Meteorology Theory

Question 1

Notes

Answer 1

• Vertical gusts increase angle of attack and will increase the load factor (g-forces) and/or stall the wing

Question 2

Explain what the four subdivisions of the atmosphere are:

Answer 2

Most significant aviation weather takes place in the troposphere - the layer of the atmosphere that ranges from the Earth’s surface to the stratosphere. The boundary between the troposphere is called the tropopause and its altitude varies with latitude, and in some cases, weather system.

• Troposphere - temperature falls with altitude gained, marked vertical air movements and mixing, warm air rising, cold air descending, contains most atmospheric water leading to cloud formation and weather systems.
• Stratosphere - constant temperature approx -57°C, limited vertical air movement and mixing, very limited water vapour and generally clear of cloud.
• Mesosphere
• Thermosphere

Note - The atmosphere extends further above the equator than above the poles.

Question 3

What gases does the air consist of?

Answer 3

78% Nitrogen
21% Oxygen
1% Other gases (argon, carbon dioxide, neon, helium, etc.)

Question 4

What is atmospheric pressure?

Answer 4

The molecules that make up the air move at high speed in random directions and bounce off any surface that they encounter. The force which they exert on a unit area of that surface is called the atmospheric pressure. Atmospheric pressure decrease with height because there are fewer air molecules at higher altitudes and the lower weight of molecules pressing down from above.

Question 5

What are isobars?

Answer 5

Places that are experiencing the same calculated sea level pressures are then joined with lines on maps known as isobars which may be depicted on meteorological charts.

Question 6

What is the pressure gradient?

Answer 6

The variation of pressure with horizontal distance is called the pressure gradient, which is at right-angles to the isobars. If the isobars are very close together rapid changes in pressure occur and the pressure gradient is said to be ‘steep’ or ‘strong; if they are widely spaced, the pressure changes are more gradual and so the pressure gradient is ‘flat’ or ‘weak’. There is a natural tendency for air to flow from areas of high pressure to areas of low pressure. This increases with the steepness of the pressure gradient.

Question 7

What is the pressure gradient force?

Answer 7

In the atmosphere, the force that is usually responsible for starting a parcel of air moving is the pressure gradient force. This acts to move air from areas of high pressure to areas of low pressure.

The stronger the pressure gradient, the greater this force will be and, consequently the strong the wind will blow.

Question 8

What is air density?

Answer 8

Density is the mass per unit volume.

Question 9

What is wind?

Answer 9

Wind refers to the horizontal flow of air across the Earth. Vertical airflow only accounts for 0.1% of wind but it results in cloud formation and is associated with severe hazards such as thunderstorms or wind shear.

• Wind Direction - The direction from which the wind is blowing and is expressed in degrees measured clockwise from the north
• Wind Velocity - Direction and strength together, usually written in the form 270/30, or 270/35kt.

Runways are described in terms of their magnetic direction, which would align to the direction indicated by a magnetic compass of an aeroplane lined up on that runway.

A steady wind velocity is known as balanced flow (also known as the geostrophic wind)

Question 10

What is veering and backing?

Answer 10

A wind whose direction is changing in a clockwise direction is called a veering wind. For example, following a chance from 150/25 to 220/30, the wind is said to have veered.

A wind whose direction is changing in an anticlockwise direction is called a backing wind. A change from 100/15 to 030/12 is an example of a wind that has backed.

Question 11

What is water vapour?

Question 12

What are the five types of heat energy transfer?

Answer 12

• Radiation - All bodies transmit energy in the form of electromagnetic radiation: the higher the temperature of the body, the shorter the wavelength of the radiation. Radiation from the sun is therefore of shorter wavelength than the re-radiation from the earth, which is much cooler.
• Absorption - Any body in the path of radiation will absorb some of its energy. How much it absorbs depends upon both the nature of the body and the radiation. A densely-forested area will absorb more solar radiation than snow-covered mountains.
• Conduction - Conduction is the process of heat transfer within a body or between bodies in contact with one another. Iron is a good conductor of heat - wood is a poor conductor, as is air. A parcel of air in contact with the earth’s surface is heated by conduction, but will not transfer this heat energy to neighbouring parcels of air instead it expands adiabatically.
• Convection - A body in motion carries its heat energy with it. A mass of air that is heated at the earth’s surface will expand, becoming less dense, and rise. As it rises, it will carry its heat energy higher into the atmosphere - which is the process called convection.
• Advection - When air moves in to replace the air that has risen by convection, this horizontal motion of air is known as advection. The air mass, moving horizontally by advection, will of course bring its heat energy with it

Question 13

What is the specific heat capacity?

