the globe

Answer 89

The mass or weight of water vapour per unit volume of air. Alternatively, it may be expressed as the humidity mixing ratio may which is a percentage expression of the mass of water vapour in grams per kilogram of dry air

Answer 90

The ratio of the weight of water vapour in a given volume of air (absolute humidity) to the maximum weight of water vapour the same volume could hold at the same temperature, expressed as a percentage

Answer 91

The maximum weight of water vapour the air can hold is generally referred to as the saturation content

Answer 92

Relative humidity œ absolute humidity / saturation content %

Answer 93

By absolute humidity is meant the amount of water vapour in the air, let it simply be called water vapour. By saturation content is meant the maximum amount of water that air can hold as a gas and the temperature was the over riding factor

Answer 94

Relative humidity is directly proportional to water vapour but it is inversely proportional to temperature

Answer 95

Dew point is defined as the temperature at which a parcel of air saturates under constant pressure - it is a temp!

Answer 96

Constant pressure. As a general rule it may be assumed that the dew point will decrease by half a degree Celsius per thousand feet of height due to the associated reduction in pressure

Answer 97

Relative humidity is affected by changes in water content and air temperature. Dew point is affected only by changes in water content (ignoring pressure)

Answer 98

Hygrometers, commonly used are the wetbulb/dry bulb, hair hydrometer and the lithium chloride element

Answer 99

This hydrometer should more accurately be referred to as a psychrometer, which consists of two thermometers mounted side by side. The dry bulb is a simple mercury thermometer, which measures the temperature of the ambient air

Answer 100

The wet bulb depression will be large, I.e the two thermometer readings are far apart which indicate a low relative humidity

Answer 101

The wet bulb depression is less, that is, the two thermometer readings are close together, which indicates a high relative humidity

Answer 102

This element changes in its electrical resistance due to vapour pressure differences, which enables it to be used for relative humidity readings. Since its construction is very light, this element is often used for radiosondes (upper air) observations

Answer 103

Assume a parcel of air is totally dry - the molecular weight of this air will have a values of 28.96. Water vapour has a molecular weight value of 18. If follows therefore that the mass of a molecule of water vapour is less than the mass of a molecule of dry air

Answer 104

The principle reason for movement of air is that variations in temperature produce fluctuations in pressure and the associated pressure gradients cause air to flow (a classic situation where heat energy due to insolation is converted to kinetic energy)

Answer 105

The horizontal flow is by far the stronger of the two, somewhere in the order of a factor of 100 for horizontal wind to one for vertical currents. The exceptions to this rule are violent convective storms such as hurricanes, also known as tropical cyclones or typhoons

Answer 106

Pressure gradient Coriolis force (also known as geostrophic force) Centripetal force Friction forces

Answer 107

Density is inversely proportional to pressure gradient. Ignoring other factors therefore, in general, that for a given isobar spacing the wind becomes stronger with height

Answer 108

At the equator, the west to east speed (sideways speed) is approximately 1,000mph. Since at the axis of rotation, the poles, this speed is zero it follows that at latitudes between the equator and the poles, the west to east sideways varies

Answer 109

Coriolis force = 2 w p v sinLat

Answer 110

Is the angular velocity of earth spin, simply its rpm of 360 degrees per 24 hours

Answer 111

Air density

Answer 112

The speed of the airflow, the wind speed

Answer 113

Is the sine of the latitude, which means that at the equator where the latitude is nil, the sine of the latitude must be nil thus coriolis force there must be nil. At the poles, sin90 is 1; coriolis force must be at its maximum

Answer 114

When a flow begins its escape from a high (to a low pressure system) under the influence of the pressure gradient, as soon as it obtains a velocity at all, coriolis force will act upon it at right angles and force it to its left in the Southern Hemisphere

Answer 115

When the wind blows along straight isobars

Answer 116

When the wind follows curved isobars

Answer 117

Pressure gradient, coriolis force and centripetal (cyclostrophic) forc. These three determine the strength of the wind and the circulation around the systems

Answer 118

There is a certain amount of leakage from a high towards a low it follows that the value of a given pressure gradient will likely decay after some time. Thus pressure gradient is rarely a constant

Answer 119

Pressure systems are seldom stationary for long, they tend to migrate generally from west to east and at the same time often move north or south

Answer 120

When pressure gradient and coriolis force are equal, isobars will be straight and the wind will be geostrophic

Answer 121

When pressure gradient and coriolis force are not equal, one or the other will be dominant, isobars will be curved, the wind will be a gradient wind and the stronger force will provide the centripetal force

Answer 122

The pressure gradient force (PG) I.e the stronger the pressure gradient the stronger the wind

Answer 123

The coriolis force is determined by the strength of the wind and by latitude, both directly proportional and when the coriolis force is greater than the PG the gradient wind is anticlockwise around a high and conversely when the coriolis force is smaller that the PG the gradient wind is clockwise around a low (both in the Southern Hemisphere)

Answer 124

At any given latitude and with identical isobar spacing, the strength of the wind is stronger around a high than around a low and given identical isobar spacing, as a pressure system moves towards lower latitudes, the wind strength increases. The opposite applies when moving to higher latitudes

Answer 125

Where strong winds following tightly spaced curved isobars are encountered near the equator, indicating balanced flow. These are tropical cyclones and tornadoes. Such systems require a very large centripetal force, which is produced by a very strong pressure gradient. The hurricane strength winds associated with these systems are referred to as cyclostrophic winds

Answer 126

The depth of the layer is generally assumed to be about 2,000ft this value fluctuates depending on a number of factors such as the nature of the surface and it must therefore be remembered that the 2,000ft commonly used is only a global average

Answer 127

Is weaker than the wind above the friction layer and flows at a relatively large angle across the surface isobars to the centre of the low

Answer 128

Is almost as strong as the wind above the friction layer and flows at a fairly small angle across the surface isobars to the centre of the low

Answer 129

In the Southern Hemisphere, the surface wind backs and increases after daybreak and veers and decreases after sunset

Answer 130

The intensity and extent of the friction layer which, in turn, depends on the type of surface, cloud cover, season, latitude and the characteristic of pressure systems involved (e.g presence of inversions)

Answer 131

The instrument used for this purpose is the geostrophic scale - a plastic ruler which, when placed at right angles to isobars, indicates wind strength. The scale uses a reciprocal graduation meaning the smaller the distance between graduations, the stronger the wind speed and vice versa

Answer 132

There are a number of instruments collectively called anemometers, which are used to give an accurate measure of the surface wind. In order to minimise ground interference as much as possible, the wind is usually measured 10m above the ground, approximately 30ft

Answer 133

For take off and landing reports, the wind is averaged over two minutes. For synoptic reports such as METARS it is customary to use a mean wind velocity over a period of ten minutes

Answer 134

The rotation can be used mechanically to activate an indicating needle through a speedometer-type cable system or it may be used to drive a dynamo or similar generator to produce electric current to activate an indicating needle. This instrument only measures wind speed

Answer 135

It's inaccuracy when wind speeds are low, or high and gusting. The inertia of the cups plus the friction in the driving mechanism, no matter how well constructed, will invariably cause some inaccuracy when the wind is light whereas at high wind speeds, the momentum of the revolving cups will resist rapidly changing speeds

Answer 136

It has a length of not more than 3.6m and an open end at the mast of not more than 0.9m (many standard windsocks are 2.82m long x 0.66m wide)

Answer 137

In the Southern Hemisphere, one stands with ones back to the wind, the area of low pressure is to ones right

Answer 138

If an aircraft descends through a shear zone below which the wind is lighter, the aircrafts airspeed will reduce, its angle of descent will steepen and its rate of descent will increase. These changes can be brought under control of course but it takes a certain amount of height to do so

Answer 139

When shear is experienced at altitude it is normally possible to vacate the region by climbing a few thousand feet or by turning away from it

Answer 140

The big problem with wind shear is that when it presents itself at low heights such as within the airspace surrounding the take off and climb out sector or the approach to land sector, it may affect the aircraft unexpectedly and cause it to descend steeper and at a greater rate towards the ground

Answer 141

Stable air will likely have fair to poor visibility, if cloud is present it will be layer type and if precipitation occurs it is likely to be rain or drizzle

Answer 142

Cloud will be in the form of cumulus (heap type) cloud that can tip at a mere few thousand feet or reach higher than 60,000ft in tropical regions. Precipitation will be showers, heavy at times and occasionally accompanied by thunderstorms. Outside cloud, visibility can be very good, indeed brilliant if the air originates from cold polar regions

Answer 143

The temperature lapse rate of the environment air and the adiabatic change in temperature of a vertically moving parcel of air

Answer 144

The temp lapse rate that applies in a given locality at a given time is known as the environmental lapse rate, which can be measured by sending a thermometer aloft. A balloon carried instrument (radiosonde) can be used for the purpose, or can be carried aboard an aircraft. Either way, as various heights are passed, the thermometer readings are transmitted to a ground station where they are recorded and plotted on a graph

Answer 145

A rising parcel of air, metering reducing atmospheric pressure and increasing its volume, will cool and thus cooling process will be an adiabatic one

Answer 146

An adiabatic process (of temperature change) is simply one where no energy (heat) is gained from or lost to the surroundings

Answer 147

One where energy (heat) is lost to or gained from the surroundings

Answer 148

One where a change in temperature occurs when the pressure remains constant

Answer 149

ELR's in excess of 3°C/1,000ft

Answer 150

The level at which condensation takes place and cloud forms when the cause of rising is through mechanical means, such as mountains, is known as the rising condensation level (RCL), also referred to as the lifting condensation level (LCL)

Answer 151

Difference between the surface temperature and the dew point, divide answer by three = RCL

Answer 152

When the ELR is steeper than the DALR, the dry are is unstable. If this rising air cools to its dew point, further ascent will produce condensation and cloud will form. The height where this occurs is referred to as the convective condensation level (CCL)

Answer 153

Convective rising of air will not produce cloud if the DALR intersects the ELR before dew point has been reached

Answer 154

Plot a vertical line up from the dew point temperature. Where the ELR and this vertical line cross, multiply this by three and add to the dew point value (on a altitude/temp graph)

Answer 155

If air is dry (consistent with a low dew point) the convective cloud base is high. If the dew point is very low, such as found in desert areas, convective cloud may not form at all

Answer 156

When a parcel of air is dry on the bottom and moist on the top it begins as unstable and the air is risen up a hill. The dry air will cool at the DALR while the moist air will cool at the SALR. The ELR is then decreased between the layers and becomes more shallow and instability has changed to stability

Answer 157

When there is a parcel of air with moist air on the bottom and dry air above, it begins as stable but as it is risen up a hill, the lower level will cool at the SALR and the higher level at the DALR, the ELR is then increased from the original and the layer which way stable initially has become unstable during the ascent

Answer 158

Refers to the behaviour of air as the surface temperature varies from day to night. As the temperature increases after sunrise the ELR steepens and instability may occur. The ELR will go through a phase where conditional stability exists. In the afternoon, when the surface temperature peaks, the instability will be at its maximum. The reverse happens during the night

Answer 159

This involves having a stable surface layer with a high moisture content and air above this layer is unstable. If the lower of air is now physically forced through the unstable layer above by some means, all the air becomes unstable. Thus the stable layer will initially produce a sheet of stratus cloud but as it enters the unstable layer aloft, the stratus turns into cumulus, which under certain conditions can be quite formidable. Thus both layer and cumulus types of cloud will be present in this type of stability

Answer 160

After sunrise, the land heats while water in the sea retains its temperature. Thus the air over the warming land will rise through conduction and convection. As it warms, the air expands and so then pressure falls off less quickly with altitude than it did during the night. As the warm air rises the pressure will be higher over the land than at the same level over the sea. This upper air will start to flow according to the pressure gradient from land to sea. As it does so the pressure falls over the land and rises over the sea due to the transfer of mass. This sets up a pressure gradient from sea to land at the surface and initiates a wind flow, the sea breeze

Answer 161

The sea breeze sets in at around 10am and peaks at around 2 to 4pm

Answer 162

A sea breeze measures between 10 and 15kts but this strength can vary somewhat. In temperate regions where the land behind the coast is sandy and prone to heating, the breeze can be quite strong. This is very common in spring when the sea temperature is still relatively cool while the land heats up quite substantially

Answer 163

This may vary from place to place, inland travel of the sea air is generally about 25 to 40km. The vertical development in temperate regions is approximately 2,000 to 3,000ft. In lower latitudes, especially on regions where sandy areas behind the coast produce very hot surfaces, the vertical extent can be many thousands of feet high

Answer 164

The presence of cloud may involve a few oktas of cumulus characterised by some turbulence and reduced visibility from the land surface up to the cloud tops. Above it, conditions can be very smooth and clear. These clouds are often referred to as fair weather cumulus and they are generally confined to land

Answer 165

In tropical regions, the warm sea will evaporate moisture into the air readily, the hot land will produce some very intense updraughts and the stage may be set for quit substantial cloud formations including cumulonimbus with associated thunderstorm activity. Many places that fall in this category experience these cloud developments and thunderstorms at a regular time of the day, normally around 3pm

Answer 166

This depends largely on the degree of heating and the supply of moisture of the sea. In tropical regions it is not uncommon to have shower activity at the peak of sea breeze activity but in most other cases precipitation is not common

Answer 167

Over the warm land, the onset of the sea breeze transports cold sea air inland, which can often produce a fairly sudden reduction in temperature around mid morning. This factor is again influenced by season in that, during the summer months, the temperature of the sea air is not as cold as during springtime and the fall in temperature is then not quite so dramatic

Answer 168

Where substantial differences exist between sea and land temperatures, the action of the entire sea breeze process is intense and a relatively steep pressure gradient can occur. This sets the stage for a strong sea breeze with the result that the coriolis effect becomes significant. A consequence of this is that the circulation of air over the land becomes cyclonic as a heat low develops. This in turn affects the direction of the sea breeze as it crosses the coast, in some cases almost causing the wind to become nearly parallel with the coast

Answer 169

The prevailing wind in a coastal area may occasionally be from land to the sea. Provided the sea breeze is strong enough, it may oppose this prevailing wind and force itself underneath it. At the top of the sea breeze a shear level will be found separating the upper sea breeze from the prevailing wind. This can sometimes be associated with moderate turbulence. In other cases, the opposition between an offshore prevailing wind and a sea breeze can be experienced right down to ground level a few miles inland so that a convergence type boundary is produced between the winds (straight up and down wind shear)

Answer 170

Sand dunes, large tree plantations, hills and cliffs on the sea breeze will often produce moderate turbulence. It will be quite turbulent from the beach and inland up to about 2,000ft to 3,000ft whereas from the beach and out to sea, conditions are totally smooth from sea level and up

Answer 171

When conditions are calm and cloud or fog covers a given area of reasonably flat countryside, it is possible for a wind similar to the sea breeze to start blowing when the ground not covered by cloud is warmed by insolation. The wind travels from the cloudy region towards the sunny area and some of the cloud or fog can actually move with the airflow. Thus a sudden drop in temperature and unexpected overcast can take place at an unusual time of the day if this has developed towards the early afternoon

Answer 172

When insolation becomes insufficient to replenish the departing terrestrial radiation, generally towards evening, there is a net loss of heat energy. Although this loss will affect the land, especially sand soils often found in coastal regions, the effect on the sea is minimal. Therefore, as the land continues to cool it will cause the air above it to become denser so that subsidence and increasing surface pressures develop

Answer 173

Pressure is less at sea level and greater at the surface over the land. A closed circuit is formed with colder land air replacing warmer, and rising sea air

Answer 174

Generally the breeze is only some 3-4kts but local topography such as sloping land towards the sea can increase this value

Answer 175

Autumn is the most likely time of the year for land breezes when the sea has still retained much of its summer warmth but the occasional inland frost starts to occur over the land

Answer 176

Factors such as cloud, turbulence, precipitation and so on do not readily occur with the land breeze but in tropical regions the rising moist air can often produce substantial cloud development at the end of the night so that around dawn cumulonimbus cloud can form accompanied by thunderstorm activity

Answer 177

Descending air in the hills and mountains. The air close to the valley floor will become cold and therefore denser, which encourages it to flow down the valley. The air touching the walls will cool through conduction and again, as the air becomes denser it starts a down slope travel, which is assisted by gravity.

Answer 178

Radiation cooling is enhanced whenever terrestrial radiation is freely allowed to escape. Thus clear winter nights almost certainly produce this type of wind in mountainous terrain

Answer 179

At any given spot on a valley wall, the air can be said to be diverging when it flows away from the location, which means that the airs pressure at a common horizontal level is greater over the centre of the valley that nearer the slopes where diverging has decreased the pressure slightly

Answer 180

The speed of the katabatic wind is normally light and non turbulent but stronger winds can develop when valley walls are relatively steep. Another factor in katabatic wind strength is the nature of slope cladding e.g snow covered slopes or bare rock walls, which can encourage substantial speeds. It can be concluded therefore that the katabatic wind is generally of light strength and smooth but exceptions do occur

Answer 181

Since most valleys have rivers or streams, it is to be expected that the air contained within the valley confines can be quite moist. Thus if descending katabatic wind brings additional cloud air it may well assist in the production of valley fog

Answer 182

After sunrise, valley walls and especially the high level east facing rocks are warmed by solar radiation. Through conduction, the air touching the walls will warm and convection will cause it to rise. Replacement air is drawn up from the valley floor and also from higher levels away from the walls. This means that at a given horizontal level, the airs pressure becomes less over the centre of the valley than nearer the walls. The overall result is a general rising of air

Answer 183

The anabatic wind is almost always rather light due mainly to the influence of gravity

Answer 184

Gusts are momentary increases in wind speed. The term gustiness includes both gusts and lulls. Gusts are caused by turbulence, which may be small scale, as the result of trees and buildings, or on a much larger scale due to rough or mountainous terrain

Answer 185

Squalls are rapid increases in wind speed, which last for some minutes, then die away again. They may be produced by the passage of individual cumulus clouds or thunderstorms or by the passage of frontal systems

Answer 186

A fohn wind may be experienced when a mountain range forms a barrier to wind flow; the orientation of the mountains determines the direction from which the fohn wind blows

Answer 187

At the approaching air arrives at the mountain range, it will be forced upward and initially cool at the DALR while from the RCL (rising condensation level) onwards the SALR takes over. Cloud of various types will form depending on stability and precipitation is almost inevitable. As well, water droplets will adhere to rock faces, trees and similar objects so that the overall result is a decrease in the water content of the rising air

Answer 188

Although condensation on the windward side of the mountain range is very commonly associated with the fohn wind, it is not an essential requirement. Some fohn winds develop without precipitation on the windward side when high level air is drawn down to the lee side, warming at the DALR all the way down.

