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Meteorology Flashcards

1
Q

Composition of the Atmosphere

A

78.08% Nitrogen
20.95% Oxygen
0.93% Argon
0.04% Carbon Dioxide

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2
Q

Average percentage of water vapour in air.

A

1%

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3
Q

Avogadro’s Law

A

At a given temperature and pressure, the number of molecules present is constant for a particular volume.

Hence the addition of water vapour to the air results in the reduction of the density of the air.

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4
Q

Layers of the Atmosphere.

A

Troposphere - <20km
Stratosphere - 20km - 50km
Mesosphere - 50km - 85km
Thermosphere - 85km - 690km
Exosphere - 690km - 10000km

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5
Q

Environmental Lapse Rate

A

The rate at which temperature falls as height increases.

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6
Q

Temperature above the tropopause.

A

Constant value of - 56 C above FL360

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7
Q

Tropical Tropopause

A

Approx 60,000ft.
Temperatures recorded lower than - 90 C

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8
Q

Polar Tropopause

A

Approx 20000ft.
Temp as low as -40 C

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9
Q

Jetstreams

A

Fast, narrow currents of air separating multiple overlapping areas of tropopause.

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10
Q

Stratosphere

A

Extends upwards from the tropopause until approx 120000ft.

Temperature reasonable constant around -56C in lower layers.

Temperature increases to near 0C at the top layer.

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11
Q

Which layer of the Atmosphere does the ozone layer sit in?

A

Stratosphere.

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12
Q

Aurora

A

A natural light display in the sky particularly in the high latitude, Arctic and Antarctic, regions, caused by the collision of highly charged particles with atoms in the Thermosphere.

The charged particles originate in the magnetosphere from solar winds and are directed by the Earth’s magnetic field towards the atmosphere at the poles.

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13
Q

International Standard Atmosphere

A

Temperature - 15C
Pressure - 1013.25 hPa
Density - 1225 g/m3
Temperature Lapse Rate - 1.98C (2) per 1000ft until 36090 ft where it remains at -56.5C
Pressure Lapse Rate - 1 hPa per 30 ft
The air is dry.

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14
Q

What is the main absorber and conductor of solar energy?

A

The surface of the Earth

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15
Q

Conduction

A

The transfer of energy through contact.

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16
Q

Convection

A

The vertical transfer of energy through the
atmosphere.

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17
Q

Advection

A

The horizontal transport of energy through the
atmosphere or ocean.

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18
Q

Radiation

A

The transfer of energy through wave or particles.

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19
Q

Latent Heat

A

The amount of energy absorbed or released during a substance change of state or phase transition.

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20
Q

Pressure

A

The weight or force of a column of air due to gravity.

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21
Q

Density

A

Mass per unit volume; that is how many molecules are in a given sample within a column of air.

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22
Q

Temperature

A

A measurement of heat energy within a
sample.

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23
Q

Warmer air vs cold air (temp/pressure/density)

A

Higher Temperature
Higher Pressure
Lower Density

Rises due to lower density than surrounding air.

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24
Q

What is a radiosonde?

A

Battery powered instrument packages attached to weather balloons that are then released into the atmosphere.

They are designed to send atmospheric telemetry to a ground receivers.

This data is then plotted on a graph called an aerological diagram that then creates a snapshot of the present atmosphere.

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25
Q

Data relayed by radiosondes

A

Altitude
Pressure
Temperature
Relative humidity
Wind (speed and direction)
Geographical position

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26
Q

Density of the air at 25000 ft relative to that sea level

A

Approximately half

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27
Q

Why is air density important to pilots?

A

The lift force supporting the aircraft’s weight is generated by the
flow of air around the wings.

Engine power is generated by burning fuel and air (oxygen).

Humans need to breathe air (oxygen) in order to live.

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28
Q

What units are used in aviation meteorology for Altitude, Pressure, Temperature, Horizontal Visibility and Horizontal Distance?

A

Altitude - Feet
Pressure - Hectopascals
Temperature - Degrees Celsius
Horizontal Visibility - Kilometres
Horizontal Distance - Nautical Miles

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29
Q

How does a change in pressure affect temperature?

A

An increase pressure will effectively force the molecules together; this creates friction and an increase in temperature.

Conversely a decrease in pressure effectively reduces friction and therefore temperature.

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30
Q

What is the adiabatic process?

A

The change in temperature due to a change in pressure.

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31
Q

What are the causes of vertical movements of air?

A

Thermal
Orographic uplift
Turbulence
Convergence and Divergence
Frontal

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32
Q

What is a thermal?

A

A rising mass of warm air.

As the land heats up it conducts its heat energy to the air in contact with it. The vibrating air molecules begin to push harder on each other, forcing them apart. The pressure remains constant and the result is a change in density. The warmer air is less dense than the surrounding air and therefore rises.

