CPL Met Flashcards
What is carbon dioxide made up of
It is a combined gas made up of components carbon and oxygen, 13 parts of carbon and 32 parts of oxygen by weight
What does carbon dioxide do in the atmosphere
Carbon dioxide in the atmosphere absorbs terrestrial (earth) radiation, which in turn has an effect on the temperature of the air surrounding earth. This greenhouse effect, as it is known, is a serious treat to life if co2 amounts are not kept within certain limits
What is atmospheric ozone (O3)
Ozone molecules, consisting of three atoms of oxygen, are formed in the upper levels of the stratosphere and the lower levels of the mesosphere. The main importance of this constituent is its ability to absorb ultra violet radiation from the sun
How much UV radiation from the sun is absorbed by the ozone layer
60% of the harmful UV radiation from the sun is absorbed by the ozone layer
How much nitrogen is in the atmosphere
78.08%
How much oxygen is in the atmosphere
20.94%
How much argon is in the atmosphere
0.94%
How much carbon dioxide is in the atmosphere
0.03%
How much neon is in the atmosphere
0.001%
How much helium, hydrogen, xenon, ozone etc is in the atmosphere
0.009%
What is the troposphere
The lowest layer of the atmosphere is the troposphere which consists of almost all the water vapour and approximately 75% of all the molecular mass of the atmosphere. This is they layer in which weather and turbulence phenomena are most marked
What are the heights of the tropopause
The tropopause varies in mean height between some 18km over the equator to just about half of that, 8km, over the poles
What are jet streams often associated with
Jet streams (high velocity horizontal wind flows) are often associated with the breaks between the main tropopauses. These jet streams have a profound influence on the development of depressions and fronts
What part of the atmosphere is an isothermal layer
The lower part of the stratosphere above the tropopause is a large isothermal layer
What is the overriding influence on the height of the tropopause
It can be said that the over riding influence on the height of the tropopause, and thus its temperature, is the temperature lapse rate
What’s the main reason for differences in height of the tropopause
Lies in the fact that density of air is less over the equator and regathering over the poles. In turn, the main reason for this dissimilarity is the difference in surface temperature due to varying angles of insulation i.e solar input
What is the stratosphere
This layer in the atmosphere able the troposphere has a thickness of some 35km. Initially above the tropopause, the layer is isothermal for some 8-10km. Above this height, the temperature slowly increases and towards the top quarter of the stratosphere, temperatures increase rather strongly with height. The main reason for this is the action of the ozone layer, which absorbs large amounts of short-wave ultraviolet solar radiation
What is the main reason for the weather in the stratosphere being generally devoid of cloud and turbulence
Since the lower stratosphere is largely isothermal and its top half associated with negative lapse rates, any up draughts caused by low level instability in the troposphere will soon be eliminated in the stratosphere. This is the main resin for the weather in the stratosphere being generally devoid of cloud no turbulence
Is the stratosphere over the poles colder or warmer than over the equator
The stratosphere above the poles is less cold than over the equator
How high is the mesosphere
The stratopause, at approximately 50km, heralds the beginning of the mesosphere, which is a layer some 30km (about 90,000ft) thick
What’s the temperature of the mesosphere
The mesosphere is found at the top of the layer and has the lowest temperature of the upper atmosphere, approaching -90°C. The pressure in the mesosphere is generally less that 1hPa and becomes almost unmeasurable towards the mesosphere
Where can noctilucnet clouds be found
Noctilucnet clouds can be found near the mesopause, most commonly over polar regions in summer. Their origin is believed to be in meteoric dust particles acting as nuclei for ice particles
What is the homosphere
Troposphere, stratosphere and the mesosphere
What alters the structure of the gas molecules in the thermosphere
The structure of gas molecules in the thermosphere alerts dramatically under the influence of ultraviolet and solar X-rays which cause ionisation (electrical charging). This process leaves oxygen atoms and nitrogen molecules with a net positive charge (ions). The ionosphere generally lies between 90km and 300km (although thus height may vary considerably)
What is the ionosphere have importance to
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
What do temperatures do in the thermosphere
Temperatures within the thermosphere rise rather steadily, especially towards the higher levels, due to very high ultraviolet radiation
Near the earth surface the atmosphere exerts a pressure of what
About 10(5) (100,000) pascals
What is the standard atmospheric pressure
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
What is the main type of mercury barometer
The Kew type
What are the errors in the mercury barometer
The location where the barometer is installed must have its ‘g’ established and incorporated in correction cards for complete accuracy to be obtained
What’s the index error on the mercury barometer
Index error is associated with the behaviour of mercury and the manufacture of the instrument
What is the advantage of the barograph in the aneroid barometer
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
What formula can be used to calculate the number of feet per hectopascal for any altitude change
Feet per hectopascal = 96T / P
T is absolute temperature, P is the ambient pressure in hPa and 96 is a mathematical factor
What is absolute temperature
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
What is absolute zero
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
My way of figuring out feet per hectopascal
96 x (273 +/- temp) / hPa
How can you obtain total and complete accuracy on weather charts
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
What are used for accurate measurement of mean column temperature
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
Where to highs and lows tend to migrate
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
What is the semi diurnal variation of pressure
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
When is the pressure drop at its maximum during the semi diurnal variation of pressure
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
What is meant by the term pressure gradient