Answer 189

A strong fohn wind can transport dust and even rock particles of substantial size many miles to the lee of the mountain range and thus is particularly likely vis gorges associated with major river valleys. The resulting reduced visibility on the lee side can be quite dramatic and from a technical point of view, the presence of dust can have a serious effect on engines and aircraft components that rely on external air intakes

Answer 190

A pressurised area forms on the windward side of the obstruction and can extend horizontally up to five times the height of the obstruction. As the airflow bends over the obstruction, a Venturi effect will accelerate the flow above and slightly to the lee of the obstruction. This effect is often greatest up to three times the height of the obstruction. The flow can be disturbed downhill as far as 50 times the height of the obstruction. Substantial turbulence can take place in the lee horizontally up to 20 times the height of the obstruction

Answer 191

A substantial mountain range, a wind blowing more or less at right angles to the mountain range and a high moisture content of the approaching air

Answer 192

Mountain waves, standing waves or lee waves

Answer 193

May cause severe turbulence under certain conditions

Answer 194

A mountain range of substantial dimension, a wind more or less at right angles to the range, the low level wind must be at least 15kts and increase in strength with height and a generally unstable atmosphere at low levels with a stable layer at altitude, normally slightly above mountain crests. The air above this layer should be less stable or slightly unstable

Answer 195

If a mountain range is too small, the wind will most likely channel around the edges rather than flow over the tops and this will be enhanced if the air in question is very stable. A similar result is likely if the wind direction onto the range is rather oblique. It is generally accepted that the wind must be at right angles to the mountains or within 30 degree of this

Answer 196

The increasing wind with height must retain a fairly constant direction, generally within 20 to 25 degrees throughout a deep layer

Answer 197

After crossing mountain tops, the air flows in an oscillating pattern which can have a wavelength of some 3 to 25km or more and an amplitude of some thousands of feet. Vertical currents can involve speeds of around 1,000 to 1,500ft/min. In extreme cases, vertical speeds of 3,500ft/min have been recorded. The oscillation axis may lean into the wind so that crests of waves at high altitude may be directly above troughs at low altitudes

Answer 198

The horizontal distance between two identical parts of successive waves such as the distance between each crest or between each trough

Answer 199

Mountain wave length depends largely on the speed of the wind in that, the stronger the wind the larger the wave length and vice versa

Answer 200

It has been shown that the upwind shape or height of mountains has relatively little effect on wavelength but the lee slope has considerable influence

Answer 201

The vertical distance between the crest and the trough of a wave. Amplitude is strongly governed by the shape and size of mountains in that the higher the mountains the greater the amplitude potential and vice versa. The largest amplitudes are gained when the width of ridges is equal to the wavelength.

Answer 202

When a series of ridges obstruct the flow of air, amplitude will be greatly increased if the distance between ridges equals the wavelength. When these distances are greater or smaller, the descending wave flow will meet rising ground with the result that successive waves are likely to be dampened

Answer 203

Generally, the wave flow is smooth except when windshear or rotor zones exist. Thus in the absence of cloud which may give visible evidence of mountain waves, the indication of an altimeter or VSI in an aircraft will provide the only evidence that wave action prevails; turbulence can not always be expected to be a sure sign

Answer 204

When a mountain wave, extending to great heights, is associated with a jet stream, windshear can be strong. I'm those circumstances, turbulence within the shear zone can be moderate to severe

Answer 205

These can often be found near the lee of mountains at more or less mountain top height when the amplitude of waves is large. The waves steep down slope can cause a strong tumbling motion directly beneath the crest of the wave and the resulting instability and turbulence can be severe. When cloud is present, it will show as ragged and rapidly rotating streaks with the tumbling axis parallel to the mountain range. In the absence of cloud, there is nothing to indicate the presence of rotor action

Answer 206

It is vital that more than adequate height is in hand above the mountains prior to crossing the tops, especially when flying into the wind. In the latter case, the heading across the ridges should be at least 90 degrees so that if an unexpected downdraught is experienced, a relatively small turn allows the pilot to vacate the zone. If sufficient height cannot be achieved and if the power available is rather limited, then an alternative route around the mountains would be advisable

Answer 207

The rising action on the leading edge of a wave causes adiabatic cooling of air and, through that, formation of cloud provided sufficient moisture is present. On the down wind side of the wave, the descending action with its associated adiabatic warming will evaporate the cloud. Thus a very smooth shaped cloud is formed known as lenticulars

Answer 208

Lenticular clouds do not drift down wind when the pattern of the oscillations is constant in both frequency and amplitude

Answer 209

A substantial change in wind direction, a change in the characteristics of the oncoming flow in terms of different stability, or a lessening of the wind speed. These changes are very often provided by the arrival of major weather systems such as fronts or depressions

Answer 210

When a strong wind blows onto a mountain range and the speed of this wind decreases in strength from crest level upward, it is likely that rotor streaming will develop

Answer 211

It's often hidden within deep cloud and precipitation

Answer 212

The layer of air above the mountain tops, moving at considerably slower speed, acts like a huge damper. Thus enhances a Venturi effect between the damper and the mountain range so that the speed of the low level wind is very high as it emerges on the lee side. A suction effect then develops in the shadow of the range below ridge height and the wind curls down in similar fashion as rotors

Answer 213

Whereas turbulence will be severe to extreme in the lee below ridge height winder these conditions, flight above ridge height (in the weaker wind zone) is likely to be less disturbed

Answer 214

After sunset, terrestrial radiation over the land continues to escape while there is no replenishment of energy from solar radiation. Thus a net loss of heat takes place so that the surface cools which has the effect of cooling a layer of air adjacent to the ground through conduction

Answer 215

The layer of air affected is quite thin because the effect of cooling is not carried aloft greatly since air is a poor conductor of heat

Answer 216

If a light wind blows, the resultant mixing will cause the cooling process to be felt up to a greater height so that a layer of air is formed (immediately above the surface) where the temperature is colder than the air above

Answer 217

The drier the air the more intense the surface cooling, the greater the likelihood of a strong radiation inversion developing

Answer 218

If the wind is light, in the region of 2-8kts, mixing will enhance the formation of an inversion but if the wind is stronger than that, the effect is to even out the temperature distribution throughout the lower layers and a radiation inversion is then invariably present

Answer 219

The reduction in air temp within the layer governed by the inversion may well cool air to its dew point. Should this happen when conditions are calm, so that only a thin surface layer cools, dew point will form but if satisfactory mixing has developed low cloud of fog may form within the deeper inversion layer

Answer 220

Should the surface temperature fall below 0°C when condensation has taken palace, frost will form. This will result in very cold surface air conditions especially in inland and sheltered areas during cloudless winter nights

Answer 221

It should be readily understood that when a radiation inversion is present, the increased density of the air will reduce the wind speed to a greater extent than it otherwise might have done

Answer 222

An aircraft climbing out into the air above the inversion may experience a sudden increase in wind speed and possibly also a change in wind direction. This involves vertical windshear found between layers of air having different wind velocity values; this shear can often be accompanied by some unexpected turbulence

Answer 223

If an aircraft descends into the inversion layer it will experience a sudden decrease in wind speed, the effect of which will be a steepening of the descent angle and an increase in the rate of descent

Answer 224

Visibility will deteriorate rather abruptly and this can involve distortion or discolouration of lights, looked at in the near horizontal plane

Answer 225

Although radiation inversions are usually associated with surface conditions, these are by no means the only causes. In certain circumstances it is possible that the top of a layer of cloud during the night may cause sufficient radiation to produce a radiation inversion there

Answer 226

Since water does not warm or cool quickly enough, it is not possible for a radiation inversion to form over the sea or over a lake. However, very shallow marshlands or mudflats at low tide can cool sufficiently quickly for an inversion to form and considering the high moisture content in the environment, fog may readily develop

Answer 227

This inversion often develops as the consequence of a surface radiation inversion

Answer 228

The presence of a shallow ELR involving the layers of air near the surface and the presence of a wind that is not too light and not too strong

Answer 229

When surface heating begins after sunrise there is an increase in mixing due to thermal currents and increased wind strength. This will cause turbulence to set in which will move parcels of air up and down in random fashion

Answer 230

The effect of wind is that if its strength is stronger than some 8-10kts, the mixing within the layer becomes too severe and the effect on the general ELR will be an overall steepening without the development of a turbulence inversion

Answer 231

If the moisture content of the surface air is sufficiently high, it is likely that the rising parcels will cool below their dew point and condensation will take place. On the other hand, the descending parcels will likely evaporate their water content. Thus a layer of cloud is formed towards the top part of the maxing layer with the inversion providing the lid. Initially this cloud will be of the layer type variety (stratus), having reasonably level base and top. As mixing increases and the ELR within the affected layer steepens further, instability develops, the layer type cloud will change into the cumulus variety, a layer of stratocumulus forms. Should the average ELR above the mixing layer be greater than the SALR, the onset of instability below the inversion will be carried on above it and larger developments of cumulus appear

Answer 232

The most common time of day when the development of a turbulence inversion takes place is after sunrise. As solar radiation becomes more effective, the ELR below the inversion will steepen. The difference between the surface temperature and dew point will widen and thus has the effect of raising the cloud base (CCL)

Answer 233

The inevitable gaps in cloud will appear, local hot spots on the surface will provide extra warming and the entire process now in place will cause further raising of the cloud base and fracturing of the stratocumulus. Provided no moisture content is carrie into the area (such as moisture transported inland when a sea breeze develops) the cloud will finally dissipate around mid morning

Answer 234

If a sea breeze should appear the development of cloud will be enhanced and the original stratocumulus may then turn into fair weather cumulus

Answer 235

When a stable air mass within an anticyclone overlies an area of flat lands especially during autumn and winter. The shallow ELR , the light wind and clear skies often associated with this type of air encourages the escape of terrestrial radiation during the night and through that, invariably produce a strong surface inversion

Answer 236

Is is possible to see turbulence inversions over oceans

Answer 237

Considering that air at lower levels has a higher pressure and greater density than aloft when air subsides from great heights, the rate of subsidence decreases as it enters the lower, more dense, altitudes. In addition, the more dense air near the surface is not as readily moved outwards through divergence as the high level thinner air moves inwards through convergence aloft

Answer 238

When air subsides, it is warmed adiabatically and since the higher air can descend though a greater depth and at a faster rate, due to its lower density, its degree of warming is greater than the lower air, which can only descend a smaller depth. Thus at some altitude, the subsiding air is warmer than the air below it and an inversion must therefore have formed

Answer 239

In some situations,a layer of stratiform cloud can spread out over many square miles just below the inversion layer. This cloud is referred to as anticyclonic gloom. Its thickness is not extensive but its horizontal spread can almost cover the entire anticyclone

Answer 240

Generally, the friction layer is shallow and consistent over ocean areas and therefore the rate at which subsiding air can escape outwards at the surface is restricted. The opposite happens over mountainous terrain. Thus the type of surface, oceanic or mountainous, can influence the formation or elimination of a subsidence inversion

Answer 241

In general, oceanic areas, where the friction layer is shallow but relatively dense, encourages the development of subsidence inversions but as the air mass in which the inversion has formed, moves over adjoining land, the inversion tends to break up due to a number of effects over the land such as greater diurnal change in temperature and local wind flows, often orographically affected

Answer 242

Since warm air wishes to rise and cold air wishes to descend it follows that when cold air moves towards warm air, the warm air will rise above the cold air. Similarly, if warm air travels towards cold air, the warm air again will over ride the cold air. Thus a warm sector will invariably exist above a cold sector whenever a front between air streams has formed. This might mean that during a vertical ascent from the surface into an over lying front, there must come a height where the cold sector is left and the warm sector entered

Answer 243

The warm front, due to its characteristics, is more likely to be associated with a frontal inversion

Answer 244

When temperature remains constant with height

Answer 245

The effects and consequences of isothermal layers are often the same as inversion i.e les cloudy, less moist and better visibility above than below the layer. As well, flight conditions are invariably smooth above the layer

Answer 246

Cloud can be defined as suspended water in liquid droplet or solid ice crystal form. Thus whenever cloud is present it must mean that at some stage, water vapour has condensed but it must be remembered that not all the water vapour has changed its state into liquid or solid form

Answer 247

What're vapour will not condense at the saturation temperature but requires a higher relative humidity than 100% and a lower temperature than its calculated dew point

Answer 248

In spite of the presence of salt in sea air, land has a far greater concentration of particles than sea air and in some cases it is shown that hygroscopic particles can be six times more numerous in land air

Answer 249

Either increasing the water vapour content or lowering the airs temperature, relative humidity can be increased

Answer 250

Through cooling of air

Answer 251

Layer type cloud

Answer 252

Cumulus or heaped type cloud

Answer 253

The RH of the air beneath the cloud, the state of the falling water and the size of the drop

Answer 254

Shape and structure, vertical extent and altitude

Answer 255

6km+ | 20,000ft+

Answer 256

2-6km | 7-20,000ft

Answer 257

Below 2km | Below 7,000ft

Answer 258

5km+ | 17,000ft+

Answer 259

2-5km | 7-17,000ft

Answer 260

Below 2km | Below 7,000ft

Answer 261

3km+ | 12,000ft

Answer 262

2-3km | 7-17,000ft

Answer 263

Below 2km | Below 7,000ft

Answer 264

High cloud consists of cirrostratus, cirrocumulus and cirrus. High cloud of the layer type is called cirrostratus while cloud of the heap type is called cirrocumulus

Answer 265

Like cirrostratus, altostratus may be an indication of approaching deteriorating weather

Answer 266

The thickness and altitude of altostratus means that icing can be a severe problem. If the freezing level lies within the layer of altostratus, the first 4,000-6,000ft above the freezing level will most likely produce glaze ice, which can accumulate rapidly. Above this height, rime ice at a considerable accretion rate can be expected. Any precipitation to sea level is generally light. In mountainous terrain however precipitation may be moderate to heavy (depending on the freezing level) may involve rain, snow or sleet

Answer 267

In global terms icing is usually considered to be light in altocumulus. Precipitation of any significance is not common

Answer 268

Stratocumulus may cover large parts of the sky when resulting from early morning mixing in the friction layer. Turbulence can, on occasions, be moderate especially if a turbulence inversion is involved. Otherwise, turbulence will be light

Answer 269

Except in latitudes where the freezing level is at or near ground level, icing is not considered a problem. However, if icing does occur, it can be light to moderate rime ice. Precipitation may be light showers or drizzle patches but will rarely be moderate to heavy

Answer 270

This cloud is typical of early morning mixing of air when a low level inversion has formed. Therefore, stratus can sometimes be seen as a turbulence cloud and it often is the forerunner to stratocumulus

Answer 271

In situations where the freezing level is at ground level, very light rime ice may be experienced. Precipitation may involve light drizzle, which accentuates the reduced viability that is sometimes associated with stratus

Answer 272

Surface heating and/or orographic ascent

Answer 273

If development is not very extensive, precipitation is most unusual but if development is substantial, a brief period of showers may take place when the clouds reach their maximum extent. This downpour has the effect of cooling the surface, which was the trigger that started the cloud forming in the first place and as a result, the clouds tend to diminish shortly after the shower activity

Answer 274

Turbulence is likely not only within this cloud but also outside of it because the surface heating associated with the phenomenon will produce an unstable lower atmosphere which encourages bumpy and turbulent flying conditions

Answer 275

Rime ice but within a shallow height band immediately above the freezing level it is possible to experience clear ice. In either case, the rate of ice accretion can be substantial due to the high concentration of water

Answer 276

Large water content

Answer 277

Pyrocumulus and condensation trails (referred to as contrails), both formed as th result of the combustion process

Answer 278

When large scale bush fire occurs and thermal heating from the fire is sufficient to carry the rising air up high enough for a cloud base to form, pyrocumulus may form. The process is aided by the injection of vast numbers of condensation nuclei and water. When the ELR becomes steep enough, the stage is set for large scale rising of saturated air

Answer 279

When sufficient moisture is available, cumulus will develop as the result of the heating source

Answer 280

The combustion process in jet engines produces a large quality of water, which are ejected via the exhaust into the atmosphere. In spite of the very cold ambient temperatures at altitudes where contrails normally form, the high exhaust temperature ensures that the water enters the atmosphere in liquid state but it soon becomes supercooled as the influence of the exhaust heat diminishes. When the supercooled water is mixed with the surrounding air, a contrail will only develop if glaciation takes place - supercooled water turning into ice crystals

Answer 281

Only if glaciation has occurred will the contrail persist because the change from ice to water vapour, sublimation (which causes the contrail to disappear), is a very slow process

Answer 282

Sometimes the prefix fracto is used. This term is usually restricted to ragged or broken low level patches of cloud of either cumulus or stratus variety

Answer 283

The lowest layer of cloud is reported regardless of its coverage, i.e even 1 oktas would be reported. The next higher layer will be included in a report if its coverage is 3 oktas or more. The highest layer will be reported if its cover is 5 oktas or more. In all circumstances, the presence of CB cloud, regardless of the amount, will be reported

Answer 284

At secondary aerodromes the cloud base is determined by a ceilometer, which forms part of an automatic weather station (AWS). During hours of darkness, when cloud cannot usually be seen or when accurate cloud base readings are required, it is possible to assess the cloud base with a searchlight or ceilometer

Answer 285

A searchlight shining vertically onto the base of the cloud. An alidade is used to measure the angle at which the spot of light appears and the height of the base can be calculated using tables

Answer 286

A photoelectric cell, facing up, is positioned a certain distance from the base of a near vertically scanning searchlight. Assuming a cloud exists, the light from the searchlight will scatter to some extent from the cloud and some of this scattering will be received by the vertically sensing photoelectric cell. Since its position from the searchlight is known, a triangular calculation can be made instantaneously

Answer 287

The system can be used during the day or night and it can also be used to provide a ceilogram record when the information is automatically plotted on a graph

Answer 288

The vaisala lasted ceilometer, which is installed at most airports in NZ. The equipment continuously measures cloud base by transmitting and receiving the echo from a reflected laser diode. The ceilometer analyses the echo reflected back from the cloud. The system can detect multiple cloud layers as well as the occurrence of rain, snowfall and fog. The measurement range is from 0 to 12,000ft and displays readings at 50ft intervals

Answer 289

The advantages of an instrumental reading of cloud height is that it removes the uncertainty inherent in visual estimations. The height measured will be very accurate

Answer 290

It doesn't scan the whole sky. The best observations will be those that combine visual and instrumental sources of information

Answer 291

The extent and type of cloud due to orographic rising depends on the RH of the air and its stability

Answer 292

If air is sufficiently moist, the lifting condensation level will be below the mountain tops and the cloud will be cumulus if the ELR is greater than 1.5°C/1,000ft or layer type if the ELR is less than that

Answer 293

If cloud is to develop, the ELR must be steeper than 3°C/1,000ft because dry air will only continue to rise above mountain tops if it is unstable dry air (absolute instability)

Answer 294

The term orographic cloud is not restricted to cloud in the immediate vicinity of mountains-lenticular clouds in standing waves of the lee of a mountain range can also be considered orographic

Answer 295

It involves an uplift of air due to interference from small features such as buildings, tree lines, small hills etc. unless air is very unstable the type of cloud associated with this kind of rising is generally limited to low level development of stratus or stratocumulus

Answer 296

This type of rising results when the ELR exceeds the DALR value, i.e when warm parcels of dry air expand and rise. There will be a requirement for a minimum surface temperature if convective cloud is to appear and this can be calculated using suitable graphs

Answer 297

If no moisture is carried along and the dew point is not altered, the height of the CCL will rise as the surface temperature increases and vice versa