Answer 13

Because of the molecular structure of any given material, a certain amount of energy is required to increase its temperature. The specific heat capacity is a measure of the amount of energy required to raise the temperature of one kilogram of a material by one degree Celsius. Water has a higher heat capacity than most land surfaces and requires more heat energy to raise its temperature. Land will therefore heat and cool more quickly than water bodies.

Question 14

How do materials differ in reflectivity and emissivity of a surface?

Answer 14

Different materials interact differently with incoming solar radiation. Surfaces with high reflectivity absorb less solar radiation and therefore heat up less quickly, examples: include snow and water.

Any particular surface will have a temperature and so also emit radiation as well as absorb it. The amount of radiation emitted is proportional to surface emissivity. Land surfaces typically have a lower emissivity than water.

Question 15

What is the Environmental Lapse Rate (ELR)?

Answer 15

The net result of heat transfer in the atmosphere is described by the ISA in which temperature decreases at 2C for each 1,000ft climbed. However, this not the real atmosphere which contains water vapour and is influenced by many different heating processes - it cools at what is known as the Environmental Lapse Rate (ELR) that is specific to weather conditions on any particular day.

Question 16

What is the Dry Adiabatic Lapse Rate (DALR)?

Answer 16

A discrete ‘parcel’ of air behaves adiabatically. If a dry, unsaturated “parcel” of air rises it will cool at 3C for each 1,000ft climbed - the Dry Adiabatic Lapse Rate (DALR).

Question 17

What is the Saturated Adiabatic Lapse Rate (SALR)?

Answer 17

Any water vapour in this parcel of air that cools to its dewpoint will condense and release latent heat energy. The parcel of air will then cool at a lower rate - the **Saturated Adiabatic Lapse Rate (SALR). Its value varies considerably, but at low levels, it is approximately 1.5C per 1,000ft climbed.

Question 18

What is general circulation?

Answer 18

The greater heating of the earth’s surface in the tropics causes the air above it to become relatively warm, expand, become less dense and rise. At the top of the troposphere this rising air cannot continue vertically and so moves outwards towards higher latitudes. As a result, new air moves in across the earth’s surface to replace the air which has risen.

In contrast, the cooler air over the polar regions sinks down, creating a large-scale vertical circulation pattern in the troposphere. This process is known as the general circulation pattern and consists of three main ‘cells’:
- Polar cell
- Mid-latitude Cell/Ferrel Cell
- Tropical Cell/Hadley Cell

The hot and less dense air rising over the tropics creates a band of low pressure at the earth’s surface known as the equatorial trough, into which other surface air will move (known as convergence). The cool and dense air subsiding in the polar regions crates a high-pressure area at the earth’s surface in the very high latitudes and the surface air will spread outwards (known as divergence).

Question 19

Explain what the Coriolis force is:

Answer 19

The Coriolis force acts on a moving parcel of air. It is not a ‘real’ force, but an apparent force resulting from the passage of the air over the rotating earth. If the parcel of air were being accelerated in a southerly direction from high-pressure area in the northern hemisphere towards a low near the equator, the earth’s rotation towards the east would ‘get away from it’ and so the airflow would appear to turn right.

The faster the airflow, the greater the Coriolis effect - if there is no air flow, then there is no Coriolis effect. The Coriolis effect is also greater in regions away from the equator and towards the poles, where changes in latitude cuase more significant changes in the speed at which each point is moving towards the east.

In the northern hemisphere, the Coriolis force deflects the wind to the right; in the southern hemisphere, the situation is reversed and it deflects the wind to the left.

Question 20

Explain what the geostrophic wind is:

Answer 20

The two forces acting on a moving airstream are:
- Pressure gradient force
- Coriolis force

The pressure gradient forces gets the air moving and the Coriolis effect turns it to the right. This curving of the airflow over the earth’s surface will continue until the pressure gradient force is balanced by the Coriolis force, resulting in a wind flow that is steady and blowing in a direction parallel to the isobars. This balanced flow is called the geostrophic wind.

Question 21

What is Buys Ballot’s Law?