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33
Q

What kind of areas produce good thermals?

A

Good conductors of heat energy such as cities, bitumen road, and darker areas.

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34
Q

What is orographic uplift?

A

Orographic uplift is a result of air flowing over mountains, following the contours and rising vertically.

As the rising air is cooled adiabatically below its dewpoint, any water vapour will condense and clouds will form.

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35
Q

What is turbulence?

A

Disturbed air.

As the wind blows over the earth’s surface, variations in the terrain (natural or man-made) can disrupt the smooth flow and create eddies. The depth of turbulence depends on the nature of the surface and wind strength. It is the vertical component of turbulence that can have an impact on weather.

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36
Q

What are convergence and divergence?

A

Convergence is the horizontal inflow of air and subsequent lifting. The air is forced together and has nowhere to go but up (lifting). As the air rises it is cooled adiabatically. Unable to rise any further the air diverges and the cooler air sinks. As it descends it warms adiabatically (subsidence).

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37
Q

What is a frontal?

A

When two air masses interact the boundary between them is referred to as a front. An example of frontal lifting would be a cooler denser air mass undercutting a warmer less dense air mass. The result is the warmer air mass sliding on top of the cooler air mass. There is a wide area uplift along the boundary and based on temperature and pressure differences, can result in spectacular weather.

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38
Q

What is DALR?

A

Dry Adiabatic Lapse Rate is the rate of temperature decrease with altitude for a parcel of dry or unsaturated air rising under adiabatic conditions.

Unsaturated air will cool adiabatically at 3°C every 1,000 feet as it rises and expands.

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39
Q

What is SALR?

A

Saturated Adiabatic Lapse Rate (SALR).

As air cools, the humidity increases. After reaching Dew Point (100% humidity) further cooling will result in condensation; clouds will form.

If the air mass continues upwards, cooling continues but not at 3°C/1,000 feet above the condensation level, the now-saturated air will continue to cool as it rises, but because latent heat is given off as the water vapour condenses into the lower-energy liquid state, the cooling will not be as great.
If continued, upwards cooling continues at 1.5°C every 1,000 feet

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40
Q

ELR

A

Environmental Lapse Rate is the actual rate of change of temperature as altitude is gained in the surrounding atmosphere (air not moving vertically).

In the real atmosphere, the actual ELR may vary from the ISA value - it might be less than 1°C/ 1,000 feet, or it might be as high as 4°C/ 1,000 feet.

The actual ELR is not uniform and varies from location to location and from day to day.

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41
Q

What does the structure and type of cloud depend on?

A

The stability of the air and its water vapour content.

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42
Q

Explain an unstable atmosphere

A

If an air mass rises and cools at the DALR, and the ELR of the surrounding air is higher i.e. the rate of cooling is more, the air mass will always be warmer than its surroundings and will, therefore continue to rise; this is referred to as an Unstable Atmosphere and results in cumuliform clouds with vertical development.

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43
Q

Explain a stable atmosphere

A

If an air mass rises and cools at the DALR, and the ELR of the surrounding air is less, the surrounding air will be warmer and this tends to resist any vertical motion; this is referred to as a Stable Atmosphere and results in stratiform cloud with little or no vertical development.

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44
Q

What drives weather?

A

The movement of heat by the oceans, combined with differential heating from the land.

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45
Q

Consequences of cloud cover during the day.

A

Reflects solar radiation away from the Earth’s surface, resulting in less heating of the earth and lower temperatures on cloudy days.

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46
Q

Consequences of cloud cover at night.

A

Reflects terrestrial radiation back to earth resulting in less heat escaping and warmer nights. The Earth is always radiating heat energy.

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47
Q

Diurnal temperature variation

A

The Earth heats up during the day and reaches its maximum temperature about 2.00 pm and cools by night, reaching its minimum temperature approximately one hour after sunrise.

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48
Q

Name the tropics and their latitudes.

A

Tropic of Cancer - 23.5 degrees north
Tropic of Capricorn - 23.5 degrees south

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49
Q

What are the three types of atmospheric cell?

A

Hadley cell
Polar cell
Mid-latitude or Ferrel Cell

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50
Q

What is wind?

A

Wind is essentially the horizontal movement of air over the earth’s surface.

Only a very small percentage is vertical flow.

Vertical movement of air can lead to cloud formation and other factors affecting aviation. It is the difference in pressure within the atmosphere as a result of temperature change that causes wind.

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51
Q

What is wind direction measured with?

A

Wind vane

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52
Q

What is wind speed measured with?

A

Anemometer

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53
Q

When, in Aviation Meteorology, is degrees magnetic used?

A

When reported by one of the following:

by the TOWER or CA/GRS;
on an ATIS, WATIR;
by an AWIS in place of an ATIS outside tower hours

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54
Q

What is a CA/GRS?