Answer 298

Cloud forming as a result of thermal rising will be cumulus cloud its height depending on the amount of moisture available and the instability of the air

Answer 299

Depressions and their troughs are of course the most common system in which widespread scent is involved but it must be remembered that these are not only ones that can produce widespread vertical motion. For instance, the inter-tropical convergence zone (ITCZ), when the trade winds of each hemisphere meet, can be seen as a huge system of widespread ascending air

Answer 300

Large scale ascent invariably involves slow rates of vertical motion. As a comparison, the ascent rate is most often only half the rate of that experienced in cumulus build ups

Answer 301

High level divergent when air near the top of the tropopause has a net outflow. This causes the pressure near the surface to decrease and a depression to form. In well developed depressions the growth of cloud through condensation will cause a substantial release of latent heat and stability will be adversely affected as a result. Thus, while the air may have been stable prior to and shortly after widespread ascent started, the warmth released in cloud will soon encourage sustained rising-a characteristic of unstable air

Answer 302

When warm air overrides cold air it creates a shallow slope. Cumulus cloud can be associated with warm fronts but the predominant cloud type is stratiform including nimbostratus. It can last for many hours, even days. They travel much slower than the cold fronts. Precipitation can be persistent and heavy; when the freezing level is low enough, snow to low levels can be encountered

Answer 303

Slopes fairly steeply. If the rising air is conditionally unstable, as is often the case, cumulus cloud of varying intensity will develop. The cloud base can be very low and in some cases can be accompanied by a squall line, which is then responsible for the sudden increase in wind strength as the front approaches. Cumulonimbus cloud, thunderstorms and hail showers are frequently encountered under those conditions

Answer 304

Due to the generally steep slope of the frontal interface, the width of the cloud band is not very great, typically in the order of 100 to 150km. Thus if the speed at which the front travels is its usual 25-30kts, the weather associated with the cold front passing a certain locality will only affect it for some two to three hours

Answer 305

Adiabatic cooling of air through rising

Answer 306

Adiabatic warming of air through subsidence

Answer 307

Large scale dispersal of cloud is often experienced when air is forced down through subsidence in an anticyclone

Answer 308

Warming of clouds through absorbti n of either solar or terrestrial radiation

Answer 309

Warming of low level air through surface heating so that the difference between the airs temp and dew point increases. As this happens, the cloud base (CCL) will rise and, provided no additional water vapour is introduced, the rising cloud base will soon equal the cloud top and cloud is said to have burned off

Answer 310

It will reduce the buoyancy of air and thereby cause subsidence of air, which may contain turbulence cloud such as fair weather cumulus. This is a common sequence of events towards the evening of fine summer days

Answer 311

Mixing of unsaturated air outside of cloud with the saturated air within the cloud. The relative humidity overall may then well go below 100% and cloud dispersal will follow

Answer 312

That the saturation vapour pressure of air over ice is less than that over liquid water. This can be rephrased by saying that air that is saturated over liquid water droplets will be more than saturated over ice crystals. In other words, water is condensed out more readily over ice than over liquid

Answer 313

If ice crystals are present in the company of supercooled water droplets, the air may be saturated relative to the ice surfaces but unsaturated relative to the water and therefore the water drops may evaporate. Consequently the released water vapour off the evaporating water drops will be attracted to the ice crystals and through the process of deposition (water vapour changing directly to ice) it will cause the ice crystal to grow at the expense of the liquid water drops. Additionally, if the ice crystals are in motion, their collision with supercooled droplets will also cause them to grow

Answer 314

At temperatures near -15°C the growth rate through deposition seems to be most efficient

Answer 315

The formation of ice crystals requires nuclei different from those needed for the formation of liquid drops, i.e condensation nuclei. Ice nuclei which are required for ice crystal birth, are far less numerous and restricted to only few constituents. Clay seems to be a ready source for ice nuclei, while fine soil particles and volcanic dust are also popular

Answer 316

The fusion of coalescence of drops when a large drop has formed and commenced its descent. For the large drop to be formed in the first place requires a large or giant nuclei which are not numerous in the atmosphere. Research has confirmed that oceanic air is more likely to have these giant nuclei and for that reason it is not uncommon for heavy precipitation to occur from clouds that have their origins over ocean areas

Answer 317

The speed at which the drop falls depends largely on their mass in that the larger the drops, the higher their terminal velocity. Other factors affecting the rate at which drops fall must be the force of gravity and drag on the drop itself. If the gravitational force is larger than buoyancy and drag, drops will continue to fall

Answer 318

If ice crystals have been formed, it is relatively certain that growth will take place and the type of precipitation then depends on the dryness and temperature of the air below. If ice crystals are not present, the onset of precipitation is then dependent on the availability of some large drops to start the coalescence process

Answer 319

Hailstones consist of layered ice of both the clear and rime variety around a cor ice crystal. The size of hail stoned can vary from some 3mm radius up to 40mm and even more

Answer 320

(Rain, drizzle, snow) implies an extensive, thick cloud sheet, which is not likely to clear in the short term

Answer 321

Temporary improvements are likely

Answer 322

(Rain, hail, or snow) can only fall from cumuliform cloud and are characterised by much brighter periods, possibly even large blue sky patches between the periods of precipitation. Showers are also characterised by sudden onset and cessation

Answer 323

In terms of light, moderate and heavy. The amount of water falling on a flat surface or into a rain gauge per unit time, will determine the rate

Answer 324

Once a drop reaches a smaller size (approx 5mm) it splits into smaller drops

Answer 325

Weather surveillance for precipitation detection over a wider area and wind finding radar

Answer 326

A network of five radar installations is required to cover NZ- three have been installed to date at AU,WL, and Rakaia

Answer 327

Unobstructed radar coverage, access to electricity and access for installation and maintenance

Answer 328

Intensity for detection and identification of precipitation and Doppler to provide speed and direction of travel of the precipitation

Answer 329

The tipping bucket type

Answer 330

The unit consists of a collecting funds from which rainwater flows to the tipping bucket - the measuring unit. The bucket is divided into two equal compartments and pivots about a central point, one side resting against the stop. The bucket mechanism is adjusted so that as soon as 0.25mm of rainfall has entered a compartment, it over-balances and will come to rest against the other stop. The water collected is emptied out and the sequence repeats itself as the other compartment fills

Answer 331

The transparency of air, which is directly related to the absence of solid or liquid particles within the air

Answer 332

The horizontal plane, in other words, how far can an observer on the ground see an object of a given size against the horizon sky

Answer 333

As the greatest distance a black object can be seen and recognised against the horizon sky

Answer 334

Whether it's day or night, if transparency doesn't change visibility doesn't change

Answer 335

Precipitation, fog and mist, haze, smoke, sea spray

Answer 336

500m or less

Answer 337

100m or less

Answer 338

4-10km. Light rain has a limited effect; visibility may well exceed 20km

Answer 339

Moderate snowfall will reduce visibility to below 1,000m while heavy snow will reduce this to less than 100m

Answer 340

Depends on the associated weather. If drizzle is accompanied by mist or fog the visibility can be far less than if it had originated from low cloud such as stratus or stratocumulus. In the former, visibility can be less than 500m while the latter visibility may be reduced to 3km or less

Answer 341

Fog is cloud on the surface when the visibility has reduced to less than 1,000m. Mist is reported when the visibility is 1,000m or more but does not exceed 5,000m

Answer 342

Haze relates to atmospheric conditions when visibility is reduced to 5,000m or less through the presence of dust particles. If dust or sand grains are blown off the surface and reduce visibility to less than 1,000m, they are referred to as sand or dust storms

Answer 343

Sand storms, due to the size of sand particles, rarely go higher than some 100-150ft above the ground. Comparable smaller dust particles however, can be blown to much greater heights, in the region of 8,000-10,000ft

Answer 344

Industrial and residential fires add innumerable particles of soot and carbon to the environment. These will reduce visibility through haze but additional problems are caused when these pollutants provide vast amounts of condensation nuclei. Thus if the airs moisture content is even remotely close to saturation, it does not take a great deal of cooling for the water vapour to condense prematurely

Answer 345

Blowing snow above 2m

Answer 346

Drifting snow within 2m of the surface

Answer 347

Slant visibility is a special problem where the surface is covered by a layer of mist or haze

Answer 348

Flying directly overhead the crew can easily see individual ground features such as runways etc. because the distance they have to see through the haze layer is relatively small when looking through it at right angles. However, having turned onto the final approach, the distance through the haze layer is now slant range, which is much greater than when overhear. Thus the visibility rapidly deteriorates

Answer 349

Depending on the density of the haze or mist layer, there is a maximum eyesight penetration range. Thus if an aircraft flies just above the haze layer the number of miles one can see ahead on the ground is not great whereas increased height will still involve the same penetration range but the distance one can see ahead is now greater

Answer 350

At some airports, the visibility prevailing along with runway, runway visual range, can be measured accurately through sensors positioned at the threshold, mid point and stop end of a runway. RVR is passed to pilots via METAR, ATIS or a controller whenever the prevailing visibility reduces below 1,500m. RVR applies to horizontal visibility along the runway as seen from 5m above the touchdown point

Answer 351

It measures the scattering of light by particles in the atmosphere and expresses the result as a voltage or frequency, which is proportional to the degree of scattering. The sensors microprocessor converts the info into a visibility range in miles and km's. The sensor projects an infrared beam while an infrared receiver detects radiation, which is scattered in the vicinity of the beam. If the air is perfectly clear no radiation is scattered to the receiver but as the air becomes more hazy, increasing amounts of scattering are received

Answer 352

The sensor measure visibility only within the airspace within the vicinity of the infrared beam. Thus if visibility is different in another quarter it is not detected by the sensor

Answer 353

Fog is cloud on the surface, whatever that surface ,at be. Since the formation of most clouds requires the initial presence of water vapour, a cooling mechanism and hygroscopic particles, it follows that when these factors are found near the surface fog is likely to form

Answer 354

When terrestrial radiation escapes and is not replenished by incoming solar radiation, a net loss of heat occurs at the surface. Thus, as the surface cools, conduction will cause the air molecules touching the surface to cool as well. Provided the RH (and thus dew point) is high, it takes only little cooling of air to reach saturation, and soon after, condensation. Air is a poor conductor of heat and therefore if no mixing of the surface air takes place, a very shallow layer of condensed water will lie on the surface, dew will have formed

Answer 355

If a light wind were to eventuate, sufficient to mix the surface air so that the cold surface temperature is distributed more extensively, the layer of air within which water drops are present thickens and fog will form. But if this wind is too strong the result is more likely to be a layer of cloud at a few hundred feet above the surface rather than fog on the surface

Answer 356

A high RH so that little cooling is needed to saturate the surface air, a clear sky is that terrestrial radiation can readily escape, a light wind so that mixing within the surface air layer is assured and a generally stable atmosphere so that mixing and cooling are confined to a shallow layer

Answer 357

In anticyclonic conditions and when moist air is readily available. The weak pressure gradient usually associated with an anticyclone will help to produce the light wind, which is normally required for radiation fog

Answer 358

The subsidence that invariably accompanies an anticyclone is likely to produce the stability of air, which prevents the fog bank from lifting off the surface while the clear skies associated with anticyclones encourage the uni pended escape of terrestrial radiation

Answer 359

While radiation cooling of the surface progresses, the surface air temp on the ELR graph moves to the left indicating a narrowing gap between it and the airs dew point. Since air is a poor conductor of heat it follows that the ELR at some height above the surface will not be affected by the colder surface temperatures and the ELR starts to show a surface radiation inversion. As soon as the surface air temp reaches dew point values saturation will occur and further cooling will cause condensation

Answer 360

Provided mixing takes place, a layer of fog will then be produced which, itself, influences the ELR. Cloud is a good absorber of terrestrial radiation, providing it with the capacity to become a source of radiation itself. This means that as soon as a fog layer has formed, the effects of escaping terrestrial radiation is offset by the warming within the layer of moisture and gentle mixing within it will cause the ELR to become isothermal rather than remain negative

Answer 361

If one ascends within this fog bank, the temp will remain more or less steady with height but near the top of the bank there will be a small increase in temperature, indicating the remnant of the inversion. Above that, the temp will decrease again consistent with the ELR prevailing in the upper air

Answer 362

The more through the mixing, the higher the position of the inversion, the deeper the layer of fog. If the wind causing the mixing is too strong however, the fog will likely be lifted bodily off the surface and a layer of low stratus is formed

Answer 363

Cooling of the surface is most common during night time, which means that the longer the night, the more intense the cooling process. Additionally, the cooler the surface before night fall, the greater the chance of the surface air cooling to and below its dew point

Answer 364

During wintertime, when nights are long and the surface already fairly old even during the day, radiation fog is more likely to occur than during summer. However, autumn air is still relatively warm which enhances the presence of water vapour and therefore fog may occur when early dips in night time temp or the occasional frost is experienced

Answer 365

Once radiation fog has formed in the early stages of the night or even towards dawn, it will likely persist until well after sunrise. Radiation fog cannot form over the sea because the water temperature does not cool sufficiently quickly

Answer 366

After the sun has risen, the particles within the friction layer become excited and mixing intensifies. The top of the fog bank will experience some of this turbulence with the result that the height of the fog bank increases. Additionally, the initial solar input into the fog encourages the droplets at and near the surface to evaporate. This process requires latent heat, which is drawn from wherever it is available, including from the air above the bank

Answer 367

The combined effect of extra mixing and demand for latent heat will produce a thickening of the fog bank after sunrise

Answer 368

Solar radiation does have a limited ability to penetrate moisture; it also has some warming effect on the water content within the fog bank. These factors combined will cause a steepening of the ELR; a positive difference develops between the surface air temp and its dew point. The fog will lift off the surface in places, a few holes will appear and direct solar input onto the surface will further steepen the ELR. As a result a layer of low cloud will form which initially takes the form of low stratus, but sometimes it turns directly into stratocumulus

Answer 369

If radiation fog forms in hilly or mountainous terrain, the effect of katabatic flow will intensify its development. Thus katabatic flow will enhance the formation of valley fog. Once formed in hilly terrain, fog will invariably settle into low lying areas and often flow through valleys and gorges to the adjacent planes

Answer 370

Although radiation fog is very often a night occurrence - it is not unusual for it to form after sunrise when the RH has been high but a lack of mixing during the night prevented its formation. The mixing produced by solar input after sunrise and the demand for latent heat as some surface moisture evaporates may cause radiation fog to form at that time

Answer 371

Dispersal of radiation fog is normally assured once solar radiation is introduced, i.e after sunrise

Answer 372

Advection fog requires a surface that is already cold so that when moist air is transported over it (advection) the air can cool to temperatures lower than dew point

Answer 373

The requirement for mixing is generally met by the action of the wind, which transports air, and a high RH assures that the airs water content is sufficient to produce condensation

Answer 374

Advection fog can form over water and land

Answer 375

Advection fog can last for days and even weeks under some circumstances

Answer 376

Radiation fog normally requires anticyclonic or col conditions whereas advection fog does not necessarily require this. Provided the wind is not too strong and moist air is blown across a sufficiently cold surface, advection fog may form

Answer 377

Two very common causes of advection fog dispersal are, firstly, a change in the direction and/or strength of the wind, and secondly, advection of less moist air

Answer 378

This fog tends to form when an abundance of moisture from rivers and streams is assured and a rapid radiation cooling process is available from rock walls, which will have been in shadow for some time during the afternoon. Valley fog can often be very persistent especially in winter when the low sun cannot penetrate down to the valley floor

Answer 379

When a light wind forces moist air up a cold slope such as a snow covered mountain, fog may form. The end of this fog bank is found where the up slope wind stops

Answer 380

When a cold ocean stream is surrounded by warmer oceans, the cross flow over the cold surface can produce fog. It is then expected that this fog will be restricted to the cold stream boundaries

Answer 381

The advection type

Answer 382

The warm air above a warm stream is likely to be very moist and if this is transported over adjacent cooler ocean surfaces very extensive fog can be produced

Answer 383

Addition of moisture from a warm wet surface to colder air above to the extent that it becomes saturated or transport of very cold land air over a relatively warm sea surface

Answer 384

Can occasionally be experienced in geothermal areas where geyser activity expels such large amounts of water that the air saturates. On rate occasions fog can develop when the sun comes out following a heavy shower or thunderstorm as moisture evaporates from the warm wet ground. Usually such fog dissipates rapidly because the warm surface tends to make air unstable but if this fog has formed towards the end of the day, it may persist for some time

Answer 385

Because fog is associated with light winds, the chances of frontal fog occurring with warm fronts are better than with cold fronts because the first mentioned are invariably more gentle in style

Answer 386

One or more electrical discharges manifested by lightening and thunder

Answer 387

The development of CB. Flight conditions will involve moderate to severe turbulence, icing, possibly lightening strikes and severe windshear at low levels

Answer 388

These dramatic and violent occurrences require vast amounts of energy most of which are thermal in origin. Development of the necessary CB cloud requires strong instability, which is often a con eques even of high surface temperatures

Answer 389

Provided the moisture content of air is sufficiently high, the convectively rising air will soon experience condensation so that cumulus cloud is formed. This process in itself is a strong provider for instability because the latent heat released during condensation adds heat to the air in the immediate vicinity. Thus the clouds temperature is warmer than the surrounding air and added impetus is provided to the rising process

Answer 390

With increasing height, surrounding colder air outside the cloud is sucked in, a process known as entrainment, which causes the warming process to diminish. Furthermore, as less moisture becomes available with height, the release of latent heat reduces so that the convection process slows down the combined result is a levelling off in instability at altitude which invariably produces high level layer type cloud in the form of the typical anvil shape often found when CB's have often passed their maximum intensity

Answer 391

An atmosphere that is unstable through a deep layer, an adequate supply of moisture from below, a trigger action that causes the initial upsurge of the unstable air and a mechanism that will produce sufficient electrical charge differences

Answer 392

There must be something that sets the ordinary CB apart from those that are associated with thunderstorms. The difference is the presence of lightening with its associated thunder. Lightening is the result or difference in electrical potential. A build up in this difference causes a spark to jump the distance between differently charged particles or between these particles and say, the earth surface

Answer 393

The ionosphere is positively charged while the earths surface is often negatively charged. Thus cloud droplets would acquire a negative charge on top and a positive charge at the bottom. Differences so caused, involving millions of drops, may be sufficient to produce the required potential

Answer 394

All draughts are upwards. The rate at which air is moved up can exceed 2,000ft/min while the horizontal extent of development can involve an area of some 10km across

Answer 395

Large quantities of latent heat are released which enhance the rising process while at the same time they strongly influence (and raise) the height of the freezing level within the cb cloud. The top of the cloud continues to rise, in some instances in the tropics this may well involve rising into the stratosphere. In mid latitudes, tops of cb's rarely go beyond 25,000-35,000ft. The cloud tops have a boiling up appearance; there is no high level stratiform development accompanying the cb at this stage

Answer 396

The growing stage takes some 15-30mins but this timing can be quite different either way under certain conditions and in different latitudes

Answer 397

The drag between the precipitating mass and the updraughts has a strong diminishing effect on the velocity of the updraughts so that after a brief period of time, some downdraughts are produced. Normally these downdraughts are restricted to the front portion of the cb while updraughts continue to function at the rear

