Met Exam Flashcards
What is the average pressure at mean sea level??
1013.25hPa
Describe the composition of the atmosphere.
Nitrogen 78.09%
Oxygen 20.95%
Argon 00.93%
Carbon Dioxide 00.03%
What are the layers of the atmosphere from highest to lowest?
Thermosphere (Ionosphere is here)
Mesosphere
Stratosphere
Troposphere
What are the depths of the troposphere?
52,000ft/16km at the equator
36,000ft/11km at 50degrees north
30,000ft/9km at the poles
Describe the conditions of the stratosphere
Extends from the troposphere to 50km with a constant temp of around 0 degrees Celsius
What is the difference between official and unofficial met observations?
Official is provided by an appropriately trained person at a unit with the necessary equipment. Otherwise unofficial.
What are the conditions of the ISA?
International standard atmosphere
.pressure of 1013.25hPa
.air density 1225gm/m3
.temperature +15°C
. 1.98°C per 1000ft lapse rate up to 36,000ft, remains at -56.5°C thereafter up to 65,000ft
Where is most of the water, weather and flight in the atmosphere?
99% in the troposphere
Most flight happens here
What is the upper limit of the troposphere called?
The tropopause
How many feet of height does a hPa equal roughly?
30ft
What is insolation?
The incoming radiation from the sun warming the earth, as the air does not absorb the energy.
What is specific heat capacity?
The energy required to heat 1kg of a substance by 1°C
What factors affect diurnal variation?
Surface- variation is greater over land and sea.
Wind speed- wind causes turbulence that spreads the cooling effect through a greater depth of air.
Cloud- reduces heat loss.
How do inversions and isothermal layers affect the lapse rate?
Inversions cause the adiabatic effect to reverse and the air actually heats up as it rises. Isothermal layers are where the temperature remains the same in that layer and doesn’t lapse.
What four ways may a parcel of air be cooled past its dew point?
Conduction
Evaporation
Mixing
Adiabatic cooling.
Define adiabatic
The change in temperature of a parcel of air as its density changes, normally cooling as the air expands when rising.
What are the adiabatic lapse rates for dry and saturated air?
SALR= 1.5 degrees/ 1000ft DALR= 3.0 degrees/1000ft
What is the ELR?
The environmental adiabatic lapse rate that exists within the atmosphere at a particular time. (ISA global average is 1.98 degrees per 1000ft)
Define stable and unstable air.
Stable air is air that doesn’t rise and unstable rises.
Describe the Celsius Scale
Freezing point H20= 0°C
Boiling point H20=100°C
Describe the Kelvin scale
Theoretical Absolute zero= -273°C (0°K)
Freezing point H20= 273°K
Boiling point H20=373°K
Just add 273 to the celsius scale
Describe the Fahrenheit Scale
Freezing point H20= 32°F
Boiling point H20=212°F
How do you convert from Celsius to Fahrenheit?
(9x°C)/5+32
How do you convert from Fahrenheit to Celsius?
5x(°F-32)/9
What is terristerial radiation?
The earth reradiating the energy it received from insolation on a lower frequency which heats up the atmosphere.
What are the three main ways heat can be transferred in the atmosphere?
- Radiation
- Conduction
- Convection (transfer of heat by vertical movement)
What is Diurnal Variation?
The daily variation in temperature
What happens to temperature as a parcel of air rises and what is this know as?
It cools
Adiabatic cooling.
Define latent heat.
The heat required to cause a change of state, the actual temperature does not change during the change of state as the energy is used to actually change the state.
A change from gas to liquid or liquid to solid will release energy into the environment and the opposite will absorb energy.
How does the temperature of the air affect its humidity/saturation? What is the point at which a piece of air that is cooling becomes fully saturated?
Warmer air holds more moisture so as a parcel of air warms its ability to retain water increases so it is less saturated and vice versa.
The dew point.
What is saturation?
Determined by how much water the air can hold and how much water vapour is actually in it.
The higher the temperature of the air the greater quantity of water vapour it can hold in suspension, this also decreases the density.
If air is cooled at a constant pressure until it is holding its maxiumum quantity of water then it is said to be saturated and the temperature at this point is know as the dew point.
What is the dew point?
The temperature a parcel of air is considered to be fully saturated.
What is sublimation?
The process where ice and snow changes into water vapour whilst skipping the liquid stage.
What is deposition?
The change of water vapour directly into a solid whilst skipping the liquid stage.
What is condensation?
When the parcel of air passes its dew point when cooling and the water vapour in it returns to liquid form.
This also warms the air slightly as latent heat energy is released into the atmosphere by the state change.
What is relative humidity?
The measurement of the amount of water in the atmosphere relative to how much would be required to cause saturation at that temperature.
What can be used to measure humidity?
- Hair hygrometers- Uses a human hair and its reaction to humidity.
- Wet and dry bulbs- Most common mehtod, where a wet bulb is covered by a muslin cloth that is kept moist.
Define adiabatic
The change in temperature of a parcel of air as its density changes, normally cooling as the air expands when rising.
What is the adiabatic lapse rates for saturated air?
SALR= 1.5°C/ 1000ft
What is the ELR?