The term pressure gradients which is the change of pressure with horizontal distance measured from high to low
What way does pressure gradient always act to the isobars
The pressure gradient always acts at right angles to the isobars
What is air said to be in the ISA
Air is dry and of constant composition at all altitudes
What is the force of gravity in ISA
Is constant at 9.8m/sec
What is the jet standard atmosphere
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)
What is absolute altitude
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
What is QNH
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
What is QFE
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
What is QNE
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
What is QFF
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
What is the relationship between QNH and QFF
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
What will happen when the altimeter sub scale is turned down
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
What will happen when the altimeter sub scale is turned up
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
What are the three main units used for temperature
The three main units of measurement of temperature are Celsius (formerly referred to as centigrade), Fahrenheit and kelvin (also referred to as absolute)
What does the mercury thermometer consist of
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
When does the mercury thermometer ceases to be totally accurate
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
What is the use of the modified mercury thermometer
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
What does the restriction in the mercury thermometer do
The restriction traps the mercury above it and the highest day temperature can be read even hours after its occurrence
When is the alcohol thermometer usually used
This type is normally used when low temperatures are involved because alcohol has a low freezing point
How does the alcohol thermometer operate
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
How does the fluid in metal thermometer work
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
How does the thermocouple thermometer work
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
How does the bi-metallic thermometer work
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
How does the thermistor work
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
Hoe does a thermograph work
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
How is the measurement of surface temperature done
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
What is radiation
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
What is solar energy
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
What is solar energy made up of
Ultraviolet approx 9%
Visible light 45%
Infrared 46%
What obstructs solar radiation
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
How much of the solar radiation does the earth receive
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
What is the total solar radiation received by the earth the sum of
The total solar radiation received by the earth is the sum of sky radiation and direct radiation, called global solar radiation
What are the three main factors that influence the amount of solar energy received by earth
The distance between sun and earth, the altitude of the sun and the length of day
What are the solstices
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
When is the solar energy strongest
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
What are the equinoxes
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
What are the instruments that measure solar radiation called
Pyranometers
How does the pyranometer work
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
What is terrestrial radiation
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
What are strong absorbers of terrestrial radiation
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
What is the atmospheric window
Not all infrared radiation is absorbed by the three atmospheric constituents. Between 8.5u and 13.0u, terrestrial radiation readily escapes
What is the energy budget
The energy budget which essentially means that the atmospheric temperature, resulting from radiation, has a relatively steady annual global average of around 15°C
How is the earths atmosphere warmed
From below
What is the hot plate effect from solar and terrestrial radiation
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
What does the release of terrestrial radiation tend to cause
Although terrestrial radiation is a major factor in warming the atmosphere, its release from earth tends to cool the surface
How does the warmed earth surface transfers its heat to the atmosphere above it
Through conduction and convection
What is conduction
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
What happens when solar energy has excited and heated the surface
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
When does the days max temp occur
Around 2pm
When does the days minimum temp occur
Just before sunrise
What soil types have greater depth of warming
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
What will any surface that heats quickly and cools just as quickly involve
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
What’s the relationship between solar insolation and the oceans
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%
What are the temperature extremes of a maritime climate
Maritime climates rarely have extremes of temperature
What is specific heat
This means the amount of thermal heat units (expressed in joules) required to raise the temperature of a unit mass by 1°C
What are the reasons why sea air temperatures are relatively steady
Specific heat, mixing, evaporation and wind
How does mixing of the oceans make temperatures relatively steady
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
How does evaporation from oceans keep temperatures relatively steady
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
How does wind over oceans make temperature relatively steady
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
How does cloud effect the diurnal temperature changes
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
How is albedo measured
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
What is condensation
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
When can the condensation process not take place
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
What are hygroscopic particles
Sea salt is particularly effective in condensation formation since it has a great affinity for water
What is super saturation
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