Answer 398

The air in the down draughts warms at the SALR because the descending air is saturated. Due to this low rate of warming, the down draughts are increasingly colder compared to the surrounding unsaturated air. This has the effect of accelerating the velocity of the down draught so that the greatest vertical speed is generally found towards the bottom of the cb

Answer 399

Between the up and down draughts

Answer 400

At the down draughts hit the surface, they have no option but to split and spread out horizontally. The speed at which this flow travels horizontally is greatest close to the cb cloud - speeds in excess of 45kts have been recorded. This high velocity squall is known as the first gust also known as the gust front. In some cases, the horizontal flow can rise off the surface some distance in front of the cloud and return to the cloud in the shape of a roll cloud

Answer 401

Although the surface deflected down draughts produce a horizontal flow all around the base of the cloud, the maximum strength is usually to the front of the cb i.e in the direction of travel

Answer 402

The freezing level oscillates, lower in the forward half of the cloud where the downdraughts prevail and higher where the up draughts are found (usually the rear). The onset of down draughts causes precipitation in the form of heavy showers of water and more often, hail. At about the same time, lightening flashes are seen

Answer 403

Towards the end of the attire stage and certainly the decaying stage, equilibrium with the surrounding air is reached near the top of the cb cell. This gives rise to horizontal outflow of ice crystals aloft which forms into stratiform cloud often in the shape of an anvil. Usually the anvil points in the direction of travel of the cb

Answer 404

Approx 15-20 mins

Answer 405

Up draughts within the particular cell cease so that a generally descending motion takes place. Turbulence and gusts decrease and in the absence of other cells at different stages, the high level stratiform cloud settles and the cb slowly disappears

Answer 406

Precipitation is common during this stage and will occur underneath the entire base of the could. The freezing level moe bulges down relative to the level outside the cloud

Answer 407

Not all cells are at the same stage of development. For instance, while one cell within the cb is in the growing stage, another one may be in the decaying stage and so on. Research has also indicated that the cycle of one cell can be interrupted or affected by the behaviour of the neighbouring cell. As well, the lifting of air at the gust front may be the trigger action necessary to set off the development of another cell

Answer 408

Is the presence of a windshear layer aloft. It is generally found that air above the shear level is drier than that below and due to the altitude where this type of shear normally occurs, the strong airflow is also cold

Answer 409

This type is found when unstable moist air is forced to rise over mountainous terrain

Answer 410

Time of day or season have no major influence although the added effects of hear during the afternoon in summer and autumn may aggravate thunderstorm conditions

Answer 411

Provided the unstable moist air is continuously transported onto the mountains, the orographic type can preserve sometimes for days on end

Answer 412

With moisture content often high in orographic cloud, the associated precipitation can be very heavy. Under these circumstance, it is not unusual for mountains and even the adjacent plains, to have a heavy coating of hail, even in the summer

Answer 413

Warm, moist air to be heated from below so that strong instability is encouraged

Answer 414

The heat type thunderstorm, under most circumstances (but not always), is diurnal and land related and must be sufficiently close to a water source for cb developments to progress

Answer 415

This favours coastal regions during the summer time and generally restricted to the early to mid afternoon

Answer 416

The heat thunderstorm can well be a daily occurrence throughout the year, starting in the early afternoon and sometimes persisting until well after sunset

Answer 417

Provided surface heating is strong enough, thunderstorms can develop from cb clouds, which have relatively high bases and little or no precipitation. Lack of rainfall onto the surface is often caused by evaporation within very warm air usually found between cloud base and the hot surface

Answer 418

This type can be found in a variety of circumstances when convergence takes place. Thus the heat type cb may well also fall within the convergence type found, for example, during a warm summers day when sufficiently strong sea breeze activity and unstable conditions force moist air aloft. Alternatively, the convergence of moist unstable air associated with low pressure systems may be sufficiently strong to produce cb's. A final example of this type is the frequent thunderstorm activity associated with the intertropical convergence zone when it is active

Answer 419

Is a common occurrence over oceanic areas approx 10 degrees latitude either side of the equator

Answer 420

The sea temp in those latitudes remain very warm throughout the year and records show that temps around 30°C are not unusual. Thus the air above the sea surface remains similarly warm but due to radiation losses of air at high levels, a steepening of the ELR follows as the air and clouds cool aloft

Answer 421

This type is at its most active towards dawn when the ELR has reached its steepest value. After day break, the thunderstorm persists for some time but under normal conditions, they cease to exist around mid morning

Answer 422

When relatively cold air moves over warm sea areas, it absorbs large amounts of water from the sea surfaces while at the same time its lower levels warm rather quickly. As the ELR steepens and vertical mixing intensifies, the stage is set for the development of cb cloud

Answer 423

This type is not unusual in the Australian region when cold polar air moves towards the continent in both winter and spring time. It can also be experienced in NZ region under similar conditions during spring and early summer. In either case, the intensity of thunderstorm activity through this cause is not very great in relative terms

Answer 424

The ups are often stronger and of greater vertical extent than the downs

Answer 425

Up and down draughts do not necessarily cause severe turbulence if the side of the draughts is large. However, maintaining altitude is a problem where the aircraft may be bodily forced to rise or descend through many thousands of feet in some instances

Answer 426

Up draughts are considered to be more forceful from the middle of the cb cloud to its top. Down draughts tend to be stronger from the middle of the cloud to the surface

Answer 427

Up and down draughts are associated with turbulence where draughts meet. Thus flight in mature stage cb clouds will more than likely produce moderate to severe turbulence on the boundaries between draughts

Answer 428

Altitude control and the secondary problem is turbulence especially when opposing draughts are closely spaced

Answer 429

The major cause of severe turbulence is the presence of gusts which are short term and localised fluctuations in both vertical and horizontal airflow, superimposed on the up and down draught pattern

Answer 430

Severe load factors imposed on the airframe; possibly large changes in airspeed which may cause a pilot to exceed the speed limits for turbulent flight and abrupt changes to the aircraft attitude which may cause structural failure if the pilot attempts to correct these changes too quickly or harshly

Answer 431

Although gusts can be found in many parts of the cb, the most likely places are in the dividing sectors of strong up and down draughts found in the mature stage

Answer 432

The cold dense outflow, which can frequently reach up to 1,500-2,000ft at speeds up to some 40kts or thereabouts, propels itself along the surface beneath the warmer environment air resulting in: a gust front forms some considerable distance ahead and a shear zone is formed between the cold outflow and the warmer air above which often travels in the opposite direction back into the lower part of the cb

Answer 433

Above the freezing level, icing conditions are worst within the first 6,000-8,000ft because within that band of height, supercooled water and droplet size combine to produce clear ice, which readily builds up and adheres strongly to the aircraft; it is difficult to get rid of

Answer 434

Towards the tops of thunderstorm clouds, and supercooled water that may have survived will likely freeze in such a way as to produce the more brittle rime ice, which is somewhat easier to remove. But then the risk of ice ingestion in turbine engines becomes a real risk

Answer 435

Flight below the cloud base will cause less icing problems but this may well be offset by the dangers of turbulence and downdraughts normally to be expected there. Aircraft performance permitting, flight through the tops or above the tops would clearly be preferable

Answer 436

Problems caused by lightening strike involve possible loss of aerials because when stuck, these will cut off neatly and instantly. Magnetic compasses will be strongly affected and large deviation errors result. It will be necessary to swing the compass before it's reliability is restored. Electronic equipment may suffer severe damage and reliance on instruments and aids that rely on such equipment may be impaired. Finally, temporary blinding may occur when crew and passengers are in dark or dimly lit environments. For this reading it is recommended that cockpit and cabin lights are turned to full bright

Answer 437

While dealing with electrical charges and charge differentials, an aircraft flying through cb clouds, heavy precipitation or even sand storms, may build up a potential charge so great that discharges can occur between the aircraft and the cloud, or aircraft and precipitation and even the aircraft and the ground. Controls attached to the airframe through hinges, can similarly be charged differently from the airframe and for this reason, amongst others, bonding is invariably provided between controls and airframe. Similarly, the use of static strips or wicks will do much to dissipate charge potential differences between aircraft and surrounding air

Answer 438

Under all the charged conditions, it is possible that highly charged air, when rapidly accelerated through a propelled disc, past a nose cone or through a heli disc, may become a visible discharge known as St. Elmo's Fire. Although the phenomenon looks very frightening, it is totally harmless

Answer 439

Statistics show that stones larger than 1/2 inch pose a serious threat especially to high speed aircraft

Answer 440

The worst parts of thunderstorm clouds, from the point of view of hail, are between some 8,000 and 25,000ft in mid latitudes. In tropical regions, these values are much higher

Answer 441

Hail at sea level is not generally a problem with CB clouds in tropical and sub tropical regions because the freezing level is fairly high and stones will have melted to a large degree by the time they reach lower heights

Answer 442

In mid latitudes, hail can persist to sea level and statistical evidence shows that even in summer time, the odd thunderstorm can produce a substantial coating of ice on the ground; admittedly, it is not likely to remain there for long

Answer 443

Flight below the base of CB, especially in the growing and the early mature stages, will involve substantial local convergence. The associated reduction in pressure may be very serious if ground clearance is not great and visibility poor because the altimeter will tend to over read i.e the crew will be lead to believe that aircraft height is more than shown on the altimeter

Answer 444

A blocked pitot source through ice or affected by heavy water ingestion, will seriously impair the workings of the ASI

Answer 445

This depends mainly on latitude, aircraft performance capability and terrain a. If possible, flight should be conducted over the tops of cloud and if that is not possible, flight beneath the base should only be attempted if visibility there is adequate and turbulence acceptable. In tropical regions, tops of cb clouds can penetrate the stratosphere but in mid latitudes, tops should be found at some 25,000-30,000ft. Thus away from tropics, flight above cloud tops should be feasible for modern jet aircraft

Answer 446

It is prudent to switch on all anti or de-icing equipment at an early stage and not wait until the symptoms of blocked intakes present themselves

Answer 447

This should be reduced to below maximum turbulence penetration speed or to a speed indicated in the aircraft flight manual

Answer 448

Switch lights to bright

Answer 449

Unless the flight manual dictates otherwise it is common practice to disconnect the autopilot. Abrupt and violent control inputs should be avoided and the main attempt should be to hold the attitude as steady as possible under the circumstances. If necessary, request for greater aircraft separation should be made

Answer 450

A line of thunderstorm clouds should be crossed as close to right angles as possible so that minimum time is spent within them

Answer 451

If airborne radar is available, this should be use to maximum advantage in order to fly between CB's rather than through them

Answer 452

Ground based radar designed for the purpose of thunderstorms is used to map areas of precipitation and to track their movement, providing information on the size and intensity of individual cells. MetService has a chain of such stations providing coverage for most of the country. Aircraft tracking radar and wind finding radar are not particularly effective for locating storms

Answer 453

Airborne radar especially colour display, is most useful in avoiding cells and intense precipitation areas. Range graticule and angle off displays are usually provided but it must be remembered that whereas at some distance, a gap between CB's my show a likely avenue for safe flight, by the time the aircraft gets there, growth of cells may have closed the gap entirely

Answer 454

The worst areas of turbulence on an airborne radar display are shown as red while areas of moderate turbulence are painted yellow. Green areas indicate where turbulence is light

Answer 455

Tornadoes are rotating funnels of air sucked up from below the base of CB cloud. They are caused by extremely strong upward motion within the cloud

Answer 456

The high rotational velocity of a tornado is produced when there is an existing rotation of air beneath the CB and this rotating air is drawn into the base of the cloud through convergence. As this convergence develops it increases the rotational velocity of air (conservation of angular momentum)

Answer 457

The rotation of air beneath a CB is very often around a vertical axis and the increase in rotational velocity through convergence as it is drawn in is therefore readily understood. But rotation due to wind shear can also produce tornadoes

Answer 458

When low level wind shear takes place beneath or in the vicinity of a CB, air is made to rotate around a horizontal axis like a roll of dough when it is rolled along a table top by hand. Should part of the rolling air be drawn into the base of the CB, the axis of rotation becomes vertical and convergence will then cause rotational velocity to increase

Answer 459

Rotational velocity in many tornadoes are often so high (values in excess of 300kph have been recorded) that atmospheric pressure in the core can drop hundreds of hectopascal's

Answer 460

The diameter of the core is only a matter of meters but some cases have been recorded where the diameter reached in excess of 200 meters. This core diameter must not be confused with the diameter of the total tornado, which can be 2 or even 3km

Answer 461

Tornadoes can form over water areas Ao that the typical water spout which sucks up water develops

Answer 462

Small scale tornadoes can develop in hot arid regions such as the Australian interior. The well known willy-willies or dust devils are a very common occurrence during the dry season. These small tornadoes can develop in a very short space of time and can travel for quite some distances before decaying

Answer 463

Turbulence can be defined as small scale, short term, random and frequent changes to velocity of air. In other words, when there are rapid changes in either the airs speed or its direction of movement or both, conditions are said to be turbulent. These changes are brought about by the imposition of vertical (or non horizontal) velocity components on the horizontal movement of air

Answer 464

When the surface is sufficiently warm, vertical currents involving irregular eddies produce turbulence, which can reach great heights in conditions of strong instability. The degree of surface heating is uneven due to different surfaces. Therefore, the intensity of turbulence can change quickly as these differing terrains are overflown

Answer 465

When thermal eddies have formed above a warm surface the effect can invariably be avoided by flying above the friction layer or above convective cloud tops

Answer 466

When absolute instability in the friction layer had been established, turbulence will occur. In many situations this instability extends beyond the friction layer and, when the moisture content of air is high enough, cumulus clouds will form which, through release of latent heat, add to the instability. Under those circumstances turbulence will be experienced from the surface up to cumulus top levels (where the inevitable inversion is found). In the absence of cloud, the top of the turbulence layer can often be seen at the level where improvement in visibility begins

Answer 467

Although steepening of the ELR is very often caused by warming of the surface through insolation, the same result will show up when a cold air stream moves into warmer latitudes i.e cold advection. Thus any factor that has the capacity to steepen the ELR may be instrumental in producing thermal turbulence

Answer 468

Turbulence caused as a consequence of insolation will be subject to diurnal fluctuations. Thus conditions will be most turbulent in the early afternoon and smoothest during the night and especially so just before dawn

Answer 469

Interference of surface features on the horizontal flow of air

Answer 470

Speed of the wind, size of the obstruction, shape of the obstruction and the prevailing ELR

Answer 471

The stronger the wind, the larger the obstacle, the rougher the obstacle and the steeper the ELR, the worse the turbulence and vice versa

Answer 472

When the speed of the wind is less than some 15kts, low hills, buildings, tree lines etc. will produce light turbulence in the lee of these obstructions. In general, under these wind conditions, small scale turbulence does not extend much above approx 1/3 the height of the obstruction above the obstruction

Answer 473

In winds slightly stronger than some 15kts it is often found that standing eddies form at both the windward and lee side of solid obstructions. The eddies in the lee tend to travel downwind so they their influence can be felt some distance away from the obstruction. However, when wind speeds become strong and gusty, the flow becomes highly irregular, standing eddies no longer occur and moderate turbulence can be experienced

Answer 474

When the moisture content of air is sufficiently high, cumulus cloud may develop from the windward side of the hills and spread over subsequent ridges

Answer 475

The wind approaching mountains will move laterally around them, over them or through valleys and saddles. In almost all cases the speed of the wind and the probability of turbulence increase where the flow bends.

Answer 476

A major factor determining the flow pattern is again stability of air. In conditions of string stability aloft the low level airflow is prevented from rising much above mountain height. In most cases this situation is linked with reduced wind shear (slower at altitude) so that rotor streaming and associated moderate to severe turbulence is likely to be encountered at low levels in the lee of the mountain

Answer 477

In less stable conditions, the interference of a mountain range on the flow of air can be felt at great altitudes as evidence in wave action and rotor turbulence. The most extensive turbulence due to large scale interference is generally to be expected from high mountain ranges when air is unstable

Answer 478

Valley winds, even when strong, tend to follow the confines of valleys up to approx 2/3 the height of the surrounding mountains. Above that height the prevailing (geostrophic/gradient) wind is usually experienced. Thus turbulence may be remarkably light near valley floors. This factor must not be relied upon however, because valleys are invariably intersected by others so that conflicting airflows and turbulence can be experienced in the vicinity of valley junctions. As well, saddles and slanting ridges can have a strong influence in lowering the cross valley wind height

Answer 479

When moisture content of air is favourable, the areas of worst turbulence can often be identified by cumulus cloud formations. Particularly between the ridges and the base of these clouds, turbulence can be moderate to severe

Answer 480

Low level shear is invariably associated with a very stable friction layer. When the ELR near the surface is shallow or negative, mixing of air is at a minimum so that the wind flow at higher levels is not carried down. Thus in conditions of strong low level stability and strong higher level instability, the shear zone is characterised by moderate local turbulence

Answer 481

When the ELR from the surface is steep, so that conditions are unstable, mixing of air will spread the stronger winds from aloft into the lower levels so that an averaging of winds occurs which prevents shear (and its turbulence) even though mechanical turbulence may occur

Answer 482

When the friction layer is at its most dense (just before dawn) turbulence in the shear zone is a maximum while at the warmest time of the day (early afternoon) it is a minimum or absent

Answer 483

Aerodynamic lift is produced when a pressure differential exists between the air above and below the lifting surface. A circular flow pattern results when (relative) high pressure air below the lifting surface moves towards deducted pressure air above

Answer 484

This pattern develops around the wing tips or blade tips and looking from behind, a clockwise rotating vortex is shed from the left tip and an anticlockwise vortex from the right tip

Answer 485

The mass of the aircraft, aircraft configuration and indicated airspeed

Answer 486

The higher the all up weight (AUW), the cleaner the aircraft configuration and the slower the airspeed, the bigger and more intense the vortices and the greater the risk to the following aircraft

Answer 487

It can persist for as long as 5-6 minutes in calm conditions

Answer 488

Wake turbulence generated by an aircraft in straight and level flight usually settles about 1,000ft beneath its flight path

Answer 489

In reports and forecasts turbulence is classified only as moderate or severe

Answer 490

This involves occasional movement of the aircraft around its three axes so that mild rolling, pitching and yawning may be experienced

Answer 491

This turbulence involves more or less persistent and abrupt movement of the aircraft around its three exes, i.e there is rather severe rolling, pitching and yawing. There is general passenger discomfort

Answer 492

This turbulence involves abrupt and large movements of the aircraft about its three axes during which pilot control is frequently (but briefly) lost. It's is quite difficult to maintain altitude and direction and poor piloting techniques may cause airframe stress damage. Passenger discomfort is marked

Answer 493

This turbulence involves frequent and prolonged periods during which control of the aircraft is lost so that altitude and direction control cannot constantly be maintained. It is likely that some airframe stress damage will be experienced. Passengers are severely discomforted

Answer 494

Polar maritime cold advection

Answer 495

Polar maritime warm advection

Answer 496

A depression has no boundaries but interacts with adjacent pressure systems such as cold and anticyclones and is also continually changing with time. It's associated frontal system will have a different structure at different points along its length. Although depressions and anticyclones have life cycles, developing and decaying over time, it is not possible to define with any precision a start point or an end point