A

Certified Ground Radio Service

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55
Q

What is an ATIS?

A

Automatic Terminal Information Service

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56
Q

What is a WATIR?

A

Weather and Terminal Information Radar (ATIS at a CA/GRS Airport)

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57
Q

What is an AWIS?

A

Aerodrome Weather Information Service

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58
Q

What is backing?

A

When the direction changes anticlockwise.

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59
Q

What is veering?

A

When the direction changes clockwise

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60
Q

What is a gust?

A

A momentary increase in speed from the average as reported, lasting only a few seconds before dying away. When wind gust values are forecast or reported to exceed 10kt above the average they are displayed as 35025G40 (Wind direction 350° at 25 knots gusting to 40 knots).

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61
Q

What is a lull?

A

A momentary decrease in speed from the average as reported, lasting only a few seconds before returning to the average.

The opposite of a gust

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62
Q

What is a squall?

A

A sudden increase in the mean wind speed by at least 16 knots to at least 22 knots and lasting for at least one minute.

63
Q

What is an isobar?

A

Lines indicating places of equal pressure on a meteorological chart.

Drawn at 4 hectopascal intervals.

64
Q

What is the coriolis force?

A

The Coriolis force is an apparent force and is really the result of the earth’s different rotational speeds at different latitudes. Imagine moving an object from a fast moving area to a slower one, the object maintaining its original speed would appear to be turning and accelerating.

The pressure gradient force between a high and low pressure system starts the air mass moving. The Coriolis force turns the air mass to the left in the southern hemisphere.
The Coriolis force will continue to act on an air mass until it is balanced with the pressure gradient force. This results in wind flowing in a direction parallel with the isobars.

65
Q

Which direction does air flow around low and high pressure systems in the southern hemisphere?

A

Low Pressure - Clockwise
High Pressure - Anticlockwise

66
Q

Which direction does air flow around low and high pressure systems in the northern hemisphere?

A

Low Pressure - Anticlockwise
High Pressure - Clockwise

67
Q

On a meteorological chart, what does the closeness of the isobars indicate?

A

The closer the isobars, the stronger the wind.

68
Q

What is Buys Ballot’s Law?

A

In the Southern Hemisphere, if you stand with your back to the wind, the area of LOW pressure will be on your right.

69
Q

Where is the friction layer considered to be?

A

Between the surface and approximately 2000ft

70
Q

Sea Breeze

A

Results from the different heating rates of land and water. Over land, the temperature during the day rises fairly rapidly as the result of solar radiation. Meanwhile, the adjacent sea absorbs the sun’s rays to a considerable depth so that the air temperature over the sea rises hardly at all.
Air over the land now rises because of the existence of an air pressure gradient between the land and the sea. This causes relatively cool air from over the sea to flow in towards the land to take the place of the warmer air that has risen. Under conditions of no pressure gradient, the sea breeze can begin to operate during mid -morning and continue until almost midnight, but it is normally expected to start soon after noon and operate until about an hour before sunset.

71
Q

Land Breeze

A

By night, the land cools more quickly than the sea, causing the air above it to cool and subside (descend). The air over the sea is warmer and will rise. A small land breeze circulation pattern may be set up, with surface air moving out to sea and upper air moving inl and.
The land breeze reaches maximum strength just after dawn when the land is at its minimum temperature.
Sometimes a land breeze holds a sea fog offshore early in the morning, but as the land warms during the day, the land breeze dies out and a sea breeze develops, bringing the sea fog inland and causing visibility problems at coastal a irports.
Since the mechanism is usually much less powerful, the land breeze seldom penetrates to more than a few miles offshore, is un likely to be more than a few knots in strength and is probably less than 500 ft. deep. If it has an associated circulation, the entire mechanism is less than 1,000ft deep.

72
Q

Katabatic Wind

A

During night-time, the Earth’s surface loses heat energy through terrestrial radiation and cools down, particularly on clear, cloudless nights.
The air in contact with the ground then also cools by conduction. Air that lies over sloping ground may be uniformly cooled so the air near the top of the slope becomes colder than the air at the same level but further away from the surface. The cooler air near the ground can now slide down the slope under the action of gravity, so that it literally drains to the bottom of the slope. This ‘drainage’ of cold air is the Katabatic wind.
In certain areas, katabatic winds can build during the night and by sunrise be blowing down the slopes of large mountains into thevalleys at more than 30kt. Shoalhaven River in NSW has strong Katabatic winds.