What is the saturation vapour pressure of moist air
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
Does evaporation add to the atmospheric pressure
Yes
What does the saturation content graph show above 0°C with respect to ice
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
Does latent heat change the temperature of the substance
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
What happens to latent heat when it is absorbed
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
When does water have the greatest density
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
What is the greatest producer of precipitation
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
What is absolute humidity
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
What is relative humidity
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
What is the saturation content
The maximum weight of water vapour the air can hold is generally referred to as the saturation content
What is the formula for relative humidity
Relative humidity œ absolute humidity / saturation content %
What is meant by absolute humidity
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
What’s the relationship between the three factors in the relative humidity formula
Relative humidity is directly proportional to water vapour but it is inversely proportional to temperature
What is dew point defined as
Dew point is defined as the temperature at which a parcel of air saturates under constant pressure - it is a temp!
What does the definition of dew point include
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
What’s the difference between relative humidity and dew point
Relative humidity is affected by changes in water content and air temperature. Dew point is affected only by changes in water content (ignoring pressure)
What are the instruments called Thant measure relative humidity
Hygrometers, commonly used are the wetbulb/dry bulb, hair hydrometer and the lithium chloride element
What is the wet bulb/dry bulb hydrometer
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
How does the wet bulb/dry bulb show low humidity
The wet bulb depression will be large, I.e the two thermometer readings are far apart which indicate a low relative humidity
How does the wet bulb/dry bulb hydrometer show high relative humidity
The wet bulb depression is less, that is, the two thermometer readings are close together, which indicates a high relative humidity
How does the lithium chloride element hydrometer operate
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
What is the density of moist air
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
What is the principle reason for movement of air
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)
Is horizontal or vertical flows stronger
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
What are the four forces which have a fundamental influence on the strength and often direction of the wind
Pressure gradient
Coriolis force (also known as geostrophic force)
Centripetal force
Friction forces
How is the pressure gradient responsive to the density of air
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
How fast does the earth spin when viewed from space
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
What is the coriolis force formula
Coriolis force = 2 w p v sinLat
What is the w in the coriolis formula
Is the angular velocity of earth spin, simply its rpm of 360 degrees per 24 hours
What is the p (rho) in the coriolis formula
Air density
What is the v in the coriolis force formula
The speed of the airflow, the wind speed
What is the sinLat in the coriolis formula
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
What is the interaction between coriolis force and pressure gradient
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
What is the geostrophic wind
When the wind blows along straight isobars
What is a gradient wind
When the wind follows curved isobars
What are the three fundamental forces affecting the air within pressure systems
Pressure gradient, coriolis force and centripetal (cyclostrophic) forc. These three determine the strength of the wind and the circulation around the systems
Is pressure gradient constant
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
Are pressure systems stationary for long
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
When will the wind be geostrophic
When pressure gradient and coriolis force are equal, isobars will be straight and the wind will be geostrophic
When will the wind be a gradient wind
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
What is the wind strength fundamentally determined by
The pressure gradient force (PG) I.e the stronger the pressure gradient the stronger the wind
What is the coriolis force determined by
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)
What happens between isobar spacing and different latitudes
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
What are cyclostrophic winds
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
What’s the depth of the friction layer
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
Land areas and particularly those involving rough surfaces with their deep friction layers will accordingly be associated with a surface wind that
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
Areas that have a shallow friction layer such as oceans will therefore be associated with a surface wind that
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
Which way does the surface wind move after sunset and daybreak
In the Southern Hemisphere, the surface wind backs and increases after daybreak and veers and decreases after sunset
What does the degree of diurnal variation depend on
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)
How is geostrophic wind measured
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
How is surface wind measured
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
How long are winds averaged over in take off and landing reports and synoptic reports
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
How does the rotating cup anemometer work
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
What is one of the disadvantages of using the rotating cup anemometer
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
What are the diameters of the 25 knot windsock
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)
What is buys ballots law
In the Southern Hemisphere, one stands with ones back to the wind, the area of low pressure is to ones right
What happens if you descend through a shear zone
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
How can you avoid shear at altitude
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