Answer 497

Norwegian theory

Answer 498

The theory was based on observations of what was actually happening in the atmosphere (known as empirical observations) rather than on an explanation in terms of physics or flux dynamics (largely used in modern meteorology)

Answer 499

The theory identified a more or less narrow zone of bad weather separating the very cold polar air from warm air of the tropics and sub tropics. This zone was called the polar front. In one place along the front cold air was seen to be advancing (the cold front) and in another palace warm air would be advancing (the warm front)

Answer 500

Often the surface pressure at the sharp end (the cusp) of the wave could be seen to fall and a depression would develop

Answer 501

A warm front and a cold front start as a stationary front, then starts a circulations. The isobars then close around the centre of the wave so that an identifiable depression is formed. The advancing cold air gives rise to the cold front while the warm air gives rise to the warm front. The higher speed of the cold front causes it to overtake the warm front so that the warm sector is squeezed up and becomes smaller. The front is said to be occluded. Since the warm sector has been lifted to mid troposphere altitudes, the associated stratiform cloud has demonised. The remainder of the occluded front is covered by various amounts of cumulus and possibly some patches of layer type cloud. The frontal system has almost decayed, the depression travels as an independent system

Answer 502

One observed fact was that most of the fronts in the Southern Hemisphere lay north-south along the meridians with their northern parts sometimes penetrating into the tropics. The term meridional front was used to describe these common fronts to distinguish them from the polar fronts of the theory

Answer 503

Air masses may be defined as large blocks of air that have uniformity in certain properties in the horizontal plane extending over many square miles. The characteristics involved in this definition relate to the airs temperature, moisture content and temperature lapse rate

Answer 504

A study of global air movement indicated that the major areas of air stagnation are found within the belts of anticyclonic subsidence, i.e the sub tropical and polar regions. These regions are therefore referred to as source regions

Answer 505

For a mass of air to absorb the characteristics of a source region, it needs to reside in that region for some 5-7 days. Weather conditions must be as static as possible, which favours the presence of anticyclones because in those systems pressure gradients are fairly slack, the wind is usually light and variable and subsidence with its slow divergence provides the imprint from the region onto the air. In addition, the requirement for a relatively steady ELR is generally met in large anticyclonic situations

Answer 506

The main deciding factor in air masses is temperature, which means that air masses are either of polar or tropical origin

Answer 507

Both polar regions, due to subsidence, are for a large part of the year affected by high inversions from the surface to about 5,000ft elevation. Beneath this inversion, small amounts of cloud may cause occasional light snow

Answer 508

Migration to different latitudes is referred to as air stream advection meaning horizontal transfer of air

Answer 509

Air streams that are judged to be cold on arrival in a new locality have the letter K appended while air streams that are judged to be warm on arrival in a new locality have the letter W appended

Answer 510

This means cold air moving towards warmer regions. Thus it could involve PmK or PcK depending on the origin of the moving air. Regardless of surface type, the temperature of the lower layers of the horizontally moving air will become warmer. It then depends on the degree of mixing as to how extensive the temperature effect will be in the vertical

Answer 511

Northbound flow towards the equator from Southern Hemisphere source regions is caught up in the generally easterly winds occurring on the equator side of the subtropical anticyclones. When confluence with the northern hemisphere easterlies takes place, often massive developments of both cumuliform and stratiform cloud occur in a zone called the intertropical convergence zone, the ITCZ

Answer 512

Southbound warm air leaving sub tropical source regions in the Southern Hemisphere travel over cooler surfaces so that lower layers will be affected by these surfaces and cool. The ELR will therefore shallow and stability is enhanced. Since on arrival at colder locations, this air will be judged as warm, the symbols TcW or TmW will apply

Answer 513

Modification of airstreams involves thermal processes when air is transported horizontally over colder or warmer surfaces. These processes are adjective and radiative in the main and, particularly in the case of warm advection, certainly not adiabatic. However, when air streams are forced to rise up in front of, or sink in the lee of mountain ranges, adiabatic factors are involved in the modification process; the associated vertical movement of air and mixing is then referred to as dynamic modification

Answer 514

Whereas thermal process changes may well be the predominant factors in air stream modification in the lower layers of the troposphere, dynamic processes are the major causes of air stream modification in the middle and upper troposphere

Answer 515

Pictures are in both the viable spectrum and infrared so that night time imagery is available. Geostationary satellites cover large areas of the globe every 3 hours (in some cases hourly) and orbiters provide very high resolution imagery so that fine details of cloud and fog distribution can be determined

Answer 516

Sensors can give approximate vertical temperature profiles in areas where upper air surroundings cannot otherwise be made. They can also give a picture of the water vapour content at various levels, sea surface temperatures, cloud top temperatures, or flight level, upper winds estimated from the movement of identifiable cloud elements, and other info. They are not forecasts themselves

Answer 517

Cloud tops against flight levels

Answer 518

FL100/FL200

Answer 519

FL300/FL400

Answer 520

FL050/FL100

Answer 521

FL200/FL300

Answer 522

FL400/FL450

Answer 523

The actual characteristics of an air mass or air stream can be resolved to a reasonable degree of accuracy from a network of upper air observations, reports from aircraft, remote sensing by satellites and by computer analysis and prediction

Answer 524

Modern meteorologists look at fronts and depressions in terms of the principles of fluid dynamics rather than as phenomena resulting from some grand battle between different air masses

Answer 525

Upper airflows are shown on contour charts, which use contour lines instead of isobars. These contour lines are read in the same way as isobars i.e closely spaced contours imply strong winds and vice versa. Furthermore, the direction of the wind flow can be assessed from the pattern of contours

Answer 526

Unlike closed isobar patterns at low altitudes, contour charts rarely show closed circulation. Instead, airflow aloft is characterised by wave flows cumulating in troughs and ridges. When a wave indicates airflow detecting to the right (clockwise), a trough is involved whereas a wave flow deflecting to the left (anti clockwise), implies a ridge. Both situations are applicable to the Southern Hemisphere

Answer 527

Jet streams are more or less horizontal ribbons of high velocity airflow near the tropopause characterised by strong values of wind shear both in the horizontal and the vertical. Thus in the centre of a jet stream the wind velocity will be very strong while above, below and to each side, the wind is less strong. These jet streams, which have a beginning and an exit, are found in preferred locations the Australasian region being one of them

Answer 528

The speed of an airflow does not necessarily increase in every wave or curve of a jet stream, speed maxima are sometimes identified along straight flows

Answer 529

Vorticity is a measure of the rotational motion or turning in an airflow (or other fluid) which can be cyclonic or anticyclonic

Answer 530

Vorticity due to curvature of the flow, vorticity due to windshear and vorticity due to earth rotation

Answer 531

Air flowing around a curve will have vorticity that will by cyclonic when airflow deflects to its right and anticyclonic when it turns to its left in the Southern Hemisphere

Answer 532

This vorticity is the result of horizontal wind shear. Where a belt of strong winds lies alongside a belt of lighter winds, as in a jet stream for example, there will be rotation of the faster flow around the slower flow. Depending on the orientation of the faster flow to the slower flow, shear vorticity can be cyclonic or anticyclonic

Answer 533

Since the earth is turning, air has vorticity due to earths rotation even when the air is calm relative to the earths surface, i.e when relative vorticity is zero. Vorticity due to earths rotation is cyclonic regardless of Hemisphere

Answer 534

Vorticity at any one point is the sum of curvature and shear components. Shear and curvature vorticity apply only to a flow of air relative to the earths surface and for that reason it is referred to as relative vorticity

Answer 535

When relative vorticity due to earths rotation at a given location and level are added together, the absolute vorticity is obtained

Answer 536

Since vorticity due to earths rotation is predominant it follows that absolute vorticity is always cyclonic. It is the change in this absolute vorticity that plays such a large part in the formation and development of major weather systems such as anticyclones, depressions and fronts because vorticity changes will produce convergence or divergence in association with conservation of angular momentum

Answer 537

Although air is not dense compared to solid structures, it has mass; many thousands of tons are involved as air flows around the curve of a trough or ridge line. The energy of the airflow is expressed by its momentum

Answer 538

For motion in a straight line, momentum is measured as M x V (MV). It is a fundamental law of nature that momentum, like other forms of energy, is conserved - it remains unchanged unless energy is lost to or gained from outside sources

Answer 539

When the flow involves curvature, the momentum is measured as angular momentum and is given by the product of the mass, the angular velocity (rpm, indicated by w) and the square of the radius of the centre of gravity (CG) of the mass from the axis of rotation: M w r 2

Answer 540

The principle of conservation of angular momentum means that if one of the factors changes, then one or both of the others must change in the opposite sense to keep the total value constant. The importance of this understanding in terms of its relationship to vorticity is that a change in angular velocity (rpm) must affect vorticity (turning motion)

Answer 541

The classic example of conservation of angular momentum is the ice skater spinning on a spot. When the arms are brought closer to the body (I.e radius of total mass decrease), angular velocity (rpm) increases to keep the total momentum constant

Answer 542

Bringing the mass closer towards the centre of rotation is mass convergence, which increases rotational velocity and therefore increases vorticity. Alternatively, moving the mass further away from the axis of rotation is mass divergence, which decreases rotational velocity and therefore decreases vorticity

Answer 543

When there is convergence in a rotating mass of air there will be an increased angular velocity (rpm) and therefore an increase in vorticity. And the reverse hold true also - when there is an increase in vorticity there must be increased angular velocity and therefore convergence

Answer 544

Going back to the ice skater, rotational velocity (rpm) only changes when the arms are in the process of extension or contraction, in other words, when the arms are steady there is no change in rotational velocity. The same principle must apply to vorticity. Only when the radius of rotation of a parcel of air is changing - only when convergence or divergence are taking place - will vorticity change. This change in vorticity is known as vorticity advection

Answer 545

When relative vorticity (curvature plus shear) happens to be cyclonic as well, absolute vorticity increases and there must then be mass convergence. Considering the principle of convergence, when convergence takes palace aloft, air will subside (in an anticyclonic system)

Answer 546

When relative vorticity is anticyclonic (or less cyclonic), absolute vorticity decreases and this decrease must be associated with divergence. When this divergence occurs at high altitude it sucks up air from low levels where surface friction often slows down the rate of surface convergence with the result that pressures near sea level reduce

Answer 547

The formation and development of pressure systems and fronts are basically caused by speeding up or slowing down of air in upper level air streams and not by events taking place at sea level

Answer 548

Large quantities of low and mid tropospheric air are transported. When this involves movement into different latitudes, conservation of angular momentum will play an important role in causing this low or mid tropospheric air to either speed up or slow down (and increase or decrease its vorticity) depending on whether the migration is towards higher or lower latitudes. E.g low level air will experience increased vorticity if it is transported to higher latitudes because the radius on which it moves becomes smaller (towards the pole). The consequence of this is that low level air will converge so that it will ultimately rise

Answer 549

One is the mid latitude depression, which brings warm moist air from low latitudes. The other is the polar depression, which forms entirely in cold air

Answer 550

This is the most common type of depression, which involves advection of warm moist air from the tropics and subtropics. When air moves towards the poles from tropical latitudes, the radius on which is rotates becomes smaller (and the poles the radius would have become zero)

Answer 551

Consistent with the principle of conservation of angular momentum, the angular velocity of the flow increases and convergence of low and mid troposphere air will cause this air to rise. When this tendency is reinforced by the shear and curvature components of vorticity, very strong upward vertical motion may result. The resulting cloud sheet will be enhanced by the release of latent heat from the condensing moisture so that an extensive cloud band is formed. Near the rear edge of this sheet there will be a temp contrast between the warm air from the north and the colder air to the rear of the trough. This is where the cold front will be drawn

Answer 552

When a southerly or southwesterly jet stream (the polar jet stream) moves over an upper trough, the curvature term (from the trough) and the shear term (from the jet stream) reinforce each other giving high values of cyclonic vorticity and hence high values of cyclonic vorticity advection (i.e change in vorticity). As the flow slows down rapidly downstream of the trough line there will be enhanced divergence aloft, accompanied by strong upward motion through the troposphere and convergence at low levels. This may result in the formation of an area of mainly cumulonimbus cloud typically in the shape of a comma. The rapidity of the process may mean that the upper divergence is not matched by the low level convergence and a small but intense depression may form

Answer 553

The comma shaped cloud structure will occasionally be seen on satellite imagery. More often, the imagery will show speckled cloud patterns of cumulus and cumulonimbus in the cold air where the decrease of absolute cyclonic vorticity at high levels is not sufficiently strong to trigger off the complete process. The cloud pattern on satellite imagery is termed a CVA-max (cyclonic vorticity advection maximum)

Answer 554

There is none

Answer 555

The amount of moisture involved The stability/instability of the rising warm air The slope of the front The speed of the front, and; The contrast in temperatures across the front

Answer 556

The cloud will be of the cu,ulus variety if rising air is unstable or if the rate at which it is forced to rise is great. Thus in steeply sloping fronts, such as in many cold fronts, it is most often found that cumulus cloud, including CB, is involved. In stable conditions or when the slope of the front is relatively shallow (as in many warm fronts), the presence of layer type cloud is the norm

Answer 557

Cloud fronts travel at about 0.8 x speed of the wind and warm fronts at about 0.6 x the speed of the wind

Answer 558

The slope of the interface between the cold and warm air (the front) is relatively steep, something in the order of 1-in-80 to 1-in-120. This means that for every 100m forward, the front rises about 1m

Answer 559

The drag caused between the moving air and the surface slows down the lower part of the front while near the top of the front, the slope also becomes less steep so that the overall effect is a curved slope

Answer 560

Firstly the rate at which the rising warm air is being displaced i.e its vertical velocity (which is generally associated with its stability and degree of high level divergence) and secondly, the steepness of the frontal slope

Answer 561

This type normally has a mean slope in the order of 1-in-120, which may be considered shallow for a cold front. There is a general sinking of warm air at high levels (hence the term kata for katabatic) which causes the vertical cloud development to be somewhat restricted. While most cloud fronts are associated with cumulus and especially CB clouds, type 1 cold front has both stratiform and cumuliform clouds

Answer 562

The mean slope of the frontal interface is somewhat shallower than the lowest part between the surface and some 5,000-6,000ft. This implies that the vertical velocity of the rising warm air from the surface is initially much more rapid that at greater altitudes. Thus stratocumulus and possibly cumulonimbus will form initially but as the frontal slope shallows and vertical velocity decreases, altostratus and nimbostratus will also be found

Answer 563

Since the slope is shallower aloft, the precipitation to be expected from the type 1 cold front is rather widespread. Some precipitation in the form of showers (possibly associated with hail and thunderstorms) takes place in advance of, and immediately behind the surface front while further behind the surface front, precipitation may involve rain from the overlying nimbostratus and altostratus

Answer 564

Cumulus cloud with associated showers will develop. However, if the cold air is not unstable or marginally so, skies should generally be clear with good visibility

Answer 565

Around 20kts, which is slow for a cold front

Answer 566

It's slope is quite steep, somewhere in the region of 1-in-150 to 1-in-180, there is instability and rising of warm air at high levels (hence the term ana for anabatic), and the speed at which the front travels can be in excess of 30kts

Answer 567

The associated cloud band is rather narrow and is often topped by an inversion near the tropopause, which causes cirrus in the shape of an anvil to develop. The predominant cloud is cumulus and cumulonimbus. It is therefore to be expected that thunderstorms and associated hail showers will be part and parcel of this front but being only some 100-150km wide, duration of precipitation will be relatively short lived

Answer 568

Often rapid clearing skies behind this type of cold front. The rapid change from heavy showers and thunderstorms to brilliantly blue skies can be dramatic

Answer 569

Conditions behind the cold front depend to a large extent on the stability of the cold air. If the flow of air has cyclonic vorticity, the air behind the front will be unstable, with cumulus and cumulonimbus cloud and showers

Answer 570

The slope of most warm fronts is in the order of some 1-in-150 to 1-in-180, quite shallow. In addition to this, the speed at which warm fronts travel is generally around 10-15kts, slower than cold fronts

Answer 571

If the advancing warm air is unstable and consistently rises relative to the frontal interface (ana type lifting), it is likely that a combination of layer and cumuliform clouds are present. However, if the warm air is stable and the air sinks at high levels, cloud is predominantly stratiform of reduced vertical extent. It is therefore convenient to subdivide warm fronts into stable kata type and unstable ana types

Answer 572

With the stable kata type warm front, the approach of the front is heralded by cirrus and cirrostratus, which may appear as anything up to 1,000km in advance of the surface front (which is that part of the front actually in contact with the surface). As the front nears the observer, the cloud layer thickens into a combination of cirrostratus and altostratus. Finally, the sector of the front within some 4-500km from the surface front combines stratiform cloud from the surface up i.e nimbostratus

Answer 573

Precipitation may fall from each type of cloud but any falling from high level layers will evaporate before reaching the ground. However, as the cloud base lowers, light rain will initially be experienced at the observers station, gradually turning into more heavy and persistent rain. Depending on the altitude of the freezing level, snow and/or sleep may be involved

Answer 574

If the advancing and ascending warm air is unstable in whole or in part, the ana-type warm front results. Cloud development is generally similar to that found in the kata type but cumulus cold, including cumulonimbus, is embedded. Thus precipitation, especially within the first 3-400 or so kilometres in advance of the surface front, will include both rain and heavy showers. In some very unstable situations, thunderstorms and hail may be experienced

Answer 575

The cold sector air below the frontal interface will become very moist as the result of large amounts of precipitation penetrating from above. The fact that the air is already cold and that evaporation of some of the precipitation demands latent heat, adds up to a situation where almost 100% relative humidity is readily reached. Thus the air in advance of the surface front will have very poor visibility and this will be made worse by development of low cloud, which will form when saturation takes place. Indeed, in the low level triangular sector between the surface and the frontal slope, frontal fog may well be experienced

Answer 576

The surface will have received large amounts of precipitation over some 24-36 hours of persistent rain and showers. Thus evaporation from these water areas will add to the moisture content of the air and development of cloud is very likely, most often low level and surprisingly extensive. Secondly, the warm air behind the front will invariably have come from a region of relatively high temp and high moisture content. Thus substantial low cloud with drizzle and even some fog can be expected. In the case of stable kata type warm front, conditions improve some heat quicker once the surface front has passed compared to an unstable ana type warm front

Answer 577

Stationary fronts are weak and often represents the remains of a decaying frontal system. Thus not a great deal of cloud, wind or weather is associated with them

Answer 578

This type normally has a mean slope in the order of 1-in-120, which may be considered shallow for a cold front. There is a general sinking of warm air at high levels (hence the term kata for katabatic) which causes the vertical cloud development to be somewhat restricted. While most cloud fronts are associated with cumulus and especially CB clouds, type 1 cold front has both stratiform and cumuliform clouds

Answer 579

The mean slope of the frontal interface is somewhat shallower than the lowest part between the surface and some 5,000-6,000ft. This implies that the vertical velocity of the rising warm air from the surface is initially much more rapid that at greater altitudes. Thus stratocumulus and possibly cumulonimbus will form initially but as the frontal slope shallows and vertical velocity decreases, altostratus and nimbostratus will also be found