73
Q

Anabatic Wind

A

Heating of a mountain slope by day causes the air mass in contact with it to become warmer than air at the same altitude but further from the slope, decreasing its density and causing it to flow up the slope. This local upslope wind is known as an anabatic wind, or a valley wind since it flows up and out of valleys. Uphill flow is opposed by gravity and the warm parcel may move directly aloft, so the anabatic wind is generally a weaker wind than the katabatic wind.
It is most likely to be observed on westward-facing slopes during a summer afternoon and is favoured by hang -glider pilots. Anabatic winds may combine with coastal sea breezes to increase the strength of the overall wind.

74
Q

Fohn Wind

A

If the air rising up a mountain range is moist enough to have a high dewpoint and is cooled to this temperature before reaching the top of the mountain, cloud will form on the windward side.
If any precipitation occurs, moisture will be removed from the airflow, and as it descends on the lee side of the mountain, it will be drier. The dewpoint will be less and so the cloud base is then higher on the lee side of the mountain.
As the dry air beneath the cloud descends, it will warm at the dry adiabatic lapse rate of 3°C/1,000 ft, which is at a greater rate than that of the rising air cooled inside the cloud (a saturated adiabatic lapse rate of 1.5°C/1,000 ft).
The result is a warmer and drier wind on the lee side of the mountains. In Australia, the Föhn wind is evident in several locations wherever a range of mountains or hills lie inland from a coastline and the wind is onshore. The west coast of Tasmania produces a Föhn over central and eastern Tasmania during winter and the Great Dividing Range results in Föhn over central and western Queensland and NSW at almost any time of year.
The wind does not always have to be onshore. In the East Gippsland district of Victoria, a Föhn wind occurs when a northerly wind operates over the Alps to its north, especially during winter. As a result conditions near East Sale are frequently better than those in the vicinity of Melbourne in winter.

75
Q

Temperature Inversion

A

In some layers of the atmosphere the temperature may increase through a small band of height. This is called a temperature inversion.

In an inversion the atmosphere is very stable with minimal vertical movement of air.

An inversion may be created by radiation cooling of the ground. After a calm, clear night the land may have cooled and this may be conducted to a small layer of air close to the ground. The atmosphere may increase in temperature up to around 1000ft or less then cool again with the lapse rate.

When flying through an inversion there may be windshear and turbulence. Visibility may be reduced in the inversion as smog cannot be dispersed into the atmosphere above.

76
Q

Low Level Jetstream

A

The low-level jetstream, commonly referred to as low-level jet, occurs within the friction layer in the leading edge of an anticyclone or ridge of high pressure when it approaches a mountain range that is tending to obstruct its eastward migration.

It is strongest in the early morning when a surface inversion under a clear sky insulates the flow from surface friction. The jet is to some extent produced by funnelling of air by the mountain range.

The jet reaches its peak of strength at about 0600 LMT and can produce strong turbulence near the top of the friction layer and at the inversion due to viscous drag.

77
Q

Diurnal Variation of Wind

A

The change of direction and speed due to the effects of night and day. During the day the land heats up causing the vertical motion of air, the air then mixes with free flowing upper level winds. This results in a stronger gradient wind that backs (turns anticlockwise). During the night thermal vertical motion ceases resulting in the wind veering and dropping in strength.

78
Q

Humidity

A

The actual amount of water vapour in the air.

When a parcel of air is supporting as much water vapour as it can, it is said to be saturated and have a relative humidity (RH) of 100%.

Air supporting less than its full capacity of water vapour is said to be unsaturated, and will have a relative humidity of less than 100%.
In cloud and fog, the relative humidity is 100% and the air is saturated. Over a desert, relative humidity by day might be as low as 10%.

79
Q

What is the maximum water vapour a sample of air can hold?

A

Approx 5%

80
Q

Can warm air or cold air hold more water vapour?

A

Warm air

81
Q

Dewpoint

A

The temperature of a parcel of air when cooled at constant pressure in order that it will become saturated i.e. reach 100% relative humidity.

Dewpoint decreases with height (due to pressure decrease) at about 0.5° per 1000 feet. When water condenses (from a vapour to a liquid) it is has reached its Dewpoint.

82
Q

Dry Bulb Temperature

A

The ambient temperature of the air.

Referred to as air temperature.

Measured by a dry thermometer.

83
Q

Wet Bulb Temperature

A

The temperature of adiabatic saturation.

Measured with a thermometer with a piece of wet cloth attached and measures the rate of evaporation and humidity.

84
Q

Three major forms of cloud

A

Cumuliform
Cirrform
Stratiform

85
Q

Cirriform Clouds

A

Cirriform Clouds are only high level clouds and composed entirely of ice crystals. Due to their height being in the upper troposphere, there is less depth of moisture so they tend to be thin and veil like.

86
Q

Cumuliform Clouds

A

Cumuliform Clouds are heaped or towering in appearance and are formed by unstable air rising and cooling. They may be of considerable vertical extent and tend to be separated from each other.