The environmental adiabatic lapse rate that exists within the atmosphere at a particular time. (ISA global average is 1.98 degrees per 1000ft)
Define stable and unstable air.
Stable air is air that doesn’t rise and unstable rises.
What is absolute stability?
When the ELR is less than both the DALR and SALR creating stable air.
What is conditional instability?
When the ELR lies between the SALR and DALR so dry air will be stable as it is cooler than the environment but saturated air will rise as it would be warmer.
What is absolute instability?
When the ELR is greater than both the SALR and DALR meaning that both dry and saturated air will be unstable.
What is the adiabatic lapse rate for dry air?
3°C per 1000ft
What other factors can cause air to rise?
- Frontal Lifting- warm air being undercut and forced up by colder air moving in
- Orographic lifting- Geographical features such as mountains causing the air to rise, behaviour of the air at the top depends on its stability
- Covergence (Depression)- Air moving into an area of low pressure eventually rises to make way for the rest of the air also coming in.
- Convergence (thermal)- Convective currents causing air to fill in at the bottom and then rise.
- Turbulence- Mechanical (caused by air flowing over rough surfaces) and thermal (caused by uneven surface haeting).
Describe the convection current method of cloud formation.
Insolation causes the earth to heat up and this energy is then reradiated out into the air causing the air closest to the ground to warm first. This air then rises until it hits its dew point at which point it condenses into cloud. The extent of the cloud is dependent on the humidity and the strength of the rising currents.
Describe the frontal activity method of cloud formation.
When a mass of air of one temperature encounters one at a different temperature the warmer air will be forced upwards and cooled adiabatically until it hits the dew point and condenses.
What is orographic ascent?
An air mass forced to rise by the terrain rising, this causes the air to cool adiabatically until its dew point is reached. If the air is stable the cloud will only form in the vicinity of the hill or mountain as the air will warm again on its descent down the other side.
Describe turbulent mixing with regards to cloud formation.
Air mass moving quickly over undulating ground has its lower levels of air retarded by friction with the ground or obstacles causing turbulence which can form low clouds. Not normally much vertical growth.
What are the four cloud families?
High level- above 20,000ft
Medium level- above 7000ft
Low level- below 7000ft
Clouds of vertical extent- those which extend over multiple levels.
What are the types of clouds?
Cirrus-wispy
Cumulus- fluffy sheep
Alto- medium
Stratus- layers
Nimbus- rain bearing
Define cloud base.
The height of the base of any cloud above the aerodrome elevation.
Define cloud ceiling.
The vertical distance from the elevation of the aerodrome to the lowest part of any cloud visible from the aerodrome which obscures more than one half of the sky.
What instruments are used to measure cloud base/ceiling and how do they work?
Ceilometer. Fires lasers up into the sky and measures the reflection.
What is used to assess cloud cover and what are the categories?
Oktas (eights of the sky)
0= NSC (no significant cloud)
1-2= FEW
3-4= SCT (scattered)
5-7= BKN (broken)
8= OVC (overcast)
What fundamental process is required for clouds to form?
Condensation
The water vapour in the atmosphere must be cooled sufficiently to reach its dew point allowing it to condense into water droplets.
This can be achieved by adiabatic processes, heat loss by conduction or evaporation, or as a result of mixing.
What four methods of causing air to rise also form clouds?
- Convection currents due to localised surface heating
- Frontal activity between air masses
- Orographic ascent
- Turbluent motion over undulating ground.
What are cumuliform clouds?
Those that are heaped, they are normally associated with unstable air
What are stratiform clouds?
Layered clouds, these are normally associated with stable air.
Moist air may be carried upwards by turbulence causing it to condense, will be a thin horizontal layer due to rise only occuring in the turbulent layer.
What are cumulonimbus clouds?
Clouds that are caused by unstable air that allows the cloud to grow quickly and are marked by extreme vertical growth
They will continue to grow until reaching the tropopause where it will then spread out into the characteristic anvil shape.
What clouds are considered high clouds?
Those at and above 20,000ft
- Cirrus (Ci)- Wispy mares tails
- Cirro-Cumulus (Cc)-Flakes or ripples
- Cirro-Stratus (Cs)- Light veil
These clouds are made of ice crystals rather than water droplets.
What clouds are considered medium clouds?
Those between 7,000 and 20,000ft
- Alto-Cumulus (Ac)- Small Flakes
- Alto-Stratus (As)- Grey Veil
- Nimbo-Stratus (Ns)- Thick dark blanket classed and coded as medium
Can be ice crystals or water droplets
What clouds are considered low clouds?
Those below 7,000ft
- Stratocumulus (Sc)- Grey or whitish
- Stratus (ST)- Grey layer with uniform base
- Cumulus (Cu)- Individual layered heaped clouds
- Cumulonimbus (CB)- Large dark Cu, anvil top
Water droplets
What height is used for cloud assessment?
Height above aerodrome elevation.
In what ways can clouds be dispersed?
Either a rise in temperature of mixing with drier air causing the relative humidity to drop below 100% allowing water droplets to become water vapour
Air can be cooled sufficiently that the water in it will be released as precipitation which will reduce relative humidity below 100%
What is precipitation and what are its dimensions?