What’s the big problem with wind shear
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
What are the characteristics of stable air
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
What are the characteristics of unstable air
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
What are the main factors that determine if air is stable or unstable
The temperature lapse rate of the environment air and the adiabatic change in temperature of a vertically moving parcel of air
How is the environmental lapse rate measured
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
What happens when a rising parcel of air enters reduced atmospheric pressure
A rising parcel of air, metering reducing atmospheric pressure and increasing its volume, will cool and thus cooling process will be an adiabatic one
What is an adiabatic process
An adiabatic process (of temperature change) is simply one where no energy (heat) is gained from or lost to the surroundings
What is an non adiabatic process
One where energy (heat) is lost to or gained from the surroundings
What is an isobaric process
One where a change in temperature occurs when the pressure remains constant
What is super adiabatic
ELR’s in excess of 3°C/1,000ft
What is the rising condensation level (RCL)
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)
How can you calculate the RCL
Difference between the surface temperature and the dew point, divide answer by three = RCL
What is the convective condensation level (CCL)
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)
When will convective rising of air not produce cloud
Convective rising of air will not produce cloud if the DALR intersects the ELR before dew point has been reached
How do you calculate the minimum surface temperature to produce convective cloud
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)
What does high and low dew points create with convective rising
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
What is convective stability
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
What is convective instability
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
What is diurnal instability
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
What is latent instability
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
How is a sea breeze created
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
When does a sea breeze usually set in
The sea breeze sets in at around 10am and peaks at around 2 to 4pm
What is the usual strength of the sea breeze
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
What is the horizontal and vertical extents of the sea breeze
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
What is the cloud formation in a general sea breeze
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
What cloud formation is found from a sea breeze in tropical regions
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
What precipitation is expected from a sea breeze
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
What temperatures are expected during a sea breeze
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
What kind of pressure gradient is found from a sea breeze
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
What kind of windshear can develop from a sea breeze
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)
When can turbulence be found from a sea breeze
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
What is the pseudo sea breeze
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
What is the land breeze
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
Which has the higher pressure in the land breeze
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
How strong are leaned breezes usually
Generally the breeze is only some 3-4kts but local topography such as sloping land towards the sea can increase this value
When is a land breeze most common
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
What are some factors with the land breeze
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
What are katabatic winds
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.
Why are clear winter nights most likely to form katabatic winds
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
What is the air said to be doing in a katabatic wind
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
What is the normal speed of the katabatic wind
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
How is valley fog formed from katabatic winds
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
What is the anabatic wind
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
What is the strength of the anabatic wind
The anabatic wind is almost always rather light due mainly to the influence of gravity
What is a gust
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
What are squalls
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
When might a fohn wind be experienced
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
How does the fohn wind work
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
What is the dry fohn
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.
What is visibility like on the lee of the mountain in a fohn wind
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
What are the effects of small scale interference to wind flow such as shelter belts or buildings
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
What does a fohn wind need
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
What is large scale interference to wind flow also known as
Mountain waves, standing waves or lee waves
What might mountain waves cause
May cause severe turbulence under certain conditions
What do mountain waves need to form
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
What happens to mountain waves if the mountain range is too small
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
What limit does the wind direction need to stay in during a gain in height of mountain waves
The increasing wind with height must retain a fairly constant direction, generally within 20 to 25 degrees throughout a deep layer
After crossing mountain tops with mountain waves, what should be expected
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
What does the wavelength of mountain waves mean
The horizontal distance between two identical parts of successive waves such as the distance between each crest or between each trough
What does mountain wavelength depend on
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
Which side of the mountain has an effect on the wavelength of mountain waves
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
What is the amplitude of mountain waves mean
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.