87
Q

Stratiform Clouds

A

Stratiform Clouds are flat layered clouds formed by the cooling of a layer of stable air. They can form sheets or layers of cloud stretching over a large part of the sky.

88
Q

Common features of high level clouds (23000ft +)

A

Detached clouds of white filaments

Patches or bands

Formed by widespread lifting

Composed entirely of ice crystals

89
Q

Common features of mid level clouds (6,500- 23,000 ft)

A

Greyish, bluish in colour

Total or partial cover of sky

Contains supercooled water droplets

90
Q

Common features of low level clouds (0 – 6,500 ft)

A

Sheet-like stratiform clouds,

Fluffy cumuliform clouds,

A mix of both,

Clouds of considerable vertical extent,

Composed of water, supercooled water and/or ice.

91
Q

Cirrus Cloud (Ci)

A

Detached cloud with white filaments, fibrous appearance, sometimes with a sheen.

92
Q

Cirrostratus Cloud (Cs)

A

A high level cloud with a layered appearance and can be wide spread. Transparent, it can sometime produce halo phenomena.

93
Q

Cirrocumulus Cloud (Cc)

A

White, patchy and layered in appearance without shading. They can appear ribbed or scaly and have little vertical development.

94
Q

Altostratus Cloud (As)

A

Grey/blue sheet of cloud, layered and can be striated. The sun can most often see the sun through the cloud.

95
Q

Altocumulus Cloud (Ac)

A

Can appear white or grey and sometimes with defined white and grey shading showing depth of cloud. Can be ribbed or fibrous and has more depth than CC due to more available moisture.

96
Q

Nimbostratus Cloud (Ns)

A

This thick dark grey cloud produces widespread continuous rain. The cloud can descend into the lower cloud levels, usually many thousands of feet thick.

97
Q

Cumulus Cloud (Cu)

A

Detached cloud with defined outlines. White and puffy in appearance they can resemble domes or towers. Single clouds can form from thermals and are generally convective clouds.

98
Q

Stratus Cloud (St)

A

Generally a thin layered cloud. May be thick enough to produce drizzle. Can be from lifting fog and can dissipate quickly.

99
Q

Cumulonimbus Cloud (Cb)

A

Bright white towering exterior of large vertical extent sometimes reaching the tropopause. Produces lightning, hail, rain and tornados. The most dangerous cloud in aviation.

100
Q

Stratocumulus Cloud

A

As the name suggests this is a combination of clouds, layered in appearance with some potential for vertical development.

101
Q

Lenticular Clouds

A

As the name suggest these have a lens shape and are formed by mountain waves. The wind flows over a mountain like an aerofoil and if the conditions are right, at the crest of the wave saturation of the air is reached. Created by the obstruction they remain stationary. They are an indicator for mountain waves, a hazard to aviation.

102
Q

How are cloud amounts reported?

A

In meteorological practice, the expression used to describe the amount of the sky that is occupied by cloud is the okta.

An okta is the equivalent of one eighth of the sky.

Sky clear is for 0 okta;
Few is for 1 to 2 oktas;
Scattered is for 3 to 4 oktas;
Broken is for 5 to 7 oktas;
Overcast is for 8 oktas

103
Q

Precipitation

A

Any liquid of frozen water that falls from the atmosphere and falls back to earth.

Precipitation can fall either as showers, or as intermittent or continuous rain, snow or drizzle.

The intensity of precipitation, regardless of its type, can be described as light, moderate or heavy.

Rain drops form when cloud particles collide and merge. On average, it requires about one million cloud particles to make up one small rain drop.

104
Q

What are showers characterised by?

A

Showers are characterised by the fact that they start and stop suddenly, and are subject to rapid and sometimes violent changes in intensity. Cloud is quite likely to break up, or even clear, between the showers.

105
Q

What kind of cloud do showers fall from?

A

Cumuliform clouds, with the heaviest rain showers falling from cumulonimbus.

106
Q

What kind of cloud does non-showery precipitation fall from?

A

Usually falls from stratiform cloud, mainly altostratus or nimbostratus.

107
Q

What is an air mass?

A

A body of air with similar characteristics (temperature, humidity) over a large area.

108
Q

What is required for an air mass to form?

A

For an air mass to form it must remain over a surface for some time. The surface must also be reasonably uniform in temperature and humidity.

These regions are referred to as source regions.

Air masses are referred to according to the source region from which they came. Since everywhere on the earth could in fact be a source region, air masses have to be given simple classifications based on whether they originated over land or water and whether they are tropical or polar.

109
Q

Types of air mass

A

According to Latitude:
- Polar
- Tropical
- Equatorial

According to Surface:
- Continental
- Maritime

110
Q

What is a front?

A

The narrow boundary between adjacent air masses is called a Front.