Any moisture that is released from the atmosphere in either liquid or solid form.
Water droplets- 0.02mm
Drizzle- 0.2mm
Heavy rain- max 5.5mm
Hail- can reach larger diameters
Why is precipitation an important factor?
Supercooled water droplets- icing
Wet surfaces- braking action
Visibility- VFR/IFR
Engines- flooding
What do you call water droplets that are below 0 degrees but have not frozen?
Supercooled
What are the types of precipitation?
Snow
Sleet
Drizzle
Rain
Hail
Detail the different levels of intensity and durations of rainfall.
Intensity- light, moderate and heavy
Duration- short, intermittent, or continuous
What types of precipitation are associated with stratus or stratocumulus clouds?
Drizzle, freezing drizzle or snow grains
What types of precipitation are associated with thick Altostratus and nimbostratus clouds?
Continuous rain or snow.
What types of precipitation are associated with thick Altostratus and stratocumulus clouds?
Intermittent rain or snow
What types of clouds can produce rain or snow showers?
Altocumulus, large cumulus and cumulonimbus
What types of cloud produce hard or soft hail?
Cumulonimbus
What do you call water droplets that are below 0 degrees but have not frozen?
Supercooled
What do supercooled droplets need to freeze?
A freezing nuclei e.g. Particulates, aircraft skins, or already frozen droplets.
Describe the Bergeron Theory of precipitation formation
Moist air cools and condenses as it rises eventually turning into ice crystals at high enough layers, these will become too heavy to be held aloft at which point they will fall to the ground melting back into water droplets on the way down.
Describe the coalescence theory of precipitation formation.
Rain drops are formed by collisions between tiny water droplets in the cloud that eventually grow until they are too heavy at which point they will fall to the ground.
What clouds are associated with Drizzle, freezing drizzle or snow grains
stratus or stratocumulus clouds
What clouds are associated with continuous rain or snow.
thick Altostratus and nimbostratus clouds
What clouds are associated with intermittent rain or snow?
thick Altostratus and stratocumulus clouds
What types of precipitation are associated with Altocumulus, large cumulus and cumulonimbus clouds?
rain or snow showers
What type of precipitation is associated with Cumulonimbus clouds?
hard or soft hail
What three conditions must be present for a thunderstorm to develop?
Moist air
Unstable air
Trigger
What are the three stages of a storm cells development and briefly describe them.
- Cumulus- growing stage, cloud grows very quickly following the trigger, strong vertical currents encourage growth, generally lasts 30 minutes.
- Mature- raindrops and hailstones grow inside the cloud until the vertical currents can no longer hold them up, they then fall to the ground bringing cold air with them, the precipitation is heavy. This stage usually lasts 30 minutes.
- Dissipating- precipitation gradually decreases, anvil shape forms as cloud growth stops, regeneration can occur if cloud joins with new cumulus clouds.
What are the hazards associated with thunderstorms?
Turbulence
Hail
Icing
Lightning
Static
Windshear
What are the four classes of trigger action?
Air mass- e.g. Polar air being warmed by warmer land.
Frontal- warm air pushed up by advancing cold front.
Orographic
Insolation- strong heating of land can lead to very unstable air.
How are thunderstorms classified?
By the source of the trigger action.
What does icing depend on?
The size, shape and speed of the water droplets. Also the airframe must be 0 degrees unless the water droplets are supercooled.
Describe hoar frost.
Affects aircraft parked overnight, as the airframe cools by radiation water condenses on it and eventually freezes if cooling continues. This can happen on the ground or in the air if the cold airframe descends through warmer air.
Describe rime ice.
Rime ice occurs when supercooled water droplets hit the airframe and freeze almost completely. This produces a weak accumulation of ice that can affect aerodynamics and small air intakes.
Describe Clear ice.
Clear ice formed by water droplets that are only just supercooled so when they impact the airframe they do not freeze entirely and a large proportion of the drop flows backwards and freezes leading to heavy, thick ice. It can be dangerous even when it breaks off as it will do so in large chunks that can be hazardous to the aircraft.
Describe rain ice.
Most dangerous form, happens when water droplets that are just above 0° hit an airframe which is 0 degrees leading to only a small proportion of the drop freezing and the rest flowing backwards and freezing rapidly. This creates a large wight penalty. This normally happens to aircraft flying into a cold front just ahead of a warm front.
This is normally encountered just ahead of a warm front.
List the types of icing in order of severity from least to most.
Hoar frost
Rime ice
Clear ice
Rain ice
What parts of an aircraft does icing affect?
Wings and control surfaces Windscreens Propellers Radio antennas Tubes and vents Carburettors Jet engines
What type of cloud is normally associated with severe icing?
Cumulonimbus or even nimbostratus in certain conditions (thick heavy layers)
What is the difference between anti-icing and de-icing?
Anti icing prevents the build up of ice in the first place De-icing gets rid of ice that has already accumulated.
Give some examples of anti icing systems.
Systems that pump alcohol (what a waste), glycol or hot air over vulnerable surfaces. Electric heating elements Etc
Give an example of a de-icing system.
Rubber boots- those rubber things attached to wings that are inflated with compressed air and break the ice.