When will the amplitude of mountain waves be greatly increased
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
When is turbulence present from mountain waves
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
What is clear air turbulence found in mountain waves
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
Where are rotor zones usually found
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
How can you cross mountains when rotor zones are present
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
What cloud is likely to form from mountain waves
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
When do lenticular clouds not drift down wind
Lenticular clouds do not drift down wind when the pattern of the oscillations is constant in both frequency and amplitude
How are mountain waves dissipated
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
When is rotor streaming likely to develop
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
What is rotor streaming associate with
It’s often hidden within deep cloud and precipitation
How is rotor streaming formed
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
Where is turbulence least in rotor streaming
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
What causes a radiation inversion
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
What size radiation inversion will form in calm conditions
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
What size radiation inversion will form with a light wind
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
What causes a stronger radiation inversion
The drier the air the more intense the surface cooling, the greater the likelihood of a strong radiation inversion developing
What are the positive and negative effects from wind on radiation inversions
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
What will happen if the reduction in air temp during a radiation inversion cools the air to its dew point
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
What will happen during a radiation inversion if surface temps fall below 0°C
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
Does the radiation inversion increase or decrease wind speed
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
What will happen when climbing out of a radiation inversion
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
What happens when descending into a radiation inversion
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
What happens to visibility from a radiation inversion
Visibility will deteriorate rather abruptly and this can involve distortion or discolouration of lights, looked at in the near horizontal plane
What are the other causes of radiation inversions
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
Can a radiation inversion form over water
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
Why does a turbulence inversion often develop
This inversion often develops as the consequence of a surface radiation inversion
What are the two main requirements for the formation of a turbulence inversion
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
What will cause the turbulence for the turbulence inversion
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
What is the effect of wind on a turbulence inversion
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
What clouds can form in the turbulence inversion
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
When is the most common time of day for a turbulence inversion to take place
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)
What happens after the cloud begins to rise in th turbulence inversion
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
What happens if a sea breeze does appear in a turbulence inversion
If a sea breeze should appear the development of cloud will be enhanced and the original stratocumulus may then turn into fair weather cumulus
What are the process for both the radiation and turbulence inversions
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
Can turbulence inversions form over water
Is is possible to see turbulence inversions over oceans
What happens to the air during a subsidence inversion
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
What form a subsidence inversion
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
What is anticyclonic gloom associated with subsidence inversions
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
How does the characteristics of the friction layer have a great influence on subsidence inversions
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
What areas encourage the development of a subsidence inversion
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
What causes a frontal inversion
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
What front is a frontal inversion more common in
The warm front, due to its characteristics, is more likely to be associated with a frontal inversion
What is an isothermal layer
When temperature remains constant with height
What are the effects of isothermal layers
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
How can cloud be defined
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
What does super saturation require to condense
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
Does sea air or land air have more hygroscopic particles
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
How can relatively humidity be increased
Either increasing the water vapour content or lowering the airs temperature, relative humidity can be increased
What is the most common way RH is raised
Through cooling of air
What type of cloud is formed in stable air
Layer type cloud
What type of cloud if formed in unstable air
Cumulus or heaped type cloud
What are the main factors that determine the degree of evaporation and height loss involved in water droplets
The RH of the air beneath the cloud, the state of the falling water and the size of the drop
What does the internationally adopted system consider cloud under
Shape and structure, vertical extent and altitude
High clouds in tropical latitudes
6km+
20,000ft+
Middle cloud in tropical latitudes
2-6km
7-20,000ft
Low cloud in tropical latitudes
Below 2km
Below 7,000ft
High clouds in mid latitudes
5km+
17,000ft+
Middle cloud in mid latitudes
2-5km
7-17,000ft
Low cloud in mid latitudes
Below 2km
Below 7,000ft
High cloud in polar latitudes
3km+
12,000ft
Middle cloud in polar latitudes
2-3km
7-17,000ft
Low cloud in polar latitudes
Below 2km
Below 7,000ft
What does high cloud consist of
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
What might altostratus be an indication of
Like cirrostratus, altostratus may be an indication of approaching deteriorating weather
What sort of icing can be expected in altostratus
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
What icing is present in altocumulus
In global terms icing is usually considered to be light in altocumulus. Precipitation of any significance is not common
What degree of turbulence can be found in stratocumulus
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
What icing should be expected from stratocumulus
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
What usually causes stratus
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
What icing is found from stratus
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