Answer 580

Since the slope is shallower aloft, the precipitation to be expected from the type 1 cold front is rather widespread. Some precipitation in the form of showers (possibly associated with hail and thunderstorms) takes place in advance of, and immediately behind the surface front while further behind the surface front, precipitation may involve rain from the overlying nimbostratus and altostratus

Answer 581

Cumulus cloud with associated showers will develop. However, if the cold air is not unstable or marginally so, skies should generally be clear with good visibility

Answer 582

Around 20kts, which is slow for a cold front

Answer 583

It's slope is quite steep, somewhere in the region of 1-in-150 to 1-in-180, there is instability and rising of warm air at high levels (hence the term ana for anabatic), and the speed at which the front travels can be in excess of 30kts

Answer 584

The associated cloud band is rather narrow and is often topped by an inversion near the tropopause, which causes cirrus in the shape of an anvil to develop. The predominant cloud is cumulus and cumulonimbus. It is therefore to be expected that thunderstorms and associated hail showers will be part and parcel of this front but being only some 100-150km wide, duration of precipitation will be relatively short lived

Answer 585

Often rapid clearing skies behind this type of cold front. The rapid change from heavy showers and thunderstorms to brilliantly blue skies can be dramatic

Answer 586

Conditions behind the cold front depend to a large extent on the stability of the cold air. If the flow of air has cyclonic vorticity, the air behind the front will be unstable, with cumulus and cumulonimbus cloud and showers

Answer 587

The slope of most warm fronts is in the order of some 1-in-150 to 1-in-180, quite shallow. In addition to this, the speed at which warm fronts travel is generally around 10-15kts, slower than cold fronts

Answer 588

If the advancing warm air is unstable and consistently rises relative to the frontal interface (ana type lifting), it is likely that a combination of layer and cumuliform clouds are present. However, if the warm air is stable and the air sinks at high levels, cloud is predominantly stratiform of reduced vertical extent. It is therefore convenient to subdivide warm fronts into stable kata type and unstable ana types

Answer 589

With the stable kata type warm front, the approach of the front is heralded by cirrus and cirrostratus, which may appear as anything up to 1,000km in advance of the surface front (which is that part of the front actually in contact with the surface). As the front nears the observer, the cloud layer thickens into a combination of cirrostratus and altostratus. Finally, the sector of the front within some 4-500km from the surface front combines stratiform cloud from the surface up i.e nimbostratus

Answer 590

Precipitation may fall from each type of cloud but any falling from high level layers will evaporate before reaching the ground. However, as the cloud base lowers, light rain will initially be experienced at the observers station, gradually turning into more heavy and persistent rain. Depending on the altitude of the freezing level, snow and/or sleep may be involved

Answer 591

If the advancing and ascending warm air is unstable in whole or in part, the ana-type warm front results. Cloud development is generally similar to that found in the kata type but cumulus cold, including cumulonimbus, is embedded. Thus precipitation, especially within the first 3-400 or so kilometres in advance of the surface front, will include both rain and heavy showers. In some very unstable situations, thunderstorms and hail may be experienced

Answer 592

The cold sector air below the frontal interface will become very moist as the result of large amounts of precipitation penetrating from above. The fact that the air is already cold and that evaporation of some of the precipitation demands latent heat, adds up to a situation where almost 100% relative humidity is readily reached. Thus the air in advance of the surface front will have very poor visibility and this will be made worse by development of low cloud, which will form when saturation takes place. Indeed, in the low level triangular sector between the surface and the frontal slope, frontal fog may well be experienced

Answer 593

The surface will have received large amounts of precipitation over some 24-36 hours of persistent rain and showers. Thus evaporation from these water areas will add to the moisture content of the air and development of cloud is very likely, most often low level and surprisingly extensive. Secondly, the warm air behind the front will invariably have come from a region of relatively high temp and high moisture content. Thus substantial low cloud with drizzle and even some fog can be expected. In the case of stable kata type warm front, conditions improve some heat quicker once the surface front has passed compared to an unstable ana type warm front

Answer 594

Stationary fronts are weak and often represents the remains of a decaying frontal system. Thus not a great deal of cloud, wind or weather is associated with them

Answer 595

A mountain range of substantial size and a wind more or less at right angles to it. Some of the air pushed onto the range will deflect around the outside edges, i.e the airflow splits up at the windward side of the barrier. Some air however, will flow over the top of the range and through the inevitable valleys

Answer 596

The air on the windward side is somewhat compressed causing a localised high pressure area. On the lee side, especially at sea level, a suction effect (and therefore convergence) takes place which causes atmospheric pressure to reduce. Thus a low pressure area forms on the lee side with winds flowing clockwise around it. It must be noted that this depression rarely extends to great heights and never higher than mountain top height

Answer 597

Generally clear of cloud, a gusting wind and possibly a fog effect producing warm temperatures. If cloud does appear, it is likely to be of minor extent and generally associated with orographic cloud formed over the mountains and drifting downwind. Similarly, some cloud may drift through valleys and penetrate into the lee side

Answer 598

The lower part of this front will lag and the formation of a lee depression will be much encouraged. If the low level air on the lee side is warm, it follows that the higher frontal sector, unobstructed by the mountain range, will be much colder than the low level air. Thus a steepening of the ELR on the lee side will produce substantial, and sometime severe instability

Answer 599

Is to cause the front at the range to stop while the parts beyond either end travel on. A good example of this is the effect of the Southern alps on a cold front travelling from west to east

Answer 600

Mountain ranges generally tend to intensify the weather (and fronts) on the windward side and diminish the effects on the lee side

Answer 601

When land masses warm in summer, the associated rising of air will produce a circulation similar to a huge sea breeze with low level pressures reducing sharply. This low pressure causes surface convergence and due to coriolis influence, clockwise air movement will result

Answer 602

Depending on the degree of surface heating, the top of the depression can be a mere few thousand feet in temperate latitudes on a fine summers day but many thousands of feet in some other situations e.g parts of the Australian interior in summer time

Answer 603

In a mild situation (e.g a summer sea breeze over large coastal plains such as Canterbury), the wind which should cross the coast at right angles, will actually cross at an angle much less than 90 degrees to the circulation associated with the thermal low. It is not unusual to have a northeasterly wind in areas near the coast while well inland the wind can be northwest. The weather in these situations involves various amounts of CU but will little or no precipitation

Answer 604

Often it is found that frontal waves develop secondary depressions involving three or four smaller disturbances. Each depression advanced in more or less the same direction as the master depression

Answer 605

Freezing nuclei are relatively rare. It is generally considered that freezing nuclei come from fine soil particles and volcanic dust

Answer 606

Latent heat of fusion is heat energy released when water changes its stable state

Answer 607

The unit of heat energy is the joule (previously the calorie). By definition, 4.18 joules (1 calorie) of heat energy are required when the temperature of one gram of water is to be raised by one degree Celsius. Again, the reverse applies in that cooling of one gram of water by one degree Celsius will release 4.18 joules (1 calorie) of heat energy

Answer 608

To change temperature of water, each gram involves 4.18 joules (1 calorie) of heat energy per degree of change. However, if the state of water is to be altered, the number or joules involved is vastly more. For example, water liquid changing into ice through the freezing process releases 334.4 joules (80 calories) of heat energy

Answer 609

At a temperature between -1°C (where into a tiny part freezes initially) and -80°C (where all the water freezes initially), the released latent heat of fusion is instrument, partly, in the freezing process of a portion of the supercooled droplet and the remainder warms the partly frozen drop

Answer 610

1/80th (4.18/334.4) of the drop freezes instantaneously for every degree Celsius that the supercooled drop is below 0°C and the remainder of the heat energy is dissipated in raising the temperature of the partly frozen drop resulting in heat losses through evaporation and conduction

Answer 611

The less cold the supercooled droplets, I.e the closer they are to 0°C, the smaller the part that freezes on impact and the larger the portion that flows back

Answer 612

Supercooled water drops are large, if drops are small, the amount of liquid water flowing back is less. Temperature between 0°C and -15°C

Answer 613

Extra weight involved, icing development may not becomes apparent quickly. Clear ice is hard. May freeze controls if allowed to flow back. If flight enters areas where the temp is above 0°C, clear ice will likely break off the aircraft in large segments and damage may be caused as a result

Answer 614

Flight in cumulus cloud within the first 6,000-8,000ft above freezing level will be very prone to clear ice accretion. The reason for this is that the up and down draughts as well as turbulence within this type of cloud are very likely to cause growth of drops while an ELR close to the SALR involves about 10,000ft between the freezing level and the -15°C level

Answer 615

Cumulus and especially CB development in frontal situations will be dangerous particularly if the aircrafts track is not at right angles to the front

Answer 616

When small supercooled water droplets are disturbed, the amount of water flowing back (freezing as it flows back) will be relatively limited. If, in addition to that the temperature of the droplets is colder than some -15°C, then freezing of the droplets is practically instantaneous. As a result, numerous pockets of air will be trapped within the accumulating ice build up, which gives the ice a milky, opaque appearance

Answer 617

Rime ice is rough and uneven in shape. As it builds up on frontal areas such as leading edges and so on, the aerodynamic qualities will be affected. If rime ice develops as far back as hinge lines, the risk of control locking is less than with clear ice because a few rapid movements of the controls concerned will break the ice

Answer 618

Cloud most likely associated with rime ice is stratiform cloud because the usual lack of turbulence within it will minimise collision and coalescence so that droplets remain small. Stratiform cloud associated with a warm front will often have some cumulus clouds embedded within it so that one must always be vigilant in anticipating conditions and likely type of icing

Answer 619

When the temperature of the skin of an aircraft is colder than 0°C and the environment air is moist but not necessarily saturated, deposition may occur. This involves the process of water vapour directly changing into the ice state without going through the liquid state

Answer 620

The boundary layer around lifting surfaces is obviously affected while the all up weight of the aircraft can be substantially increased

Answer 621

Hoar frost can form on an aircraft in flight whenever the skin of the aircraft is very cold and the flight path takes the aircraft rapidly into warmer and moist air. Aircraft don't need to be in cloud for this type of ice to form. It may instantly clover the windscreen so that vision is lost at a critical time

Answer 622

When a warm layer of cloud lies above air which has a temperature of less than 0°C and rain falls from this warm cloud into the cold zone, an aircraft can be covered in ice from freezing rain in a matter of minutes

Answer 623

Typical conditions for this happening in flight the cold sector beneath the slope of a warm front or in the cold sector just behind a front. Alternatively, the presence of a strong inversion intersecting the freezing level can sometime produce the same results

Answer 624

If freezing rain conditions are encountered, the best remedy is to vacate the area forthwith by either descending, climbing or altering heading

Answer 625

It will not generally adhere to aircraft parts. However, when supercooled water is mixed with snow, the combination will increase the rate at which ice accumulates

Answer 626

Snow has proved to be a serious problem for aircraft on the ground waiting for take off. Even light snow accumulation can be sufficiently severe for aircraft to be placed at risk during the take of and initial climb out phase

Answer 627

Hail is defined as precipitation of circular hard pellets of ice that fall from cumulonimbus cloud

Answer 628

Close examination of a hail stone indicates that it consists of a number of layers of clear and rime ice which supports the theory that hail is formed as the result of strong up and down draughts in which a formative stone is repeatedly exposed to temperatures ranging from positive to grossly negative

Answer 629

Hail formation is generally restricted to only part of a cloud cell so that aircraft in flight only experience relatively brief encounters

Answer 630

Soft hail consists mainly of rime ice, which has formed in clouds that have limited water content

Answer 631

Cloud water content, aircraft characteristics, kinetic energy and adiabatic heating

Answer 632

In mid latitudes, icing will be more severe in summer than in winter. Flight above the freezing level in polar regions is less likely to produce high accretion rates. The very low temperatures encountered in polar regions often produce rime ice or dry ice crystals, which rarely adhere in sufficient quantities to cause serious problems

Answer 633

Areas where pressures decrease due to higher flow velocity, such as over the wing or around a pointed nose cone, will experience a drop in temperature and a better opportunity for ice to form

Answer 634

The higher the aircraft speed the greater the rate of accretion but this is generally applicable only to aircraft that have a speed range below 200-250kt mark

Answer 635

Friction between the aircraft skin and passing air molecules causes the skin to heat. Clearly, the higher the aircraft speed, the greater the temperature increase. At about 500kt, skin temperature can increase by some 25°C

Answer 636

In accordance with bernoullis theorem, a fluid or airflow slowed down or brought it rest will increase in pressure and an associated increase in temperature is then involved. Thus, any parts or components of aircraft, which tend to slow the adjacent airflow down, will experience this warming and through that reduce or avoid icing

Answer 637

Rime ice may be worse that clear ice due to its ragged nature

Answer 638

Aerofoil shapes can be badly distorted by a cover of ice (especially rime ice) so that production of lift can be severely reduced (and drag increases)

Answer 639

Aerodynamic distortion and increase in all up weight have a dramatic influence in increasing an aircrafts stall speed

Answer 640

It is highly unlikely for ice accretion to be the same on all blades, thus an unbalancing factor is introduced

Answer 641

Flight in icing conditions will almost certainly block the aerodynamic pitot intake (if not heated) so that airspeed readings are in error. Under certain conditions the static intake may also block which will then deprive the crew from useful readings from all pressure instruments

Answer 642

Especially in older type aircrafts, which do not have the modern smaller and sturdier aerials, ice accretion may be sufficient to break them off and deprive the crew of radio or navigation aid reception/transmission

Answer 643

Often these bends are built into the fuel cap and when blocked, ambient pressure will be prevented from entering the fuel tank and fuel starvation will soon follow

Answer 644

Normally referred to as de icing boots and involves rubber or plastic tubing manufactured within rubber or plastic skins installed on leading edges of wings, tailplanes or fins

Answer 645

Fluids such as alcohol and glycol which have a freezing temperature well below 0°C may be used to combat icing. Strategically placed nozzles eject spray over parts of the aircraft most vulnerable to icing

Answer 646

Weight of the fluid and when fluid has been exhausted the aircrafts capacity to combat ice has been eliminated. Some of the fluids are night corrosive

Answer 647

Perhaps the most common method, involves either electrical heating through heat coils and pads or through ducting of hot air over affected aircraft parts. Electrical elements are used for items such as pitot tubes, windscreens and often, propeller blades. If the required equipment is not fitted, has malfunctioned or exhausted, flight below the freezing level will remove it

Answer 648

Flight at high altitude will involve either dry ice crystals or greatly reduced water contents which means that icing problems there are virtually nil

Answer 649

This ice problem is caused by two fundamental processes: the reduction of temperature when air is de pressurised and the reduction of temperature due to demand for latent heat when fuel vapourises

Answer 650

Any moisture present will turn to ice and cling to the various components found in side the Venturi

Answer 651

When fuel is injected into the air stream flowing through the Venturi, it vapourises and a demand for latent heat occurs. The associated reduction in mixture temperature can cause ice to form, which is referred to as refrigeration icing

Answer 652

OAT +25°C and -15°C

Answer 653

The ice of carburettor heat, supplied by hot air ducted from around the exhaust manifold

Answer 654

The main problem with turbine engines is the build up of ice on and near intakes, which can lead to compressor stall and flame out

Answer 655

Cirrus, cirrostratus and cirrocumulus

Answer 656

Middle and low level clouds

Answer 657

Middle and low level clouds

Answer 658

High level clouds

Answer 659

Cirrus, cirrostratus and cirrocumulus

Answer 660

Altostratus, altocumulus, stratocumulus and cumulus

Answer 661

Cumulonimbus and nimbostratus

Answer 662

Altostratus is usually associated with rime ice although within the first 5,000ft above the freezing level (where the mean temperature is between freezing and -10°C) some clear ice may encountered

Answer 663

Altocumulus can similarly produce clear ice but due to the generally larger drop size, the most likely zone is the layer from the freezing level to the -15°C level

Answer 664

CB in the growing and mature cells can have moderate to severe icing up to the -15°C level but records show occasional severe icing conditions in vigorously growing cells at levels of -20°C. Summertime and tropical CBs are the worst in these respects

Answer 665

NS can produce moderate to severe clear ice within the first few thousand feet above the freezing level and this likelihood is enhanced when the water content of the cloud is high such as in tropical or summer nimbostratus development or when orographic causes are involved. The fact that NS is generally widespread points to a high amount of ice accretion because the aircraft is in the cloud for a longer period of time

Answer 666

For instance, in mid-high latitudes in winter, when the freezing level is below ground or sea level, even stratus cloud can produce a light coating of ice. Ice and snow can be found in large quantities in AS, AC and CU cloud developed in and around mountainous terrain in winter and sometimes even in summer

Answer 667

Minor coverage of aircraft parts by either rime or clear ice. Airspeed loss is very small or nil and, in the absence of a deterioration in ice accretion, no change or altitude or track is needed

Answer 668

Substantial build up of clear or rime ice on wings and other major aircraft parts. Airspeed loss may become sufficient to cause concern. The safety of the aircraft may be jeopardised if rate of accretion increases or is anti icing equipment is not coping adequately. It may be desirable to alter altitude or heading

Answer 669

Accretion is sufficiently great to endanger the aircrafts performance. Change of altitude and/or heading are deemed essential if anti icing equipment is to adequately coping with the problem

Answer 670

The tropics generally are regions where the air is tending to ascend due to the intense heating under a sun, which at some time of the year will be vertically overhead. The rising air spreads pole wards at high levels but because of the coriolis effect it does not travel all the way to the poles and as a consequence it sinks towards the surface in the belt of sub tropical highs in each Hemisphere. The surface air then moves towards the equator as the trade winds, blowing from the southeast and the northeast in the southern and northern hemispheres respectively

Answer 671

A zone of ascending air within the regions at or near the meteorological equator caused by convective action from warm surfaces. These regions contain the equatorial trough and are referred to as the doldrums

Answer 672

Subsiding air within the regions covered by the semi stationary anticyclones centred at average latitudes 30°N and 30°S. these regions are referred to as the horse latitudes

Answer 673

High level divergence over equatorial regions and high level convergence over the anticyclonic regions

Answer 674

Low level air flows (winds) from the anti cyclonic regions towards the equatorial trough. These winds are known as the trade winds

Answer 675

The slanted orientation of the earths spin axis causes maximum insolation from a vertically overhead sun to oscillate annually either side of the geographical equator, giving rise to the opposing seasons of each Hemisphere. Thus the terms thermal equator or meteorological equator were introduced to seperate the geographical equator from those latitudes where solar input is temporarily a maximum

Answer 676

The heated surfaces at or near the thermal equator obviously produce a high degree of instability and convection so that the main thrust of air movement within the affected regions is upwards. This results in the establishment of the equatorial trough, a system of comparative low pressure and surface convergence

Answer 677

A few degrees of latitude away from the equatorial trough on the geographical equator

Answer 678

The equatorial trough is a more or less permanent feature of convergence and convection whereas the ITCZ is an erratic feature, the presence and intensity of which depends on the variable degree of trade wind convergence. Put differently, the equatorial trough is a general term for the belt of comparatively low pressure around the globe in the tropics. The ITCZ is a synoptic weather feature within this trough