Why do supercooled droplets not freeze instantly on impact with aircraft?
Due to energy released by the droplet as it changes state from liquid to solid (latent heat), meaning the drop will not freeze entirely until it has lost this energy to the cold aircraft wing.
What is HOT?
The Hold Over Time
The amount of time a de-icing fluid remains on the airframe and continues to be effective, depends on fluid type and the outside air temperature.
What is cold soaked fuel cooling?
Conductive cooling of airframe caused by the fuel onboard being very cold.
What factors are significant in terms of icing?
- Temperature
- Drop size
- Airframe shape
- kinetic heating
- Concentration
At what distance is visibility measured in km instead of m?
More then 5000m
Define visibility
The ability, as determined by atmospheric conditions and expressed in units of distance, to see and identify prominent unlighted objects by day and prominent lighted objects by night.
Either;
A) the greatest distance at which a black object of suitable dimensions, situated near the ground, can be seen and recognised when observed against a bright background;
(B) The greatest distance at which lights in the vicinity of 1000 candelas can be seen and identified against an unlit background; (SERA)
Define fog?
a suspension of small water droplets reducing visibility; To less than 1000m for fog
What is haze?
A suspension of solid particles in the air which reduce visibility to not less than 1000m where the relative humidity is less than 95%
Describe radiation fog and what are the conditions required for it to happen?
Rapid cooling of the land on a clear night through radiation, this cools the layer of air closest to the ground causing it to reach its dew point and condense.
Clear sky- allows earth to radiate heat and cool
Land surface- to allow cooling of the air layer by conduction
Moist air- so there is moisture to condense
Light wind- between 2-8kts to prevent dispersal of air
Long night- allows time for fog to form
Describe advection fog
A warm, moist mass of air being forced over a cold surface (due to advection which is caused by convection) causing its temperature to drop and the air to condense. Wind speed not critical as air mass is moving anyway.
Under what conditions would smog form?
Normally under a temperature inversion causing the air to become stable and therefore preventing the dispersal of the pollutants.
What is the prevailing visibility?
The maximum value reached or exceeded in at least 50% of the circle around the aerodrome, the sectors of visibility do not need to be contiguous.
The lowest visibility is also reported in the METAR when it is less than 1500m (providing it is not the prevailing visibility) or less than 50% of the prevailing visibility, along with its general direction.
What is the minimum visibility?
The minimum value of visibility, this will be reported on a METAR is the value is less than 50% of the prevailing vis or less than 1500m along with a general direction.
Define RVR and how it can be measured.
The actual visibility along the runway as viewed from the perspective of the pilot in the cockpit. It can be measured by counting;
- A series of marker boards along the side of the runway.
- A series of lights of known intensity along the side of the runway.
Human observer method above
-or by a photo-optical measurement of atmospheric transparency along the runway (IRVR), can have three Transmisometers at the touchdown zone, midpoint and stop end.
When is an RVR reported?
When local visibility is less than 1500m
or RVR readings indicate less than 1500m
What are the conditions for CAVOK?
The visibility must be more than 10k (9999)
No cloud below 5000ft or the minimum sector altitude (whichever is greater)
No significant weather at the aerodrome
No Cb or TCu
How would you measure visibility during the day?
A met observer would eestimate the vis based on identifying suitable objects at known distances
How would you measure visibility during the night?
A met observer would attempt to estimate the distance by attempting to identify suitable light at known distances.
What increments is visibility measure in?
- <500m= 50m intervals
- 500-5000m= 100m intervals
- 5km-30km=1km intervals
- 30km-70km= 5km intervals
- 70km>= greater than 70km (but who gives a shit if its that high)
When do you have to report the reasons for visibility reductions?
When the vis falls below 5000m
Define mist
a suspension of small water droplets reducing visibility; To not less than 1000m
What is the minimum number of lights a IRVR needs?
10% runway lights on
How many IRVR units does a Cat I ILS require?
Requires none of them but a touch down one is recommended
How many IRVR units does a CAT II ILS require?
Touchdown and Midpoint
How many IRVR units does a CAT III ILS require?
All of them
What is oblique visibility?
An aircraft flying over an airfield in fog may be able to visually acquire it from directly above but when flying in the layer will be unable to see the aerodrome owing to the greater thickness of the fog from an oblique angle.
What is reported on a METAR for cloud if the sky is obscured due to fog, falling or blowing snow?
VV///
What happens to an air mass as it rises?
It will decrease in pressure and density.
How many inches of mercury is the ISA?
29.92 (1013.25hPa)
What pressure are aircraft altimeters calibrated to?
ISA of 1013.25hPa
How does an aircraft’s altimeter work?
An aneroid barometer (without liquid) measures the difference in pressure between the datum pressure on the subscale and the ambient pressure at that level, this is then converted into feet to be displayed on the scale.
What effect does temperature have on pressure at certain heights?
As cold air is denser than warm air the height at which a pressure has fallen to a specified value will be lower down. Vice versa for a column of warm air.
What is QFE?
Pressure at aerodrome level (or another specified datum on the surface of the earth)
This gives you height
What is QNH?
Pressure at mean sea level
this will give you your altitude
What level type do you get when using 1013.25hPa?