The name given to the front is that of the Advancing air mass, either warm or cold

111
Q

How are warm fronts depicted on a weather chart?

A

A line with semicircles aligned with the direction of travel, indicating a warm air mass coming up behind a cold air mass.

112
Q

How are cold fronts depicted on a weather chart?

A

A line with triangles pointing in the direction of travel, indicating a cold air mass coming up behind a warm air mass.

113
Q

True or False. Warm fronts tend to result in a more dramatic change in weather than cold fronts.

A

False.

The leading edge of a cold front rises at a much steeper angle than a warm front, resulting in a big change in temperature and pressure in a short time and therefore dramatic weather. A warm front tends to be shallow resulting in small changes over a longer time, and therefore less dramatic weather.

114
Q

Warm Fronts

A

As a warm front approaches, an observer on the ground may first see high cirrus clouds, which are followed by a lowering base of cirrostratus, altostratus and nimbostratus. Rain may be falling from the altostratus, possibly evaporating before it reaches the ground (virga), and from the nimbostratus. The rain from the nimbostratus may be continuous until the warm front passes and may, due to its evaporation, cause fog. Visibility may also be quite poor.

The atmospheric pressure decreases continuously as the warm front approaches and, as it passes, it either stops falling or falls at a reduced rate. The air temperature rises as the warm air moves in over the surface. The warm air can hold more moisture than the cold air, so the dewpoint decreases. As the front passes, the surface wind backs. Once the front has passed, there is likely to be some stratus, which will clear, and visibility may still be poor.

115
Q

Cold Fronts

A

The air that is forced to rise with the passage of a cold front is often unstable and so the clouds that form are more likely to be cumuliform; cumulus and cumulonimbus. Severe weather hazardous to aviation, such as thunderstorm activity, line squalls, severe turbulence and windshear, may accompany the passage of a fast-moving cold front.

The majority of cold fronts in Australia tend to be fast moving and have a relatively steep slope producing the severe effects just described. However, slow moving cold fronts can occur and these produce less severe weather conditions.

The slope of a slow-moving front is shallow and similar to that of a warm front, and the cloud formation is similar but in a reverse order; that is, the low cloud will appear first, followed by the high stratiform cloud as the front moves away.

116
Q

Quasi-stationary front

A

A front that is not moving or is tending to move along its own length is called a quasi-stationary or stationary front. Such fronts align themselves with the isobars and tend to lie roughly east-west.

Quasi-stationary fronts usually have an area of high pressure on both sides of the front. There is not likely to be any significant weather associated with the front. The front is depicted on a weather chart as a solid line with barbs and semicircles.

117
Q

Occluded Front

A

Because cold fronts usually travel faster than warm fronts, it often happens that a cold front overtakes a warm front, creating an occlusion (or occluded front). The reverse is a warm occlusion - these are not seen in the Australian region.

This may happen in the final stages of a frontal depression. Three air masses are involved and their vertical passage, one to the other, will depend upon their relative temperatures. The occluded front is depicted by a line with alternating barbs and semicircles pointing in the direction of motion of the front.

The clouds that are associated with an occluded front will depend upon what clouds are associated with the individual cold and warm fronts. It is not unusual to have cumuliform clouds (Cu, Cb) from the cold front as well as stratiform clouds associated with the warm front.

Sometimes the stratiform clouds can conceal thunderstorm activity. Severe weather can occur in the early stages of an occlusion as unstable air is forced upwards.

118
Q

What would you expect to see during the approach of a warm front?

A
  • Cloud developing
  • Lowering of cloud-base
  • Thickening cloud
  • Pressure falling
  • Rain
119
Q

What would you expect to see during the approach of a cold front?

A
  • Cirrus cloud
  • Wind backs and strengthens
  • Pressure falling
  • Cumulus and/or Cumulonimbus forming
  • Increasing showers
120
Q

What would you expect to see during the passage of a warm front?

A
  • Wind backs
  • Rain eases and then stops
  • Cloud clears
  • Temperature rises
  • Relative humidity decreases
  • Pressure stabilises
121
Q

What would you expect to see during the passage of a cold front?

A
  • Wind backs
  • Temperature falls
  • Humidity rises
  • Pressure starts to rise
  • Possible thunderstorms
  • Low cloud-base
122
Q

What would you experience behind a cold front?

A
  • Weather clears rapidly
  • Temperature cools
  • Light showers may occur over oceans
  • Generally stable atmosphere
123
Q

What is a trough?

A

Can be thought of as a valley of low pressure extending from the centre of a low and is represented as a dashed line.

124
Q

What is a ridge?

A

Can be considered like a mountain of high pressure.

125
Q

What is a col?

A

A region of little or no pressure gradient and is usually a large area of equal pressure between two highs and two lows.
Cols are occasionally seen in Australia during the summer and results in light variable winds and high temperatures.