Answer 679

The latitudes covered by the equatorial trough are known as the doldrums. Within these areas, winds are generally light and variable mainly due to the fact that the predominant movement of air is upward rather than horizontal. However, the convergence associated with doldrums is not of a constant nature

Answer 680

The entire trough is located in the northern hemisphere but, due to the strongly differing nature of the surface, the latitudinal change in position and intensity of the trough varies greatly

Answer 681

Over the major oceanic regions, the equatorial trough is fairly constant in terms of latitude but the influence the continental land masses, their differing mountainous natures and local pressure systems cause the trough to shift considerably and even fracture in a few localities

Answer 682

There is a totally different picture. A large deflection takes the trough south through South Africa while a smaller wave brushes over the northern parts of Australia for similar reasons as stated for the northern hemisphere i.e influence of variations in surface characteristics

Answer 683

The major difference is the position of the trough in the central pacific and Atlantic oceans where the equatorial trough remains in the northern hemisphere. The main reasons for this are firstly, stronger trade wind strength in the northern compared to the Southern Hemisphere and secondly, the influence of oceanic surface temperatures

Answer 684

Considering the high moisture content of air especially when the ITCZ is positioned over oceanic regions, any lifting will almost immediately involve the SALR when air saturates at low levels. Thus the zone becomes associated with massive cloud developments, which can involve a horizontal width of some 5-600k. And a depth of some 50,000-60,000ft

Answer 685

Cloud consists mainly of the cumulus variety such as CU, CB, and AC but also some stratiform cloud at high levels (CS) where tops of CB clouds reach a balance with the surrounding cold air. It is not uncommon for tops of CBs to exceed the height of the equatorial tropopause through simple momentum

Answer 686

Turbulence, icing and other factors associated with CB activity

Answer 687

Can be moderate to severe. It will be difficult to avoid this phenomenon unless the flight path can be maintained above the tops of CBs. Heavy precipitation associated with the cloud presence will strongly reduce visibility and especially when flying over ocean areas, the horizon will appear merged with the cloud base. Depending on the stage of cell development, strong up or downdraughts can be expected below the cloud base. Strong convergence below growing cells will reduce ambient pressure with the altimeter over reading

Answer 688

The high water content experienced in ITCZ cloud developments poses a risk of moderate to severe icing above the freezing level and especially within the first 10,000-15,000ft above it

Answer 689

Exposure to dangers associated with thunderstorms e.g wind squalls, lightening strikes etc. when the ITCZ extends over continental areas, its charcoal will be influenced by the nature of the land, the temperature differences of the air masses either side of the zone and the presence of major airflows

Answer 690

The zine is associated with various oktas of cumulus and some shower activity from the more developed clouds. Winds are light and variable

Answer 691

A zone of intense cloudiness due to convergence of two major airflows is frequently identified from the eastern tip of Papua New Guinea to slightly southeast of French Polynesia. This is known as the South Pacific convergence zone, which like the ITCZ, varies in intensity

Answer 692

The semi stationary anticyclonic system in the eastern Pacific. The easterlies on the equator side of this system turn north easterly on approaching the western Pacific; and, The anticyclones originating and travelling eastward from the Australia/New Zealand regions. The south easterlies preceding these systems extend to the region where SPCZ forms

Answer 693

When the north easterlies and the south easterlies meet they form a convergence zone with weather characteristics very similar to the ITCZ. The zone is mor active in summer than winter

Answer 694

The SPCZ appears to be associated with the northeast passage of mid latitude cold fronts when the cloud band of the latter merges with that of the convergence zone. Thus cloud fronts appear to strengthen or re-activate the SPCZ

Answer 695

The northern limit of the SPCZ is often taken to lie over Papua New Guinea and fluctuations in its intensity may bring cloudiness and rainfall down into Australians Northern Territory and northern Queensland from time to time. It is thus occurrence that may seem to indicate incorrectly that the ITCZ extends into Australia

Answer 696

Trade winds are quite steady and persistent in strength. In the pacific region they vary throughout the year from 10-20kts with the occasional increase to 30kts. In global terms, trade winds are generally slightly stronger in winter than summer. The winds above the trade winds are usually from the west, often light

Answer 697

Although their direction in the Southern Hemisphere is mainly from the south-east there are minor variations in direction due to underlying terrain (islands versus ocean for instance) and pressure gradient fluctuations

Answer 698

The weather, especially at the source of the trade wind zone, is for a large part influenced by subsidence inversions, which vary in height from 6,000-8,000ft above sea level

Answer 699

Below the inversion, visibility is usually fair to good with varying amounts of cumulus with bases between 2,000-3,000ft and tops at some 8,000ft. These amounts and vertical limits are prone to increase when trade winds cross the windward side of islands so that increased wind speeds and shower activity are likely in those localities

Answer 700

Directional change depending on season. For instance, in mid Pacific, the trough never enters the Southern Hemisphere, which means that the south-east trade winds (of the Southern Hemisphere) cross the equator to flow to the trough (in the northern hemisphere)

Answer 701

In the northern hemisphere north of Australia so that trade winds in that region also cross the equator. However, in summer time in Australia, the trough is south of the equator so that the northern hemisphere trade winds will have to do the crossing

Answer 702

They will be subjected to the coriolis force applicable to the northern hemisphere i.e veers to the right. Admittedly, the strength of the force is weak but present nonetheless. Therefore, after crossing the equator, the trade winds blow initially towards the trough from the south and ultimately from the count west

Answer 703

Will initially become northerly and ultimately north-west. Arriving over land after travel over very warm waters, these northwest winds (or from other northerly quarters depending on local topography) can produce some intense cloud developments in coastal regions. This, coupled with the activity of the equatorial trough, a heat low and occasionally the SPCZ, produces the wet season over the northern parts of the continent between December and march

Answer 704

In summertime the land of major islands warms to a greater degree than the surrounding seas so that sea breeze effects develop over the land. Consistent with this, air pressure will be slightly lower over the land than over the sea and instability increases. Thus coastal regions exposed to the southeast experience an increase in cloud development and possibly heavy precipitation depending on the nature of terrain. In some situations this effect will be felt even at the lee side of islands

Answer 705

The land is colder than the surrounding seas so that pressures are somewhat higher inland and stability is enhanced. Although seem cloudiness may still occur at the southeast coasts from exposure to the trade winds, generally conditions are much drier and more stable

Answer 706

Individual cumulus, mesoscale convective areas (MCAs) wave disturbances, monsoons; and, tropical cyclones

Answer 707

These are the smallest disturbances, measured in a life span of only a few hours and a dimension some 5km across, resulting from turbulence experienced in the trade wind friction layer. These individual clouds often align themselves in cloud streets more or less parallel to the wind direction, or they are arranged in honeycomb patterns

Answer 708

These are systems comprising of grouped clusters of cumulus and CB developments within areas from 100km-1,000km across. MCAs are characterised by instability through deep layers - they can often be part of the equatorial trough. Although many MCAs last no longer than one or two days, others can persist for extended periods of time

Answer 709

Disturbances demonstrate themselves in wave form in the equatorial east elise where waves travel from east to the west but not necessarily at the same speed as the trade winds

Answer 710

The formation of a wave generally originates some two to four degrees of latitude on the pole ward side of the equatorial trough. Of all the wave disturbances that form in the tropics throughout a year, approximately 25% develop to the tropical depression stage while some 10% become named tropical cyclones. Easterly waves are more frequent in the northern hemisphere than in the South Pacific

Answer 711

Easterly waves normally have a life span of about 10 days, have an average wavelength of some 3,000km, travel approximately six degrees in longitude per day and slope eastward with height

Answer 712

When large land masses of the world heat during their summer season, intense instability and convection cause air to rise so that replacement air from the oceans travel towards the land and island. The great water content of this air produces large cumulus developments so that thunderstorms, heavy precipitation and squalls result. Mountains barriers accentuate the development. This reflects the large scale wind regime of the wet monsoon, which may well involve winds contradicting the prevailing wind

Answer 713

When landmasses cool in winter the reverse development takes place. Subsidence on a grand scale over the land causes air to flow outward towards the oceans, which have retained a great deal of their stored heat. Thus the weather over the land is generally clear due to adiabatic warming. This reflects the dry monsoon

Answer 714

With the exception perhaps of the north Australian continent, the major monsoon regions are found in the northern hemisphere. The most developed monsoon winds are concentrated along the south Asian continent when in summer time the transport of moist air from the Indian Ocean causes widespread precipitation and cloudiness in India, Pakistan, Myanmar etc. the wet season begins around June and ends towards late September. In the case of India for instance, about 75% of its annual rainfall is concentrated within this monsoon period

Answer 715

In the Bay of Bengal region. These are systems extending up to some 25,000ft and lasting about three days on average. These depressions have a strong influence on the distribution of monsoonal rain

Answer 716

From the south-southwest but local topography has strong influences

Answer 717

In the Australian situation, mention was mad before of the influence of the equatorial trough, the South Pacific convergence zone and the northwest winds on the northern parts of the continent during summer. The wet monsoon takes effect in December and peaks during February. During this period, heavy precipitation and thundery conditions may prevail for many days. In the dry winter season, the weather in northern Australia is governed by the southeast trade winds, which are generally dry

Answer 718

Tropical cyclones are low pressure systems that from over warm tropical waters and have gale force winds (sustained winds of 34kts or greater and gusts in excess of 49kts) near the centre. Technically they are defined as a warm core, non frontal low pressure system of synoptic scale developing over warm waters having organised deep convection and a maximum mean wind speed of 34kts of greater extending more than halfway around near the centre and persisting for at least 6 hours

Answer 719

The gale force winds can extend hundreds of kilometres from the cyclone centre. If the sustained winds around the centre reach 64kts (gusts in excess 89kts), then the system is called a severe tropical cyclone

Answer 720

The circular eye or centre of a tropical cyclone is an area characterised by light winds and often by clear skies. Eye diameters are typically 40km but can range from under 10km to over 100km. The eye is surrounded by a dense ring of cloud about 16km high known as the eye wall which marks the belt of strongest winds and heaviest rainfall

Answer 721

Tropical cyclones derive their energy from the warm tropical oceans and do not from unless the sea surface temperature is above 26.5°C, although once formed, they can persist over lower sea surface temperatures. Tropical cyclones can persist for many days and may follow quite erratic paths. They dissipate over land or colder oceans

Answer 722

Many tropical cyclones originate as the result of a wave disturbance particularly in the northern hemisphere. In the pacific, tropical cyclones often appear to develop in situ with no detectable wave disturbance. Most cyclones originate in the equatorial trough

Answer 723

Relationship with the equatorial trough, thermal energy, high level divergence, formation of the cyclone eye and the presence of a warm cyclone core

Answer 724

Maximum cyclone activity is normally attained when the trough is at its most northerly or southerly position which means that in the southwest pacific, the tropical cyclone season starts about October and peaks around February

Answer 725

Generally lower sea surface temperatures due to the cold humboldt current and the almost permanent anticyclone, giving rise to the regional dry zone

Answer 726

Most cyclones form within an area from northwest of Australia to approximately 160°W. due to the total absence of coriolis formed at the equator it is not feasible that cyclones form and develop within 5 degrees latitude of the equator. Due to other influences, such as ocean temperatures, it's not usual for cyclones to form beyond latitude 20°S

Answer 727

The immense kinetic nerdy involved in the high velocity airflows around a tropical cyclone is fundamentally derived from thermal energy. Although part of this is obtained from the required high ocean surface water temperatures which must be at least 28°C for the formation of a tropical cyclone, the warm ocean condenses into liquid water

Answer 728

This upper air outflow must be present to facilitate the fall of pressure at sea level so that the depression can develop into the cyclone stage. It appears that apart from the action of the upper air high pressure system, the eastern limbs of high level troughs in the westerlies can produce the necessary divergence as well. Thus upper air circulation has a great influence on the growth and also on the direction of travel of tropical cyclones

Answer 729

The narrow cylindrical column from the surface to the very top of the storm involves subsidence of air so that adiabatic warming dissipates much of the cloud. It is also clear that without the presence of the eye, pressure at sea level is unable to fall much below 1,000 hPa

Answer 730

The warming action is provided by immense latent heat released from a few hundred cumulonimbus towers within a small radius from the centre. The warm core is instrumental in the stimulation of the storm because it encourages the development of an upper level high pressure zone, which, through the associated high level divergence, causes lower air to rise and surface air to converge

Answer 731

The formative stage, the immature stage, the mature stage and the decaying stage

Answer 732

The surface pressure within a wave disturbance drops below 1000hPa, the eye begins to form and the wind in at least one quadrant increases to gale force strength (34kts). The affected area encompassed by a few closed isobars has a radius of some 50km. The cyclone is designated a tropical depression

Answer 733

Surface pressure continues to fall to under 1,000hPa. A closely spaced isobar pattern forms with a clearly defined eye in the centre (which will ultimately be of some 40km diameter) and winds reach hurricane force strength (64kts) within some 50km radius. Spiral bands are established and weather conditions commonly associated with CB developments are experienced within an expanding area. Now classified as a tropical cyclone and gets names

Answer 734

Surface pressures settle at its lowest value, which can be as low as 950hPa. Winds of hurricane force and stronger are experienced within a radius of some 200km from the centre while gale force winds are experienced up to 500km radius. Torrential rain, thunderstorms and violent squalls. The quadrant of the cyclone where winds and weather are at their worst is at the left forward side in the Southern Hemisphere

Answer 735

Surface pressure begins to rise and the tropical cyclone diminishes in intensity and is soon classified as a tropical depression

Answer 736

Movement over substantial land areas so that the fundamental source of energy, latent heat from an abundant water source, is cut off. Movement into latitudes where ocean temperatures are too cold for the required evaporation to be provided. Ingestion of cold air from whatever source. Separation of the cyclone from the high altitude divergence pattern

Answer 737

The average life span of tropical cyclones is 10 days nut substantial variations either side of this have been recorded

Answer 738

In the southwest pacific tropical cyclones tend to move southeast wards initially, particularly in the early part of the season (December to April). In the latter part of the season they may move west of southwest before re-curving to the southeast

Answer 739

The mean track of most tropical cyclones is from west to east and the average speed is 20kph but erratic changes in direction and speed are often noted. It is not possible for tropical cyclones to cross the equator

Answer 740

When cyclones move into higher latitudes, there strength is still such that even ordinary depressions they cause can cause a lot of harm and damage. When moving away from tropical latitudes, the decaying cyclone is referred to as extra tropical depressions

Answer 741

Approximately 1hPa per 6km

Answer 742

An increasing sea swell due to the outward flow effect of squalls and strong winds and a change in the semi diurnal variation of surface pressure which, in tropical regions, is readily evident due to the normally static nature of sea level pressures

Answer 743

An average occurrence of 6 tropical cyclones per annum in the southwest pacific region. For the entire Southern Hemisphere the number is close on 25 per annum which is only half the recorded occurrences in the northern hemisphere where in excess of 50 hurricanes/typhoons are experienced each year

Answer 744

The entire Pacific Ocean south of the equator is influenced by a circulating weather system known as the walker circulation. The system balances about two pivotal areas, high pressure off Peru and low pressure to the northwest of Australia

Answer 745

The Pacific Ocean along the west coast of South America is strongly influenced by a cold ocean current, originating from polar regions and flowing north, known as the Peruvian current (also known as the humboldt current). Off the coast of Peru this cold current forces itself to the surface with the result that the sea water temperature there is colder than it ought to be considering its position in the tropics

Answer 746

A consequence of this is that the air above the region becomes more dense and develops into a large subsiding anticyclonic system rather than a rising air mass, which is more characteristic of tropical environments. Under the influence of the subsiding motion of the air off the Peruvian coast, surface divergence towards the west (into the centre of the pacific) strengthens the trade winds so that the general low level easterly flow across the ocean is enhanced

Answer 747

Under normal conditions the sea temperature in the western Pacific is warmer than in the east, this difference can amount to as much as 5°C. Thus the less dense air over these warmer sea surfaces produce a region of comparative low pressure within which air rises and at altitude some of this air flows eastwards towards the subsiding air mass off the Peruvian coast

Answer 748

A huge closed circuit has developed right across the Pacific Ocean with westbound air travelling at low lev Los and east bound air travelling at altitude. These are the elements of the walker circulation

Answer 749

When the pressures are somewhat lower in the west than in the east the walker circulation is said to be a neutral or normal state. This is reflected by a mildly neutral or very slightly positive ENSO index (El Nino - southern oscillation index), which can be measured by comparing the prevailing sea level pressure at Darwin in the west to that at Tahiti in the east

Answer 750

When the pressure in the west (Darwin) is lower than normal and that in the east (Tahiti) higher than normal and stronger easterly surface winds in tropical latitudes of the pacific are the norm

Answer 751

When the pressure in Darwin is greater than Tahiti and easterly surface winds are reduced with strengthened westerly winds in mid latitudes, there is said to be a strong negative index

Answer 752

When pressures decrease further than normal in the west and increase further than normal in the east, the walker circulation intensifies, the index becomes more positive and the La Niña event takes place, enhancing easterly winds

Answer 753

When pressures increase in the west and decrease in the east the walker circulation weakens the index becomes more negative and the El Nino events take place, enhancing reduced easterlies in tropical latitudes and stronger westerlies in mid latitudes

Answer 754

It is believed to be a change in the state of the oceans surface water. The ocean surface in the western Pacific can be more than 30cm higher than in the east

Answer 755

The walker circulation is normal (or neutral) when the pressure index is very slightly positive/mildly neutral. The circulation intensifies, when an associated increase in the trade wind strength, when the index becomes positive and the La Nina event is established. When the circulation weakens and sometimes even reverses itself, the index becomes strongly negative mid latitude westerlies become stronger and the El t is established

Answer 756

When the index becomes strongly negative (low pressure in the east, high pressure in the west, El Niño) the overriding wind blows from the west-southwest and regions exposed to that direction such as both west coasts become wet while dry conditions are experienced on the lee of the main reaches

Answer 757

When the index is positive (high pressure in the east, less pressure in the west, La Niña) the overriding wind is from the north/north easterly quarter. Regions exposed to the north and northeast such as bay of plenty, northland, Nelson and the sounds experience wetter conditions than normal

Answer 758

Used for tropical latitudes because the coriolis force is so small that the pressure gradient is the overriding force which means that air will flow more or less directly from high to low pressure and across the isobars

Answer 759

In the tropics, the only method whereby the wind can be presented in useful form on a chart is by observing the actual wind over a wide area and drawing these observations on streamline charts. Directional streamlines show the direction of the wind for a given level

Answer 760

The information is gathered by a network of observation stations and from aircraft. Streamline analysis is also provided for upper air charts such as the 700,500hPa levels etc

Answer 761

It only provides information on the direction of the wind and nothing else. There is not similar link between the spacing of streamlines and windspeed

Answer 762

There is a need for wind speed as well as direction and the use of isotachs satisfies this requirement. An isotach is a dashed line that is generally curved or closed around regions where a given wind speed has been observed. They use wind vanes to indicate wind strength