Answer 21

“If you stand with your back to the wind in the northern hemisphere, the low pressure will be on your left”

Question 22

Explain surface wind:

Answer 22

Surface wind is measured at 10 meters (30ft) above level and open ground, i.e. where windsocks and other wind indicators are generally placed. Wind is usually less strong near the surface than at higher levels because of the friction exerted by the Earth’s surface - the rougher the surface, the greater the slowing down. Friction forces will be least over oceans and flat desert areas, and greatest over hilly or city areas with many obstructions.

Frictional forces caused by the earth’s surface decrease rapidly with height throughout what is known as the atmospheric boundary layer - a larger version of that which forms over the surface of a wing. The top of the boundary layer is defined as the height at which frictional and thermodynamic effects of the earth on the atmosphere become negligible. The height of the top varies with stability of the air and roughness of the surface. However, a good rule of thumb is that frictional effects generally become negligible at between 2000 and 3000 feet above ground level.

Question 23

Explain what turbulence is:

Answer 23

Turbulence is the unsteady, abrupt movement of the air, characterised by chaotic changes in wind speed and direction.

Question 24

The main four groups of clouds are:

Answer 24

• Cirriform
• Cumuliform
• Stratiform
• Nimbus

Clouds are further divided according to the level of thier bases above mean sea level, resulting in ten basic types.

Question 25

What are the high-level clouds?

Answer 25

High level clouds have a base above 20,000ft and looks quite fine and spidery because it is usally formed in the coldest region of the traoposphere and composed of ice crystals rather than water particles.

1. Cirrus (Ci).
2. Cirrostartus (Cc).
3. Cirrostartus (Cs).

Question 26

What are the middle-level clounds?

Answer 26

Middle-level cloud has a base above about 6,500ft but below 20,000ft amsl.

4. Altocumulus (Ac).
5. Altostratus (As).

Question 27

What are the low-level clouds?

Answer 27

Low cloud has a base below about 6,500ft amsl.

6. Nimbostratus (Ns).
7. Stratocumulus (Sc).
8. Stratus (St).
9. Cumulus (Cu).
10. Cumulonimbus (Cb).

Question 28

The three states of water are:

Answer 28

Water in its vapour state is not visible, but when the water vapour condenses to form water droplets we see it as cloud, fog, mist, rain or dew. Frozen water is also visible as cloud (high-level), snow, hail, ice or frost. Water, therefore, exists in three states - gas (vapour), liquid (water) and solid (ice).

Under certain conditions, water can change from one state to another, absorbing heat energy if it moves to a higher-energy state (from ice to water to vapour) and giving off heat energy if it moves to a lower-energy state (vapour to water to ice). This heat energy is known as latent heat and is a vital part of any change of state.

Question 29

What is humidity?

Answer 29

The amount of water vapour present in the air is called humidity, but the actual amount is not as important as whether the air can support that water vapour or not.

Question 30

What is relative humidity?

Answer 30

When a parcel of air is supporting as much water vapour as it can, it is said to be saturated and has a relative humidity of 100%. If it is supporting less than its full capacity of water vapour. it is said to be unsaturated and its relative humidity will be less than 100%. In cloud and fog, it is 100%, whereas over a desert it may be 20%.

How much water a particular parcel can support is dependent on the air temperature - warm air being able to support more than cool air. Saturated vapour pressure is proportional to temperature. If the temperature falls, the amount of water vapour that the air can support decreases and so the relative humidity will increase. In other words, even though no moisture has been added, the relative humidity of a parcel of air will increase as its temperature drops.

Question 31

What is Humidity Mixing Ratio (HMR)?

Answer 31

This is the ratio of the mass of water vapour in a parcel of air to the mass of dry air in the same parcel. The relative humidity of a rising unsaturated parcel of air hold as much water vapour. However, the humidity mixing ratio remains constant during ascent because the actual amount of moisture in the parcel remains constant.

Question 32

What is dewpoint temperature?

Answer 32

Dewpoint is the temperature at which a parcel of air becomes saturated if it cools, i.e. the temperature at which it no longer able to support all of the water vapour that it contains; the more moisture in the air, the higher its dewpoint temperature.

A parcel of air that has a temperature higher than its dewpoint will be unsaturated, i.e. its relative humidity is less than 100%. The closer the actual temperature is to the dewpoint, the closer the air is to being saturated.