126
Q

What are thunderstorms?

A

More aggressive cumulonimbus clouds with the main difference being thunderstorms produce lightning - cumulonimbus does not.

Generated only from cumulonimbus clouds, and usually result in spectacular weather including heavy rain, hail, squalls, microbursts and tornadoes.

127
Q

Types of thunderstorm?

A

Supercell
Multicell
Single Cell
Squall Line

128
Q

What kinds of weather do thunderstorms exhibit?

A
  • Spectacular weather
  • Heavy showers
  • Hail
  • Squalls
  • Downbursts
  • Windshear
  • Lightning
  • Tornadoes
129
Q

What are the conditions required for a thunderstorm to form?

A

High humidity.

A trigger action to start the air rising. e.g. a cold front, orographic uplift, thermals, convergence

Deep atmospheric instability. A layer above the cloud base (approx 20000ft deep) in which the ELR is greater than the SALR.

130
Q

What are the three stages of a thunderstorm?

A

Cumulus/Developing Stage - continuous updrafts, a duration of 10-20 minutes and no rain.

Mature - commencement of downdrafts, rain starts to fall, hail GR likely

Dissipating - downdrafts only, heavy precipitation, cloud disintegration with the possibility of AS, NS

131
Q

Hail

A

Hailstones form in the upper portions of a thunderstorm when super-cooled water droplets freeze onto ice crystals, which then grow in size as they are held aloft by updraughts within the storm. While most hail does not reach the ground (it melts during de scent and falls as rain), almost all cumulonimbus contain hail.

Hail falls when it becomes too large to be supported by the storm’s updraught or when it moves outside the storm’s updraught area. Hailstones can grow to the size of an orange in a storm with strong updraughts. Small hail could be experienced below or clear of the thunderstorm as it could be thrown out of the top of cumulonimbus cloud and carried along by strong winds, sometimes several kilometers.

132
Q

Thunderstorm Descriptors

A

ISOL - Isolated means they are individual,
OCNL - Occasional are well separated,
FRQ - Frequent have little or no separation or
EMBD - Embedded are in layers of other clouds or concealed by haze.

133
Q

Downburst

A

A downburst is a sudden downward rush of air accompanying the onset of a shower of rain, hail or snow from a mature cumulonimbus cloud.

Macroburst - widespread
Microburst - smaller and less widespread

Aircraft do not have the ability to combat turbulence and windshear in downbursts and microbursts close to the ground, and can be severely affected in critical phases of flight like take-off and landing.

134
Q

What phenomena allow for the identification of microbursts?

A
  • virga;
  • lowering cloud base;
  • bulbous cloud base;
  • dust rings and waves on the surface below the cloud; or
  • a dark shaft of rain falling from the cloud.
135
Q

Adverse Effects of Icing

A
  • Loss of lift
  • Increase drag
  • Increase in weight
  • Interruption of controls
  • Blocked pitot system
  • Interruption of other sensors
  • Engine surging and failure
  • Degradation of Communications and navigation equipment
136
Q

Four Types of Icing

A

Clear - the most dangerous form of airframe icing. It forms when an aircraft fly’s through freezing rain. Large supercooled water droplets spread out and freeze on contact with an airframe that is 0°C or below.

Rime - the result of tiny supercooled water droplets freezing on the aircraft surface. Because the droplet size is very small the ice takes a long time to build-up and has an opaque appearance due to air being trapped in the ice. It is generally rough and fairly brittle, it can interrupt laminar flow of the wing and air flow into engines. The temperature range for the formation of rime ice can be between 0°C and -40°C, however, it is more likely to occur in the range of -10°C to - 20°C.

Mixed - The difference between clear ice and rime ice is droplet size. Different cloud types contain different droplets sizes. Mixed ice is a combination of large and tiny supercooled water droplets and is the most common airframe icing. Pure Rime Ice would be experienced in medium level clouds with less moisture such as Altostratus and Altocumulus. Clear Ice would be experience in freezing rain below Nimbostratus or in Cumulonimbus.

Frost - Hoar frost occurs when moist air comes in contact with a freezing surface. The water vapour freezes instantly without transitioning through a liquid change of state. Its appearance is white crystalline and reasonably easy to remove. Typical conditions for frost are clear nights with below freezing temperatures, calm winds and high humidity.

137
Q

Engine Ice

A

Due to the venturi effect within a carburettor, there is a decrease in pressure and temperature. This can result in the throttle body’s temperature decreasing to below freezing and any moisture in the air or fuel will freeze. This can choke the throttle and cause rough running and engine failure.

138
Q

What is the threshold at which, and below, horizontal visibility is reported in metres?

A

5000m

139
Q

At what threshold is horizontal visibility simply reported as ‘greater than ____’?