Answer 763

843hPa +5°C

Answer 764

697hPa -5°C

Answer 765

572hPa -15°C

Answer 766

456hPa -25°C

Answer 767

377hPa -35°C

Answer 768

301hPa -45°C

Answer 769

239hPa -55°C

Answer 770

226.3hPa -56.5°C

Answer 771

The large scale pattern of pressure systems and winds, which endure worldwide throughout the year

Answer 772

Low latitude (equatorial) regions and high latitude (polar) regions

Answer 773

Constant angular momentum of the atmosphere and vertical and horizontal circulations

Answer 774

If large masses of warm air leave their tropical origins and move into higher latitudes, the smaller radius upon which they travel will cause their velocity (from west to east) to increase. This increase gives added stimulus to the poleward transfer of heat

Answer 775

There must be a groans balance between easterlies and westerlies otherwise the angular velocity of earth would alter

Answer 776

Vertical transfer in the two tropical Hadley cells (one in each hemisphere) and predominantly horizontal mixing of mid latitude and polar air rough the continued action of travelling extra tropical highs and lows, semi stationary highs and particularly their associated wave patterns at high altitude

Answer 777

The long Rossby waves and jet streams

Answer 778

In the Southern Hemisphere summer, jet streams, semi-stationary highs and mid-latitude pressure systems tend to move towards the south pole whereas in winter, the systems move equator ward. Due to the more intense thermal gradient in winter, the winter hemisphere experiences a stronger general circulation than is the case in summer

Answer 779

These are long waves on contour charts that are break offs from jet streams

Answer 780

By interpreting the direction of curvature of long waves it is possible to determine where the ridges and the troughs are. The importance of these waves in their function of transferring heat pole-ward and cold equator-ward

Answer 781

The principle cause of long waves are firstly, the influence of major mountain features such as the Andes in South America and the high ground of the eastern Antarctic and secondly, the influence of major ocean currents and their cold upwelling in coastal areas

Answer 782

Analysis over long periods has shown that the wave pattern in the Southern Hemisphere is more or less the same summer and winter, which is due mainly to the predominance of oceans, about 80% of the total area

Answer 783

Ocean and land coverage of the northern hemisphere is almost evenly divided, it comes as no surprise that the long wave pattern involves a greater number of ridges and troughs due firstly, to strongly differing heat and cold sources such as warm ocean currents in winter and hot land masses in the summer and secondly, large mountain barriers

Answer 784

The term zonal index refers to the strength of the mid latitude upper level westerly winds, which are also referred to as zonal winds

Answer 785

When the zonal winds are strong and the wave pattern does not display large fluctuations there is said to be a high zonal index. Under these conditions, surface and low tropospheric systems such as highs, lows and their fronts, travel at relatively high speed from west to east. The upper level westerlies and jet stream associated with high index situations are characteristically strong and homogeneous (uniform in structure), with the jet stream showing little change in speed along its axis and little curvature

Answer 786

When the zonal winds are less strong and when the long waves have larger northward and southward fluctuations. In low index situations the upper level westerlies are weaker overall in mid latitudes and the jet stream shows considerable variation in speed along its axis and also shows strong curvature. Localised high speed maxima occur in the regions of strong cyclonic and anticyclonic curvature

Answer 787

Pressure systems in the lower troposphere move towards the east at slower speeds than was the case in high index situations

Answer 788

Complete breakdown of zonal westerly winds with the result that closed contour patterns are formed at altitude. Deep lows close to the subtropics and deep anticyclones at high latitudes are commonly found under these conditions

Answer 789

Wave number 1 means one wave around the hemisphere i.e a wavelength of 360 degrees. Wave number 2 means two waves around the hemisphere with a wavelength of 180 degrees. Wave number 6 means 6 waves around the hemisphere with a wavelength of 60 degrees and so on

Answer 790

Wavelengths 1-3 are considered long and wavelengths 4-7 are short. Long waves are very stable features of the hemispheric circulation and may persist for weeks with little Chaney or movement. Short waves are less stable, they have a life cycle of days and show greater mobility

Answer 791

In the synoptic scale and therefore last a short time

Answer 792

Sudden loss of airspeed, sudden reduction in lift, increase in the rate of descent, increase in the angle of descent, increase in take off and landing path when head wind/tailwind changes, shallower climb angle when headwind changes to tailwind, might to moderate turbulence

Answer 793

Descending into a dense friction layer, most likely around dawn. Descending into a cold zone i.e aerodrome and surroundings covered in snow. Descending into the lee of sand dunes when a sea breeze blows. Descending into low levels where the reported wind is lighter and/or blowing from a different direction than that above. Ascending or descending through a sea breeze boundary

Answer 794

A microburst often involves downward spiralling of air usually within a small circumference. The vertical velocity within microbursts can be as high as 6,000ft/min

Answer 795

On hitting the ground, microbursts form a vortex ring, with winds flowing outwards at speeds exceeding 50kts up to a radius of some 1-2km from the core. Not all microbursts involve spiralling down draughts - in some cases the draughts are straight down so that, on contacting the ground, they form (vertical) vorticies about a horizontal axis. The height to which these vorticies reach can be over 2,000ft AGL

Answer 796

More than one

Answer 797

From the time the microburst first contacts the ground, the speed of the wind continues to intensify for approximately five minutes. After that time period the intensity decreases and the event should have dissipated after some 20 minutes

Answer 798

Microburst may be associated with convective cloud developments and/or heavy precipitation but it is not necessary that the base of the cloud is low

Answer 799

Research into microbursts and microbursts is continually carried out and there are indications that very heavy precipitation from clouds with temperatures at their base of +22°C or warmer can produce very powerful downdraughts

Answer 800

Anti icing is a preventive measure, and de icing is a remedial measure. Pre flight anti icing can normally be achieved by applying a solution of cold, rich freezing point depressant (FDF) fluid which under certain circumstances can prevent snow, from adhering to aircraft skins for up to 45 minutes in ambient temperatures as low as -14°C. Pre flight de icing can also be achieved by application of FDF fluids

Answer 801

Cooling of surfaces can be aggravated by very cold fuel affecting the temperature of metal surfaces after long periods of flight at altitude. Rapid and sometimes instant development of hoar frost is then likely

Answer 802

When stable, saturated air is forced up orographically, the freezing level will lower over the mountains. When unstable attracted air is forced to rise orographically, the freezing level will rise over the mountains

Answer 803

VFR flight in sleet can cause a build up of snow in areas where components join such as near wing roots, wing strut attachment points and around undercarriage components

Answer 804

It may cause engines to flame out due to blocking of intakes. It may cause extensive damage inside the engine due to the abrasive action of the ash particles and also because the particles may melt at the high temperatures to form a layer of glass which totally distorts the shape of all components. It may affect the operation of instruments due to blocking of pitot heads and it may cause total loss of visibility through the windscreen due to the sand blasting effect

Answer 805

AHRS is a greater hazard at the high altitudes used by jet aircraft as compared to piston engine aircraft. At lower levels, the ash disperses quite quickly due to turbulence

Answer 806

The presence of moisture or dust in the air has an adverse effect on the persons ability to judge distances, both vertically and horizontally

Answer 807

Precipitation and low cloud very often have the effect of changing the appearance of ground features such as valleys, saddles and ridges

Answer 808

It is possible to experience serious loss of depth perception through grey out in unsaturated air when flying in mountainous terrain close to sunset when the friction layer (which will extend above mountain tops) is dense as in stable atmospheric conditions. The shadow sides of ridges and particularly cliffs merge with the dimming light and it then becomes almost impossible to determine distances

Answer 809

A partial remedy to this situation is to select a height equal distance between the cloud base and the surface

Answer 810

The execution of an immediate 180 degree turn will navigate the aircraft quickly into the clearer airspace outside the cloud

Answer 811

When jet aircraft experience severe turbulence or heavy rain there is a risk of flame out. For that reason, many pilots turn engine igniters into continuous when experiencing these conditions

Answer 812

Use altitude, fly above turbulence cloud, select a route on the windward side of mountains or ridges, slow down and read the clouds

Answer 813

In calm conditions wake turbulence will affect areas within approximately 750 meters from the centre line of the runway

Answer 814

If wake turbulence cannot be out climbed after take off, a delay of 6 minutes in totally calm conditions will normally be sufficient. This time frame can be reduced as wind speed increases so that in winds of more than some 20kts a time delay of around one minute is normally more than adequate

Answer 815

When water has accumulated on a runway, a hydrodynamic pressure is developed between the footprint and the surface; thus pressure increases with increasing aircraft groundspeed. When the lift produced by hydrodynamic pressure equals the weight resting on the tyre, any further increase in aircraft ground speed will lift the wheel cleanly off the surface. Under certain circumstances this can cause the wheel to stop rotating or even start rotating in the opposite direction when the water build up in front of the foot print produced the required rotational force

Answer 816

The loss of contact between the tyre and runway surface reduces or eliminates braking effectiveness and directional control

Answer 817

It is necessary that substantial water accumulation exists on the runway e.g a flooded or heavily puddled runway

Answer 818

The smoother the runway and the smoother the tyre, the less the water required to develop aqua planning. Studies have revealed that only 3mm of water is needed between a smooth tyre and smooth surface for the problem to develop whereas a rough runway surface and a threaded tyre needs some 8mm of water

Answer 819

Vp = 9 square root p | Vp is the dynamic aqua planning speed in knots, p is the tyre inflation

Answer 820

Can occur on a wet runway that is very smooth either by design such as smooth asphalt, or due to the rubber accretion such as found in the touchdown area. The amount of water required to develop viscous aqua planning is dramatically reduced and the speed at which it can occur is at least 35% below dynamic aqua planning speed

Answer 821

This type of often the consequence of dynamic or viscous aqua planning. It occurs when the wheels are locked and the aircraft aids along the wet runway. The heat produced by the friction between tyre and runway surface wears off the rubber and causes particles to accumulate at the rear of the tyre foot print. This forms a barrier that prevents water from escaping. As the trapped water is heated, its stream pressure lifts the tyre off the runway

Answer 822

When a pilot experiences dynamic or viscous aqua planning after touchdown and on sensing reduced or nil braking effectiveness, applies progressively greater brake pressure so that the wheels ultimately lock. The prevention of this type of aqua planning is not locking the brakes

Answer 823

Worn or poorly inflated tyres lower the ground speed at which aqua planning can occur and it is therefore important that pilots check their tyres throughly for wear and pressure prior to flight when the destination airfield is likely to have a wet runway

Answer 824

Increase landing roll and reduction or loss of directional control. The latter is particularly important to remember because nose wheel tyre pressure is normally less than main wheel tyre pressure

Answer 825

The aircraft should be landed at the minimum safe touchdown speed as close to the opening threshold of the runway as possible. Use of braking devices such as spoilers, reverse thrust etc should be used with care and initial contact with the runway should be firm

Answer 826

Use of coarse chips in the runway sealing coat greatly reduces the risk of aqua planning and the best protection against aqua planning is lateral grooving of the runway cover

Answer 827

Plastics, nylons, paints, dyes and rubber. In contrast, metals are non organic which means that metal aircraft, with the exception of their organic components, are not subject to UV degradation

Answer 828

The distance from the source of radiation, the exposure time and the presence or absence of screening

Answer 829

The higher the latitude the less the radiation per area and therefore the risk of exposure is somewhat less

Answer 830

The use of suitable screens between the sun and an organic substance can reduce ultraviolet damage considerably

Answer 831

Experience has shown that dark coloured materials, such as Dacron used in many microlight aircraft, has a better protective quality from UV radiation than light or white coloured cloth

Answer 832

Differences in discolouration of surfaces facing up compared to those facing down, or those facing vertically

Answer 833

Latitude, oceanic surroundings and topography

Answer 834

It produces a climate there that is characterised by small annual fluctuations in temperature, a generally high relative humidity, reduced visibility and, apart from the occasional tropical disturbances passing by, light to moderate winds

Answer 835

The north island experiences greater fluctuations in annual temperatures where an average around 14°C. The South Island has similarly fluctuating annual temperatures with the average hovering around 10°C

Answer 836

The country lies at the northern edge of the mid latitude westerlies, which causes the prevailing winds to be from the west. Auckland/Northland experience westerlies about 50% of the time whereas Southland experiences these winds some 70% of the time

Answer 837

Between 1,000hPa and 1,020hPa

Answer 838

The usual track of decaying tropical cyclones is southeastward, with the centre of the depression passing to the north of northland

Answer 839

The influence of the oceans around NZ is to temper the prevailing temperature and produce a relatively moist climate. This is reflected in the generally high average relative humidity values and cloudiness pertaining throughout the country

Answer 840

In terms of variation, the steadiest rate of rainfall is in the southwest of the South Island whereas the most variable pattern is found on the east coasts of both islands and the north

Answer 841

Most rainfall is recorded in the northern and central regions of New Zealand during the winter and least in the summer

Answer 842

The axis of the main mountain ranges lies SSW to NNE which is very close to right angle obstruction to the prevailing westerlies. When the high moisture content due to surrounding oceans is added to the orographic effect, it can be seen that regions exposed to the west experience a greater degree of cloudiness and precipitation than those to the east

Answer 843

Turbulence will be more prevalent on the east of both islands

Answer 844

A funnelling (and Venturi) effect is readily established through cook trait whenever the wind has a west or northwest component. In similar vein, when the prevailing wind blows from the south, a southeast wind will funnel through the affected area. The effect on both winds is to increase their strengths, often to gale force

Answer 845

There is often little cloud at low levels other than stratocumulus on the hills to the north of Wellington. Also, turbulence is generally restricted to the friction layer. When the wind is forced through as a south easterlies, cloud bases are then lee while turbulence is nil or light over the water to the South of Wellington but often moderate over the land

Answer 846

On the west: increasing stratiform cloud, rain developing in SW, flying conditions generally smooth On the East: little or no clouds turbulent flying conditions, fohn winds likely in South Island and wairarapa/Hawkes bay, good visibility

Answer 847

On the west: showers, generally smooth flying conditions and visibility good to excellent outside showers On the East: mainly fine, turbulent flying conditions and visibility good

Answer 848

On the West: fine weather, turbulent flying conditions, good visibility On the East: showers, generally smooth flying conditions and visibility good to excellent outside showers

Answer 849

Over NZ, the passage of an anticyclone followed by a cold front followed by the next anticyclone, the entire sequence involving some 6-8 days

Answer 850

The most common winds are south west and north-north east with the first mentioned the more predominant south westerlies have their greatest percentage occurrence during spring and summer while the north easterlies are spread fairly evenly throughout the year

Answer 851

The main direction of the sea breeze is from south-south west

Answer 852

Surface friction induced turbulence is not very common but thermally induced turbulence is often experienced in and around the nearby hills

Answer 853

Radiation fog is common in winter (between April and late august) when synoptic situations indicate a high pressure system over the area

Answer 854

The most common cause of low cloud formation is a moist north east air stream when a cloud base of some 500-600ft and sometimes as low as 300ft can be experienced. When formed, this cloud can persist for anything up to 24 hours

Answer 855

Due to the orientation of the mountain ranges, the prevailing wind in the region is north/north west or south/south east

Answer 856

Fog is very uncommon at Wellington airport

Answer 857

The prevailing wind at the airport is from the north east (about 20% of the year). Southerly winds occur about 15% of the year and the north westerlies blow approximately 13% of the year

Answer 858

The onset of hot and turbulent northwesters can be anticipated by monitoring the pressure difference in a northerly and westerly direction. When the QNH difference between Cape Campbell (high) and Dunedin (low) exceeds 4hPa and when the QNH difference between hokitika (high) and Christchurch (low) exceeds 5hPa and the air is unstable, the surface wind at Christchurch will be a north westerly

Answer 859

In terms of season, the months of May to September have the highest incidence of low cloud and/or poor visibility

Answer 860

METAR, SPECI and METAR AUTO

Answer 861

Global en-route forecasts for international air navigation are provided by the world area forecast centres (WAFC) London and Washington, which distribute forecasts of upper wind, temperature, humidity, tropopause height, maximum wind, icing, turbulence, CB cloud and significant weather to meteorological offices throughout the world

Answer 862

The AWS consists of a central data collection, processing and communications system in which a range of sensors can be incorporated to monitor specific weather elements

Answer 863

Surface wind speed and direction, visibility, present weather elements, cloud, air temp, dew point, atmospheric pressure (QNH)

Answer 864

Pilots should be aware that the information on cloud and visibility obtained from automatic weather stations may not be representative of the conditions over the entire aerodrome and immediate vicinity

Answer 865

By radiosondes, which are small lightweight (200 grams) packages sent aloft by balloon

Answer 866

Temperature: measured by a thermistor consisting of a small element coated in a special chemical Humidity: measured by a small capacitor (resistor), which contains a lithium chloride element Atmospheric pressure: measured by a small aneroid capsule Wind velocity: obtained by radar tracking of metal refractors attacked to radiosondes or by GPS radiosondes

Answer 867

METAR, SPECI and METAR AUTO are reports of weather conditions at a given time and are not forecasts, therefore they will have an issue time but cannot have validity times. They describe conditions at a certain time within a radius of 8km from the aerodrome reference point

Answer 868

METAR and SPECI reports, based on human observations are available in New Zealand only from Milford Sound and the RNZAF aerodromes

Answer 869

METAR AUTO reports are based on AWS data available for the international and secondary aerodromes. SPECI are not reported from METAR AUTO sites

Answer 870

METAR and SPECI are provided in New Zealand only at the RNZAF aerodromes (Whenuapai and Ohakea) and at Milford sound. METARs are issued routinely on the hour during the hours of ATC service. SPECI are issued between routine reports when changes in weather conditions meet certain defined criteria. When SPECI conditions exist at the time of a routine hourly report, the report is coded as a METAR and not as a SPECI

Answer 871

Wind velocity, gustiness, visibility, cloud, the onset or cessation of moderate or heavy precipitation or obscuration and air temperature

Answer 872

A SPECI is issued immediately when one or more of the phenomena goes through predetermined values. When an improvement above predetermined values has persisted for 10 minutes a new SPECI is issued with the words SPECI CONDX CEASE or SPECI CEASES added as a remark

Answer 873

If a report is missing

Answer 874

If a report is corrected for any reason

Answer 875

Degrees true and knots

Answer 876

When gusts exceed the mean speed by 10kts or more they are reported using the letter G after the wind speed

Answer 877

When the direction of varying by 60 degrees or more, a second group is used showing the extremes separated by the letter V

Answer 878

The term VRB is used when the wind direction is variable and the wind speed is less than 3kts

Answer 879

If the lowest visibility is observed in more than one direction, then the most operationally significant direction is reported. When the visibility is fluctuating rapidly and the prevailing visibility cannot be determined, only the lowest visibility will be reported with no direction indicated

Answer 880

The abbreviation VC (in the vicinity) is used to identify phenomena between 8km and 16km from the aerodrome reference point

Answer 881

Used using the abbreviations FEW, SCT, BKN, OVK. The type of cloud is not reported unless it is CB or TCU (towering cumulus without the anvil). The base of reported cloud is given as a 3 figure group in feet above aerodrome level

Answer 882

No dew point reported

Answer 883

Moderate or heavy rain in the past hour (which was heavier than currently included in the METAR or SPECI)

Answer 884

Wind shear is reported using the format WS RWY followed by the runway designator