Flight levels
This is used because the separation between aircraft at these levels is more important than their separation from the ground.
What are Gusts and lulls?
Gusts are increases in wind speed and lulls are the opposite, these happen over a matter of seconds.
What is a gale?
Prolonged high speed wind with a mean value of 34kts or is gusting to 43kts or greater.
What is veering and backing in regards to wind velocity?
Veering is a clockwise change in wind direction.
Backing is an anti-clockwise change in wind direction.
What is an isobar?
A line delineating areas of constant pressure, usually at mean sea level.
The closer they are the larger the pressure gradient and vice versa
Describe pressure gradient force.
The movement of air from an area of high pressure to low pressure. Creates wind.
What effect does the coreolis force have on wind in the northern hemisphere?
It deflects winds to the right, it does the opposite in the Southern Hemisphere. This is otherwise known as geostrophic wind
What is geostrophic wind?
The result of the Coriolis force acting on pressure gradient wind.
What is a cyclone?
Otherwise known as a depression, it is an anti clockwise rotation of air around an area of low pressure. Caused by the air flowing inwards to the low pressure area from all directions and deflecting to the right.
Air converges from the surface and is forced upwards before dispersing, adiabatic cooling of this air will produce thick cloud and precipitation. These can lead to tropical cyclones.
What is an anticyclone?
A clockwise rotation of wind around an area of high pressure. Air converges and then descends to the surface before diverging. Creates stable and clear weather. In summer these create periods of fine settled weather. In winter radiation fogs.
Why does wind at the ground differ from higher winds (at 2,000ft)?
Due to friction with the earth that slows it down, geostrophic force is also reduced. This means the wind will decrease and back to around one third of the geostrophic wind over land and two thirds over the sea.
What effect does heating and cooling have on surface wind?
Heating increases the turbulence which cancels out land irregularities and lessens the effect of friction, leading to the wind increasing and veering.
Cooling has the opposite effect.
How do sea breezes form?
It is caused by the land heating up faster than the water and transferring the heat to the air above it which rises, cooler air from the sea then is pulled in to replace this air (advection). Geostrophic wind will cause the wind to veer meaning that the wind will not blow at right angles to the coast.
Will be strongest on a sunny day with a weak pressure gradient with these winds being felt as far as 30km inland.
What is a land breeze?
The opposite of a sea breeze, the land cools faster than the sea and this causes a movement of air out to sea.
Describe katabatic wind
The air at the top of the mountain or hill cools quicker, becomes denser and sinks to the bottom of the mountain.
Describe anabatic wind
Insolation causes transfers heat to the air and this rises up the topography.
What is a fohn wind?
A wind down the leeward side of a mountain which is warmer than the windward side owing to the differences in temperature changes with the SALR and DALR. The air drops its moisture at the top of the mountain causing it to descend down the leeward side at DALR which warms it up quicker.
Describe mountain waves
Air that has travelled over an obstacle descends immediately and then rises again with lesser amplitude as if it were bouncing. This creates lenticular clouds and rotary clouds.
These are hazardous to aviation owing to the strong vertical currents which will be experienced.
How is wind direction measured?
The direction from which the wind is blowing given as true north
however for readings from an anemometer it will be from magnetic north
What is wind speed measured in?
Knots (KT), Kilometres per Hour (KMH) or metres per second (MPS)
What is a squall?
An increase in wind speed that lasts for minutes, normally associated with CB’s
Describe buys ballot law
If you stand with your back to the wind in the northern hemisphere the area of low pressure will be on your left hand side.
Does wind flow parallel to the isobars?
Not exactly, it will be at an angle around 10°-30° to the isobar dependent on whether the wind is over land or the sea.
What effect will cloud cover have on turbulence and therefore the wind velocity?
It will decrease the turbulence, increasing the surface friction which will cause the wind to decrease and back.
What are valley winds?
Winds that are cooler and faster in a valley due to the venturi effect and bernoullis principle.
What is a roll cloud?
A cloud created on the leeward side of a mountain due to mountain waves, it exists where the vertical turbulence is at its most extreme and is very hazardous to aircraft.
What is a thermal wind?
A thermal wind is one that blows as a result of a difference in temperature between adjacent atmospheric areas.
What is the Jetstream?
A permanent band of geostrophic wind at the boundary between two air masses, these are west to east in the northern hemisphere. These exist owing to difference in temperatures between adjacent air bands and the thermal wind this produces.
Blow west to east in the northern hemisphere.
These can reach speed of 200kts
They are found immediately below the tropopause owing to the air temp above there being constant.
What is Anti-Cyclonic Gloom?
One necessary condition for fog, or cloud to form at low levels, is that the relative humidity of the atmosphere close to the surface is high - the temperature of the atmosphere is already close to its dew point. Fog or low cloud will then occur if the Earth cools rapidly on a clear night, by radiation. Prolonged surface cooling is commonplace in anticyclone conditions over the continent in winter, when following a fine day, a cloudless night leads to such surface cooling.
Given sufficient turbulence to mix the air which has been cooled by contact with the surface, a layer of cloud will form. Sometimes a layer of cloud has already formed over the sea and on drifting inland, if the surface of the Earth is cold, will not disperse but instead create a stable layer. This is because the top of the cloud layer is being evaporated by insolation and creating a minor inversion.
