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.