At its dewpoint, the air will be fully saturated and if it becomes cooler than its dewpoint, then the excess water vapour will condense as visible water droplets (or, if it is cool enough, as ice). This process can be seen both when moist air cools at night to form fog, dew or frost, and as air rises and cools, the water vapour condenses into the small water droplets that form clouds. If the air is unable to support these water droplets (for example if they become too large and heavy), then they fall as precipitation (rain, hail or snow).

Question 33

How is mist and fog formed?

Answer 33

Mist and fog occur when small water droplets are suspended in the air and the relative humidity is above 95%. In both cases the visibility is reduced; mist 1km or more, fog below 1km.

The condensation process that causes mist/fog is usually associated with cooling of the air by an underlying cold surface or by the interaction of two air masses (frontal fog). It is usual for mist to precede fog and to follow fog as it disperses, unless an already formed fog is blown into an area (advected) - for example from the sea.

Question 34

What is a radiation fog?

Answer 34

Conditions suitable for the formation of radiation fog are:
* Cloudless Night - Allowing the earth to lose heat by radiation to the atmosphere and thereby cool, also causing the air in contact with it to lose heat.
* Moist Air - Only requires little cooling to reach the dewpoint temperature.
* Light Winds - (2-8 knots) To mix the lower levels of air with very light turbulence, thereby thickening the mist/fog layer.
These conditions are commonly found with anticyclone (high-pressure system).

Question 35

What is an advection fog?

Answer 35

A warm, moist air mass blowing across (advecting) a significantly colder surface will be cooled from below and advection fog will form if its temperature is reduced to the dewpoint temperature.

The onset of advection fog can be quite sudden and be in the day or night, for example, a moist maritime airflow over a cold land surface can lead to advection fog over land during the afternoon. Furthermore, advection fog can persist in much stronger winds than radiation fog.

Question 36

What is an orographic fog?

Answer 36

As moist air is forced to rise over a hill, it may condense and form hill fog or stratus. The Channel Islands are a classic case where this happens.

Question 37

What is steaming fog?

Answer 37

Steaming fog forms when very cold air (0° Celsius or below) moves over warmer water. It occurs when a light wind of very cold air mixes with a shallow layer of saturated warmer air immediately above the warmer water. There must be a marked temperature inversion in the air before it moves over the water.

The warmer air is cooled beyond the dew point so water condenses out. The effect is similar to the ‘steam’ produced over a hot bath. It is usually only a thin layer because the fog is confined to the layer of warm air above the sea. However, sea smoke columns can be higher. Because this type of fog requires very low air temperatures, it is uncommon in temperature climates, but this is common closer towards the poles.

Question 38

What does the term ‘air mass’ mean?

Answer 38

The term ‘air mass’ is used to describe a large region of the atmosphere (continental scale) that has similar thermal and humidity properties throughout.

It is used to classify an air mass according to:
* Its origin - Maritime air flowing over an ocean will absorb moisture and tend to become saturated in its lower levels. Continental air flowing over a land mass will remain reasonably dry since little water is available for evaporation.
* Its path over the earth’s surface - Polar air flowing towards the lower latitudes will be warmed from below and so become unstable. Continental air flowing to higher latitudes will be cooled from below and so become more stable.
* Whether the air is diverging or converging - An air mass influenced by the divergence of air flowing out of a high-pressure system at the earth’s surface will slowly sink (known as subsidence) and become warmer, drier and more stable. An air mass influenced by convergence as air flows into a low-pressure system at the surface will be forced to rise slowly, becoming cooler, moister and less stable.

Question 39

What are the three necessary conditions for a thunderstorm to form?

Answer 39

Three conditions are necessary for a thunderstorm to develop:
- Deep Instability in the atmosphere, so that once the air starts to rise it will continue rising.
- High Moisture content
- Trigger Action to start the air rising, from:
- A front forcing the air aloft
- A mountain forcing the air aloft
- Strong heating of the air in contact with the earth’s surface
- Heating of the lower layers of a polar air mass as it moves to lower latitudes
- Convergence of airflow, eg. a pressure trough or a sea breeze, which forces air to rise.

Question 40

What is a microburst?