The conditions described are normally associated with an inversion of temperature which creates this layer of cloud, and if the atmosphere is polluted, the resultant reduction in visibility and thick low cloud is called anticyclonic gloom.
What is convection turbulence?
Insolation transfers heat to air near the surface which produces a convection current of varying strength.
What causes mechanical turbulence?
Large vertical variations at the ground level such as mountain ranges, it is also dependent on the strength of the surface wind, its stability and the nature of the surface.
What is the definition of wind shear?
A rapid change of wind speed and/or direction over a short distance horizontally or vertically. Can occur at any height.
When must you report windshear?
Whenever it is reported to you, report it to landing and departing aircraft until you are certain or it has been confirmed that it no longer exists.
What should a windshear report contain?
A warning of the presence of it
The height or height band where it was encountered
The time at which it was encountered
Details of the effect of the windshear on the aircraft.
What are microbursts?
Localised down drafts of short duration, high velocity winds. Diameter equal or less than 2.5 miles
What is clear air turbulence?
Turbulence that occurs without clouds indicating its presence. It can occur anywhere but is normally associated with the upper troposphere of lower stratosphere as well as jet streams. They will be experienced flying between the boundaries between warm and cold air. It can also be caused by standing waves.
What system installed at an aerodrome can detect Microbursts?
Low Level Wind Shear Alert Systems (LLWSAS)
have been installed at some airports to warn pilots of possible microbursts. Consists of five anemometers situated around the airfield perimeter and one centrally located. These are connected to a monitoring and comparator unit that transmits a warning when a predetermined difference in wind velocity across the airfield is sensed.
What is a Macroburst?
A downdraft of air, a microburst with a diameter of more than 2.5 miles
What is Clear Air Turbulence?
CAT is turbulence that occurs without any cloud to indicate its presence.
It can occur anywhere in the atmosphere but is normally associated with strong winds in the upper troposphere or lower stratosphere.
Wind shear and strong vertical currents that can cause CAT are frequently associated with jet streams where the turbulence is produced by rapid changes in wind speed over very short distances.
Can also happen high above mountain waves.
What conditions are most likely to cause windshear?
The situations most likely to cause wind shear are;
- climbing or descending through an inversion.
- climbing or descending through a warm front.
- in the lee of high ground or large buildings when close to the ground
- in the vicinity of standing waves
- flying near, especially under, cbs
What is low level windshear and why is it particularly hazardous to aircraft?
Strong air currents close to the ground, especially those caused by CB’s, are capable of producing this sort of wind shear.
The changes of wind velocity associated with this may occur faster than an aircraft can accelerate or decelerate which when this close to the ground can be disasterous.
Under what conditions is a low level wind shear alert issued?
When one or more of the following are experienced;
- Mean surface wind of at least 20kts
- A difference between surface and 2000ft winds of at least 40kts
- Thunderstorms or heavy showers within 5nm of the airfield
- Pilot reports or local wind shear conditions.
What are the classifications of air masses?
Polar or tropical
followed by;
continental or maritime.
What is associated with tropical maritime air masses?
Characteristics- high temps, high dew point, high humidity
Modification- continuously cooled from below so often forms stratus clouds, stratocumulus or fog with associated drizzle. Lower layers become stable
Summer- warm, moist air and hazy with broken cloud over land. Advection fog over coasts and sea.
Winter- warm and moist with low cloud or fog and drizzle.
What is associated with tropical continental air masses?
Characteristics- high temp, low dew point, low humidity.
Modification- continuously cooled from below and humidity increased by evaporation over water. Usually remains dry enough to prevent cloud.
Summer- warm and dry overland with little cloud but often hazy due to suspension of dust particles from air mass source.
What is associated with polar maritime?
Characteristics- low temperature, high dew point, high humidity
Modification- heated from below and acquires moisture due to evaporation. Rapidly becomes unstable at lower levels permitting formation of large cumulus and cumulonimbus clouds with associated showers, gust winds and thunderstorms.
Summer- cool with showers and possible thunder near the coast. Calm and clear inland. Good visibility except for precipitation.
Winter- suffers surface over the cold land and becomes stable. Possible layer of cloud and fog overnight.
Polar continental, what is associated with this type of air mass?
Characteristics- low temp, low dew point, low humidity
Modification- acquires moisture as it crosses the North Sea. May be sufficient to cause instability in lower layers.
Winter- rain or snow showers but often cold and clear inland.
What is an Artic maritime air mass?
An artic maritime air mass has similar characteristics to a polar maritime air mass but is less moist and colder due to the shorter tracking across the sea. Uncommon during the summer but when it does occur it brings thunderstorms and unseasonably low temperatures.
In terms of pressure what is a ridge?
An are of high pressure which elongates to form a ridge of high pressure. Shown as a zig zag on charts.
What is a pressure trough?
An elongated area of low pressure marked on charts. All are associated increasing cloud and risk of precipitation.
What is faster, a cold or warm front?
A cold front
What are the characteristics of a warm front?
1:100 slope
What does a cold front look like?
1:50 slope
What are occluded fronts?
