Aviation Weather Flashcards
What are the two main constituent groups of the atmosphere?
Permanent constituents (Nitrogen, Oxygen, Trace Gases) Variable constituents (Solids, Liquids and Gases, ie. dust, ash, clouds, water vapour)
What main gases make up the atmosphere?
Nitrogen (78%) Oxygen (21%) Trace Gases (1%)
What trace gases are present in the atmosphere?
Argon, Carbon Dioxide, Neon, Helium, Methane, Krypton, Hydrogen, Xenon, Ozone)
In the high atmosphere what happens to oxygen?
Oxygen can dissociate from the molecular form (O2) to the atomic form (O), under the influence of very short waves of solar radiation.
What form can water take in the atmosphere?
Fog, Mist, Cloud, Ice and Precipitation.
How can solid particles in the atmosphere affect precipitation?
Solid particles can promote the formation of water droplets by providing condensation nuclei.
What effects can large quantities of solid matter in the atmosphere have?
Large amounts can reduce visibility (sand storms, smoke haze).
How is Ozone formed?
Ozone is the combination of O and O2 O3 can absorb radiation to produce warm layers in the upper atmosphere
What is the lowest layer of the atmosphere?
Troposphere
What is the order of the layers of the atmosphere?
Troposphere Stratosphere Mesosphere Thermosphere
What are the boundaries between spheres called?
Pauses
Within which atmospheric layer is most of the atmospheric water present?
Troposphere
Within which atmospheric layer does the majority of weather occur?
The Troposphere, as nearly all atmospheric water is present within this zone.
What is the average height of the Troposphere?
36 000 ft
What is the height range for the Troposphere?
25 000 ft - 55 000 ft
What atmospheric zone is located above the Troposphere?
Stratosphere
What is the average height of the Stratosphere?
160 000 ft
What atmospheric zone is located above the Stratosphere?
Mesosphere
What atmospheric zone is located above the Mesosphere?
Thermosphere
What is the height of the Mesosphere?
280 000 ft
What is the height of the Thermosphere?
2 000 000 ft
At what rate does temperature decrease within the Troposphere?
Decreases by 2C per 1000 ft up to the tropopause
What is the average temperature of the Stratosphere?
0 C
What features are associated with the Tropopause?
Rarely clouds above the Tropopause Wind speed is often at its maximum just below the Tropopause Severe Turbulence is often associated with the Tropopause
Where are contrails usually formed?
At the Tropopause
What is CAT?
Clear Air Turbulence
Where are Gas Turbine engines at their most efficient?
At the Tropopause
Where are Tropopause heights at their greatest?
At the Equator
Where are Tropopause heights at their smallest?
At the Poles
What is the average height of the Tropopause over the UK?
35 000 ft
What is the average height of the Tropopause over the poles?
25 000 ft
Why are Tropopause heights greater in the Northern Hemisphere?
There is a greater percentage of land which results in higher temperatures.
Does atmospheric pressure increase or decrease with altitude gain?
As height increases, pressure decreases as the weight of the air above is less.
What is the rate of pressure decrease near the ground?
1 hPa per 30ft
What is the rate of pressure decrease above 20 000 ft?
1 hPa per 50ft
Does air density increase or decrease with elevation gain?
Density decreases with altitude due to the reduction in pressure.
What is the formula for calculating air density?
ρ = p / RT where ρ is the ratio of mass to volume of air (kg/m^3) p = pressure R = gas constant T = absolute temperature
What is the definition of a Pascal?
A Pascal is defined as the pressure exerted by a force of one newton per square metre.
What value is the standard pressure setting?
1013.2 hPa
What are the disadvantages of a Mercury Barometer?
They must be kept stationary and are therefore used in labs or weather stations.
How does a Mercury Barometer work?
The pressure increases on the mercury in the base of the barometer. This pressure forces the mercury up the vacuumed glass tube. As the pressure reduces, the mercury starts to move back down the tube.
What is an Aneroid Barometer?
A compact barometer that can be easily transported and read.
How does an Aneroid Barometer work?
The aneroid barometer consists of a flexible sealed metal chamber which is partially evacuated. The chamber expands as pressure decreases and contracts as pressure increases.
What is the ISA?
International Standard Atmosphere
What are the conditions for the ISA?
1013.2 hPa = Surface Pressure 15C = Surface Temperature Up to 36 000 ft the temp decreases at 2C/1000ft At sea level the density is 1.225 kg/m^3
For the ISA what is the density at sea level?
1.225 kg/m^3
What is the surface temperature for the ISA?
15 C
What is the atmospheric pressure for the ISA?