What is cumulus cloud often caused by
Surface heating and/or orographic ascent
What precipitation can be found in cumulus
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
Is turbulence found in cumulus cloud
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
What is the most common type of ice found in nimbostratus
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
What does nimbus mean
Large water content
What are the artificial clouds
Pyrocumulus and condensation trails (referred to as contrails), both formed as th result of the combustion process
When will pyrocumulus form
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
What cloud does pyrocumulus form
When sufficient moisture is available, cumulus will develop as the result of the heating source
When will contrails develop
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
What process must occur for contrails to persist
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
What is meant by fracto when cloud amounts are referred to
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
How is the reporting of cloud usually done
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
How is the assessment of cloud base done
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
How does the cloud searchlight measure cloud base
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
How does the ceilometer measure cloud base
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
When can the ceilometer be used
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
What is the most modern ceilometer
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
What are the advantages of the ceilometer
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
What is the disadvantage of the ceilometer
It doesn’t scan the whole sky. The best observations will be those that combine visual and instrumental sources of information
What does this extent and type of cloud depend on due to orographic rising
The extent and type of cloud due to orographic rising depends on the RH of the air and its stability
What’s happens if air is sufficiently moist during orographic rising
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
When can cloud develop from orographic rising if the air is dry
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)
What does the term orographic cloud refer to
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
What is mechanical rising
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
When does convective rising result
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
What happens during convective rising if no moisture is carried along
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
What cloud will form from convective rising
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
What are the most common systems in which widespread ascent is involved
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
What does large scale ascent involve
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
What is the initial cause of widespread ascent
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
What is the warm front
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
What is the cold front
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
How big is the width of the cloud band of a cold front
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
What is the most common cause for the formation of clouds
Adiabatic cooling of air through rising
What is the most common cause for dispersal of cloud
Adiabatic warming of air through subsidence
When is large scale dispersal of cloud often experienced
Large scale dispersal of cloud is often experienced when air is forced down through subsidence in an anticyclone
How is cloud dispersed from radiation
Warming of clouds through absorbti n of either solar or terrestrial radiation
How is cloud burnt off
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
How does a decrease in convective activity disperse cloud
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
How does mixing of unsaturated air disperse cloud
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
What’s is the Bergeron theory
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
What is the process of the Bergeron process
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
When is the rate of deposition most efficient
At temperatures near -15°C the growth rate through deposition seems to be most efficient
What nuclei do ice crystals require
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
What is the fusion of coalescence
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
What are the factors affecting the rate at which a water droplet falls
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
What type of precipitation will form if ice crystals have been formed
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
What does hail consist of
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
What is continuous precipitation
(Rain, drizzle, snow) implies an extensive, thick cloud sheet, which is not likely to clear in the short term
What is intermittent precipitation
Temporary improvements are likely
What are showers
(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
How is the rate of precipitation measured
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
What size drop will split into smaller drops
Once a drop reaches a smaller size (approx 5mm) it splits into smaller drops
What are the two types of weather radar currently used by the meteorological service of NZ
Weather surveillance for precipitation detection over a wider area and wind finding radar
What is the weather surveillance radar
A network of five radar installations is required to cover NZ- three have been installed to date at AU,WL, and Rakaia
What is the main requirements of the weather surveillance radar
Unobstructed radar coverage, access to electricity and access for installation and maintenance
What are the two modes of the weather surveillance radar
Intensity for detection and identification of precipitation and Doppler to provide speed and direction of travel of the precipitation
What is the most commonly used rain gauge
The tipping bucket type
How does the tipping bucket rain gauge operate
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
What is the deciding factor in visibility
The transparency of air, which is directly related to the absence of solid or liquid particles within the air
What does meteorological visibility relate to
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
How is visibility defined
As the greatest distance a black object can be seen and recognised against the horizon sky
Does visibility change during day or night
Whether it’s day or night, if transparency doesn’t change visibility doesn’t change
What are the predominant factors of visibility
Precipitation, fog and mist, haze, smoke, sea spray
Mid latitudes visibility from precipitation
500m or less
Tropical latitudes visibility from precipitation
100m or less
Moderate rain visibility
4-10km. Light rain has a limited effect; visibility may well exceed 20km
Snowfall visibility
Moderate snowfall will reduce visibility to below 1,000m while heavy snow will reduce this to less than 100m
Visibility in drizzle
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
Visibility in fog and mist
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