This happens when a cold front catches up to a warm front, the type of occlusion depends on the relative temperatures of the cold air on either side of the warm sector,
- cold front occlusion (cold front colder than the cold air in front of the warm front)
- warm front occlusion (coldest air in front of the warm front, warm front followed by cooler air).
What is the definition of an air mass?
A body of air with a similar temperature throughout, separated from an adjacent body of air by a sharp transition zone.
What is the intertropical convergence zone?
The zone of interaction between the equatorial region and tropical maritime air, creates monsoons.
What are continental artic ari masses?
Very cold and very dry air masses, they are also very stable.
What is a circular area of low pressure known as?
a depression or cyclone
What is a circular area of high pressure know as?
an anticyclone
What weather is associated with an anti cyclone?
Summer- Prolonged periods of settled, fine weather. Bumpy for flying, poor vis.
Winter- Radiation fog
May cause anticyclonic gloom under a layer of stratocumulus clouds.
What is a front?
A weather pattern at the transition area between adjacent air masses.
What are the main frontal zones?
Polar Front
Arctic Front
Mediterranean Front
Intertropical front
What is the speed of a warm front?
2/3 of the geostrophic wind speed
What characteristics are associated with an approaching warm front?
- Pressure falls steadily
- Wind may back and increase slightly
- Temperature begins to rise
- Cirrus clouds become Cs
- Cloud base lowers
- Rain commences and becomes continous
What characteristics are associated with a passing warm front?
- Pressure remains steady or continues to fall slowly
- Wind decreases and veers
- Temp and dew point rise
- Low nimbo stratus and stratus persist but light rain lightens to drizzle or ceases.
- Poor visiblity persists
- may take hours to pass
What characteristics are associated with a receding warm front?
- Wind is light
- Temp, pressure and dew point remain steady
- Little or no precipitation
- Low stratus usually remains
- Mist or fog may persist
What speed do cold fronts travel at?
The geostrophic wind speed
What are the characteristics of an approaching cold front?
- Wind speed increases to moderate
- Pressure falls
- Dew point and temp generally remain steady
- Precipitation increases
What are the characteristics or a receding cold front?
- Visibility very good
- Rain and cloud replaced by scattered cumulus and showers
- Wind moderate and steady
- Pressure rising slowly
- Temperature and dew point unchanged.
What are the characteristics of a passing cold front?
- Wind veers quickly and increases
- rapid pressure rise
- temperature and dew point fall sharply
- Heavy precipitation, takes around an hour to pass
What is a line squall?
Air overunning the air at ground level at the front of a cold front, which imparts a rolling motion to the cloud immediately ahead of it.
This creates individual heavy showers along the line of the front with strong turbulent wind conditions.
Who provides the forecasting and climatological services in the uk?
The met office.
What are the met offices functions?
.Maintain a watch over meteorological conditions affecting rations within FIRs .Prepare sigmets and other information relating to its area of responsibility. .disseminate sigmet info
What are the types of Met Reports?
Metars (meteorological weather report)
SPECIS (special report)
TAFs (terminal aerodrome forecast)
Aerodrome warnings
Windshear Warnings
What is a METAR and what does it include?
A report of actual weather conditions at a particular aerodrome at a specified time. Includes;
Surface wind
Surface vis
Any weather
Cloud
Air temp/ dew point
QNH
Remarks
- In Europe they include a trend forecast for the 2 hours following the METAR, in the UK this is not always the case.*
- Auto indicates that the METAR is prepared by Automated Observation systems.*
What is a SPECI?
An amendment to a METAR that draws attention to an improvement or deterioration of a condition that might affect the aerodrome ops.
Disseminated locally
Required following an accident.
What is a TAF?
Terminal Aerodrome Forecast
A concise statement of expected meteorological conditions at an aerodrome during a specified period.
What are the time periods for TAFS?
9 hour TAFs, prepared every 3 hour intervals.
24 or 30 hours, prepared at 6 hourly intervals
What does NOSIG signify on a TAF?
That no significant change in met conditions is expected within the next two hours, otherwise a trend would have been recorded.
What sort of met conditions trigger an aerodrome warning?
Gales/strong winds
Squalls
Thunderstorm
Hail
Fog
Frost/snow
Freezing precipitation
These warnings are valid for the period stated on them!
What conditions trigger SIGMETS?
Thunderstorms
Heavy hail
Freezing precipitation
Tropical cyclone
Heavy sand/dust storm
Volcanic ash
Severe mountain waves
Severe icing
Severe turbulence
What is an ATIS?
A repetitive recording that is continuously transmitted on a VHF frequency, includes;
aerodrome status
current weather
ATIS info code.
What is a Windshear report?
Windshear reports are based on recent pilot reports of windshear on the approach or climb-out.
Pilots should give concise information concerning wind shear conditions, whether by reference to speed loss or gain or in terms of its effect on the aircraft.
- Cancelled when aircraft reports indicate that windshear no longer exists.
- Pilot should report windshear even if already reported or warned.
To whom are SIGMETS issued?
To ACC’s or FIC’s
Who then pass it onto to any a/c that will encounter those conditions that triggered the SIGMET 500nm ahead of their flight path or 2 hours flying time.