1013.2 hPa
What is the lapse rate?
The rate at which temperature and pressure falls with an increase in height.
What are the rate of lapse with an increase of height for pressure and temperature?
Pressure: 1 hPa decrease for every 30 ft increase in altitude. Temperature: 1.98 C decrease for every 1000 ft increase in altitude.
Solar radiation that reaches the Earth is known as…
Insolation
What percentage of solar radiation is absorbed by the Earth?
70% The remaining 30% is lost to space.
What is the rough pressure range (hPa) to be expected when airborne?
950 - 1050 hPa
Why may the ASI under-read at high altitude?
Due to reduced air density. Corrections may therefore be required to give the TAS.
What is the term used for how much solar radiation makes it to the Earth’s surface?
Insolation
INcoming SOLar radiATION
What factors affect the level of Insolation?
Earth/Sun distance elliptical orbit
Seasonal Variations
Latitude
Time of Day
Atmospheric Conditions
What is Insolation?
INcoming SOLar radiATION
What is the tilt of the Earth’s axis?
23.5°
What is the inclination of the Earth’s North-South axis?
66.5° to the ecliptic
What is the effects of latitude on Insolation?
Lower concentration of insolation at higher latitudes over larger area.
Greater concentration of insolation at lower latitudes over smaller area.
The angle of incidence determines the area of coverage of the insolation.
How does time of day affect Insolation?
There is greatest insolation at noon.
Dawn and Dusk results in low insolation.
What atmospheric conditions affect Insolation?
Clouds, Smoke, Pollution.
They all diffuse sunlight.
Briefly describe Radiation, Conduction and Convection
Radiation heats the Earth’s Surface.
Conduction is the Earth’s surface warming the air.
Convection is the rising of warm air.
What is sublimation?
Sublimation is the transition from solid to a gas state. (Vapourisation)
What is Deposition?
Deposition causes water vapour to condense from gas to solid.
What is the definition of Rain?
Rain is frozen preciptation that falls through warm air, melts and reaches the ground as rain or drizzle.
What is latent heat?
Latent heat is the heat required to change the state of a substance without a change in temperature.
What is absolute humidity?
Absolute humidity is the measure of water vapour (moisture) in the air, regardless of temperature. It is expressed in g/m^3
What is relative humidity?
Relative humidity is a ratio, expressed as a percentage, of the actual water vapour content of a sample of air to the maximum it could hold at its temperature.
What is the dew point?
The Dew Point is the temperature, in degrees, at which the air is saturated with water vapour.
What is Virga?
A virga is an observable streak or shaft of precipitation falling from a cloud that evaporates or sublimates before reaching the ground. A shaft of precipitation that does not evaporate before reaching the ground is a precipitation shaft.
How does humidity affect air density?
Higher humidity results in lower air density.
What is Vapourisation?
The transition phase from liquid/solid to gas phase.
What are the phases of the Water Cycle?
Accumulation (In ice and snow)
Surface Runoff
Infiltration
Evaporation
Condensation
Precipitation
What device is used to measure atmospheric conditions?
Radiosonde
What is the standard temperature lapse rate?
1.98°C per 1000ft
What is an Inversion?
An Inversion is a layer within the atmosphere where temperature increases with height.
What is an Isothermal Layer?
Isothermal layer is a zone in the atmopshere where the temperature is constant over height.
What is Environmental Lapse Rate?
ELR is the actual rate of temperature change with altitude for a specific time and location obtained from the radiosonde.
What types of Adiabatic Lapse Rate are there?
2 types:
Dry Adibatic Lapse Rate (DALR)
Saturated Adiatbatic Lapse Rate (SALR)
What value is the Saturated Adiabatic Lapse Rate?
It is 1.5°C / 1000 ft.
What value is associated with the Dry Adiabatic Lapse Rate?
3°C / 1000ft
What is Absolute Stability?
Absolute Stability is when:
ELR is less than SALR and DALR.
Rising Air is cooler than the surrounding atmosphere.
Air will sink without a lifting force.
The atmospher is stable.
What is Absolute Unstability?
Absolute Unstability is when:
ELR is greater than the DALR and SALR.
Rising air is warmer than the surrounding atmospher.
Air will continue to rise without a lifting force.
The atmosphere is unstable.
The displaced air is always warmer (less dense) than the surrounding atmosphere and will continue to rise.
What is Conditional Instability?
Conditional Instability is when:
ELR is between DALR and SALR.
Dependent air saturation:
Unsaturated is cooler, will sink and is stable.
Saturated is warmer, will rise and is unstable.