Answer 40

A microburst is a localised column of sinking air (downdraft) within a thunderstorm and is usually 2 nautical miles in diameter and lasts between 5 and 15 minutes. There are two primary types of microbursts:
* Wet Microbursts - Servere windshear accompanied by significant precipitation.
* Dry Microbursts - Severe windshear with no visible signs.

**Microbursts produce severe windshear and are extremely hazardous.

Sometimes an updraught is so strong that it suspends large numbers of these droplets and hailstones in the upper portions of the thunderstorm. When the updraught weakens it is no longer capable of holding the large core of rain/hail up in the thunderstorm which descends very rapidly towards the ground from where it spreads out in all directions. The location in which the microburst first hits the ground experience the greatest divergence and strongest windshear.

Question 41

What are supercooled water drops?

Answer 41

Liquid water drops can exist in the atmosphere at temperatures well below the normal freezing point of water, possibly at -20C or even lower. This is known as being supercooled, and such drops will freeze on contact with a surface - the skin of an aeroplane, or the propeller blades, for example.

Question 42

What is rime ice?

Answer 42

Rime ice occurs when small, supercooled liquid water droplets freeze on contact with a surface whose temperature is sub-zero. Because the drops are small, the amount of water remaining after the initial freezing is insufficient to coalesce into a continuous sheet before freezing. The result is a mixture of tiny ice particles and trapped air, giving a rough, opaque, crystalline deposit that is fairly brittle.

Rim ice often forms on leading edges and can affect the aerodynamic qualities of an aerofoil or the airflow into the engine intake. It does not usually cause a significant increase in weight.

Question 43

What is clear ice?

Answer 43

Once the freezing process actually begins, large water drop with a temperature between 0C and -20C will not freeze instantaneously. The freezing process could be triggered by the drop striking a cold aircraft surface, where it will start to freeze. Because latent heat is released in this process, the rate of freezing will decrease, and the reaming liquid water will spread back and coalesce with water from other partially frozen water drops. before freezing on the cold airframe or propeller surfaces. The result is a sheet of solid, clear, glazed ice with very little air enclosed.

The surface of clear ice is smooth, usually with undulations and lumps. Clear ice can alter the aerodynamic shape of aerofoils quite dramatically and reduce or destroy their effectiveness. Together with the increased weight, this creates a hazard to safety. Clear ice is very tenacious but, if it does break off, it could be in large chunks capable of doing damage.

Question 44

What is hoar frost?

Answer 44

Hoar frost occurs when moist air comes in contact with a sub-zero surface, the water vapour, rather than condensing to form liquid water, sublimates directly to ice in the form of hoar frost. This is a white crystalline costing that can usually be brushed off.

Hoar frost will form in clear air when the aeroplane is parked in sub-zero temperatures or when the aeroplane flies from sub-zero temperatures into warmed moist air - for example, on descent, or when climbing in an inversion. Although hor frost is not as dangerous as clear ice. it can obscure vision through a cockpit window, and possibly affect the lifting characteristics of the wings.

Question 45

Cloudy or Mixed ice

Answer 45

It is common for the drops of water in falling rain to be of many sizes and often, if ice forms, it will be a mixture of clear ice (large drops) and rime ice (small drops), resulting in cloudy or mixed ice.

Question 46

When can you expect ice to form?

Answer 46

• Cumulus-Type Cloud - Nearly always consists predominantly of liquid water droplets down to about -20C, below either liquid drops or ice crystals may predominate. Newly formed parts of the cloud will tend to contain more liquid drops than mature parts. The risk of airframe icing is severe in cumuliform clouds in the range 0C to -20C, moderate to severe in the range -20C to -40C, with the chance of airframe icing below -40C is only small. Since there is a lot of vertical motion in convective clouds, the composition of clouds may vary considerably at one level, and the risk of icing may exist throughout a wide altitude band in (and under) the cloud. If significant icing does occur, it may be necessary to descend into warmer air.
• Stratiform cloud - Usually consists entirely or predominantly of liquid water drops down to about -15C, with the risk of airframe icing.
• Raindrops and drizzle - From any sort of cloud will freeze if they meet an aeroplane whose surface is below 0C, with a severe risk of clear ice forming, the bigger the water droplets are. You need to be cautious when flying in rain at freezing temperatures.
• Cirrus Clouds - Usually composed of ice crystals, and the risk of airframe icing is therefore only slight. However, turbine engine icing may be experienced, particularly in dense cirrus clouds associated with cumulonimbus.