What is an AIRREP?
An Air Report
A report issued by a pilot in flight saying that he has encountered the following conditions;
- Severe turbulence or icing
- Severe Mountain wave
- Thunderstorms with or without Hail
- Volcanic ash or pre eruption
- Any other phenomena that, in the opinion of the pilot, may adversely affect the safety of aircraft operations.
What is VOLMET?
VOLMET is a voice broadcast of a set of METARs broadcast on four frequencies covering different regions of the UK with each frequency transmitting a different METAR set. The four regions are London VOLMET (Main), London VOLMET (South), London VOLMET (North) and Scottish VOLMET. Details can be found in the UK AIP GEN 3.5.8.
What must a controller do before starting operationally?
Prior to watch must self brief using;
- Study forecasts, prognostic charts
- Briefing for clarification
- Handover controller briefing
- Local knowledge
When are temperature inversions included in aerodrome warnings?
For specific aerodromes whenever marked temperature inversions with a difference of 10°C or more exist up to 1000ft AAL.
What additional phenomena are considered relevant to a/c travelling transonic or supersonic?
- Moderate or severe turbulence
- CB’s
- Hail
- Volcanic Ash
How long are SIGMETs valid for?
4 hours and they are reissued as necessary.
What differenct types of report can be identified from the heading of a message on the AFTN regarding Met?
- SA- Routine aerodrome report
- SP- Selected special report
- FT- Aerodrome Forecast valid for 18 or 24 hours
- FC- Aerodrome forecast valid for 9 or 12 hours
What is a world area forecast centre?
One of two centres (one in UK and one in USA) responsible for provision of global forecasts of significant weather and the following global point data;
- Wind
- temp
- humidity
- Tropopause height and temp
- Maximum wind speed, direction and height.
What are AIRMETS
General Aviation Weather Briefing Service
- Routine forecasts in plain language
- Valid for 6 hours
- Comprehensive Selection of current and forecast weather conditions with less severe weather than SIGMET
- Internet
What can ATC transmit weather wise?
Only information which has been supplied by the MET office, the exceptions are:
- Indicated wind speed
- RVR obs
- Controller observation of sudden deteritoration
- A/c reports of weather hazards
- Cloud echoes observed on radar
- Official met observations
- Unoffical met obs
What type of north is used to describe wind speed on a met report?
True
When is directional variation included on a met report?
If greater than 3kts and 60 degrees or more.
What does VRB mean on a met report?
Variable wind direction.
What does 0000 signify on a met report in regards to vis?
Vis less than 50m
When is vis variation reported?
When it is less than 1500m in one direction and greater than 5000m in another.
How is RVR denoted on a met report?
R followed by the runway and measured vis distance.
Distance in metres measured from 50m to 1500m
Letter P before distance means that it is higher than can be assessed.
Letter M before distance means that it is lower than can be assessed.
Mean values, U for trend improving D for opposite and N for no trend .
RVR tendencies and significant variations; and Wind shear groups (WS………) are not reported in the United Kingdom.
What types of clouds are notified on a met report?
Cumulonimbus and towering cumulus.
What does prob mean on a TAF?
Likelihood of proceeding change happening, 30 unlikely, 40 likely.
What validity periods can TAFs have?
9, 24 or 30
When are METARs issued?
Every 30 minutes at 20 past the hour and 10 to.
+
Heavy
-
Light
Bc
Patches
Bl
Blowing
Br
Mist
Ds
Dust storm
Dz
Drizzle
Fc
Funnel cloud
Fg
Fog
Fu
Smoke
Fz
Freezing
Gr
Hail >5mm
Hz
Haze
Ic
Ice crystals
Mi
Shallow
Pl
Ice pellets
Pr
Banks
Ra
Rain
Sh
Showers
Sg
Snow grains
Sn
Snow
Sq
Squalls
Ts
Thunderstorms
Va
Volcanic ash
Vc
Vicinity
Re
Recent
For what types of reports is abbreviated plain language used in met?
SIGMETS
TAF’s for some information
METAR or SPECI (trend group only)
What language is used in a METAR?
Codes, eg RA= Rain but this is not an abbreviation.
What does 00000KT mean on a METAR?
No wind
What are the conditions for a gust?
When the wind is gusting to a speed 10 or more knots faster than the mean wind speed.
How are clouds reported on a METAR?
- Lowest cloud of any amount
- Next highest cloud of more than 2 oktas
- next highest cloud of more than 4 oktas
- Any CB/TCu present if not already reported.
- No significant clouds- NSC
What does M mean before temperatures on a METAR?
That the value is below 0°C
How is the QNH reported on a METAR?
Rounded down to the nearest whole hPa
What does NOSIG mean on a METAR?
No significant change
What does BECMG mean?
Becoming, from and till
What does TEMPO mean?
temporary change in conditions- from and till.
What does AMD mean on a TAF?
TAF amendment, used if a revised forecast is issued covering the remaining period of original TAF.
What does NSC mean on a TAF?
No significant cloud, can be used if all requirements of CAVOK or SKC cannot be met?
What does SKC mean?
Sky Clear
What does NSW mean on a TAF?
No Significant Weather