A

10km

140
Q

How is vertical visibility reported?

A

Reports of vertical visibility are reported in hundreds of feet with the
abbreviation VV e.g. VV020 means vertical visibility is 2000 feet

Vertical Visibility is only given from a staffed observation when the sky is obscured.

141
Q

What is the prevailing visibility?

A

Visibility can vary in different directions so it is the maximum visibility over more than half the horizon that is reported.

142
Q

What are some examples of hydrometeors?

A

Fog - The result of very small suspended water droplets reducing visibility below 1000 metres, and is essentially a cloud on the ground. Fog is caused by cooling of the air below its dew point temperature.

Mist - Mist is identical to fog, however, the visibility does not reduce below 1000 metres and the humidity is also slightly lower at around 95%

Spray - the addition of salt particles in the air and can significantly reduce visibility. It is common in coastal areas and can travel several miles inland. Rough seas, strong winds and an onshore breeze all contribute to its severity on visibility and impact of aviation.

Haze (caused by moisture) - reduced visibility below 5000 metres due to dust particles in the air.

143
Q

What are some examples of lithometeors?

A

Sand

Dust - When visibility is reduced to below 1000 metres it is classified as a dust storm or sandstorm. Dust storms can be seen preceding a strong frontal system during times of drought and requires an unstable atmosphere. They can vary in size from a couple of kilometres to many thousands of kilometres.

Smoke Haze - Smoke from bushfires can reduce visibility to below 1000 metres over a very large area and can suddenly appear with changes in the wind direction.

Pollen

Salt

144
Q

What is mechanical turbulence?

A

The result of obstructions in the boundary layer and its intensity is based on:
- Wind strength
- The obstructions
- Air stability

145
Q

What is Inversion Turbulence?

A

Can exist in areas of inversion due to abrupt temperature changes through a reasonably shallow area. The strength of the turbulence is dependant on the depth of inversion and the rate of change of temperature and wind throughout; aircraft performance can also be affect due to temperature changes.

146
Q

What is Frontal Turbulence?

A

The passage of a front can create some of the worst turbulence regardless of the turbulence associated with thunderstorms. A horizontal windshear change of up to 180 degrees with gale force winds are not uncommon. The severity is based on:
- Width of the front
- Speed of the front
- Amount of instability
- Any thunderstorms
- Air mass temperature differential

147
Q

Clear Air Turbulence

A

Is associated with Jetstream’s and occurs above 15,000 feet. CAT is not associated with cumuliform clouds but is the windshear boundary between upper level winds and the fast narrow currents of the Jetstream. CAT has caused many aircraft incidents resulting in injury to passengers and structural damage to aircraft. CAT is forecast on an upper level SIGWX chart.

148
Q

Mountain Waves

A

Mountain Waves are generally the smooth flow of wind over a mountain range, creating a wave formation which can extend many thousands of feet above the ground and for many miles downwind. A stable atmosphere is required along with a wind blowing perpendicular to a mountain range. The effect can be similar to the airflow over an aerofoil with an increase of speed over the top and a down draft on the leeside; lenticular clouds may be present. Rotors can be present on the leeside and must be avoided.

Conditions required are:
- A mountain range usually 1000ft or greater above the ground
- A steady wind stream through upper levels
- A wind stream blowing at right angles to the range
- A wind strength 25kt of greater
- And inversion or stable upper levels of the atmosphere

149
Q

How Are Cold Fronts Hazardous?

A

Flying through a cold front may require diversions to avoid weather. There may be thunderstorm activity, violent winds (both horizontal and vertical) from cumulonimbus clouds, line squalls, windshear, heavy showers of rain or hail, and severe turbulence.

Icing could also be a problem especially in winter. Visibility within heavy precipitation may be severely reduced. Away from the showers and clouds, visibility may be quite good, but it is still worth considering avoiding the weather activity that accompanies many cold fronts. A line squall may form ahead of the front, often accompanied by severe turbulence.

150
Q

What is windshear?

A

Windshear is a change of wind direction and or speed within a volume of air. It can be vertical or horizontal. Updrafts and downdrafts are moving vertical columns of air and when close to the ground present a real danger to aviation. With the passage of a front the wind can back 180 degrees creating a horizontal shear and is sometimes defined as undershoot or overshoot shear. The examples of turbulence mentioned earlier all involve windshear.

151
Q

What instrument is used to measure air pressure?

A

Barometer

152
Q

What is the name of the instrument shelter used to protect instruments from the rain and direct sunlight?

A

Stevenson Screen

153
Q

What instrument is designed to indicate wind direction and relative wind speed?

A

Wind sock

154
Q

What is the name of a helium filled balloon launched into the atmosphere to collect atmospheric data?

A

Weather balloon

BAK (6 decks)

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