Unsaturated stable air can be forced to rise and cool to its dew point becoming saturated and unstable.
If Warm Air is cooled from below what happens to the stability?
It becomes more stable.
If the ELR was 4°C per 1000ft what would the stability be?
Unstable
Dry Parcel Rises
Saturated Rises
Therefore the ELR is Unstable
If cool air is heated from below what happens to the stability?
It becomes more unstable.
The parcel of air is warming than the environment so it continues rising.
What is the average ELR (Environmental Lapse Rate) in the atmosphere?
2C per 1000 ft
If the ELR was 2C per 1000ft what would the stability be?
The dry parcel will sink
The saturated will rise
Therefore conditionally unstable.
If the ELR was 5C per 1000ft what would the stability be?
Unstable
If its warmed from below its unstable.
If it is cooled from below its stable.
What components does a wind velocity reading have?
Direction from true
Speed in knots
i.e. 270/35
What is veering?
Veering is when wind direction i changing clockwise.
What is backing?
If wind direction is said to be backing, it is changing in an anti-clockwise direction.
How does the Coriolis effect affect wind?
In the Northern Hemisphere air flow is deflected right.
In the Southern Hemisphere air flow is deflected left.
How does Coriolis effect differ at the poles and the equator?
There is no deflection at the equator, zero.
Deflection is greatest at the poles
What is a geostrophic wind?
A theoretical wind is formed when ‘pressure gradeient force’ and ‘coriolis effect’ are exactly balanced.
These move parallel to the isobars.
ADD MORE TO THIS
What is Buys Ballot’s law?
In the Northern Hemisphere, standing with your back to the wind the low pressure is on your left.
What types of turbulence are there?
Clear Air Turbulence
Thermal Turbulence
Friction Turbulence
What are standing/mountain waves?
Mountain Waves is defined as oscillations to the lee side (downwind) of high ground resulting from the disturbance in the horizontal air flow caused by the high ground.
Rotor clouds behind the peak.
Lenticular clouds above and down wind of peak.
Rotor turbulence.
The wind must be blowing ±30° to perpendicular to the ridge.
15-30 kts+ wind speeds at the crest.
What are valley winds?
Strong winds that pass through narrow passes or valleys. Two sides of differing pressure funnel the air through resulting in greatly increased wind speed and turbulence.
What means of dispersing clouds are there?
Heating
Mixing
Reduction in Relative Humidity (RH)
What is the latin meaning of Cirrus?
Cirrus
Lock or tuft of hair
What is a low pressure system also known as?
Cyclone
Spin anti-clockwise in the Northern hemisphere.
What is a high pressure system also known as?
Anti-cyclone
Spins clockwise in the Northern Hemisphere.
What are the three types of low?
Thermal low - associated with thunderstorms
Polar low - form over the sea with potential for thunderstorms and snow.
Orthographic (lee) - form on the leeward side of mountains. Produce little weather, but can produce wind and turbulence.
What is a trough?
Indicated by isobars extending out from an area of low pressure. Often V shaped.
What is a front?
Front
Boundary between two air masses having uniform characteristics of moisture and temperature.
What is the definition of Met Vis?
Met Vis
Greatest horizontal distance known unlit objects can be seen and recognised by an observer at ground level (2m) with normal eyesight in normal conditions.
What is the definition of Prevailing Visibility?
Prevailing Visibility
Greatest horizontal distance covering at least 1/2 airfield (in continuous or non-continuous sectors)
What is Min Vis?
Min Vis
When it changes colour state (Military).
When it is 1/2 prevailing and less than 10km (Civil).
What is the definition of a Red weather colour state?
Red
Visibility below 800m
What is the definition of an Amber weather colour state?
Amber
Visbility greater than 800m, less than 1600m
What is the definition of a Yellow 2 weather colour state?
Yellow 2
Visibility greater than 1600m less than 2500m.
What is the definition of a Yellow 1 weather colour state?
Yellow 1
Visibility greater than 2500m less than 3700m.
What is the definition of a Green weather colour state?
Green
Visibility greater than 3700m less than 5000m.
What is the definition of a White weather colour state?
White
Visibility greater than 5000m less than 8000m.
What is the definition of a Blue weather colour state?
Blue
Visibility greater than 8000m.
What is the difference between fog, mist and haze?
Fog - 100% humidity, <1000m, min vis 0m
Mist - >95% humidity, 1000-10000m vis
Haze - <95% humidity, 1000-10000m vis.
What is radiation fog?
Radiation fog
Air will generally need to cool below its dew point to condense into fog. Suitable conditions include high relative humidity, clear night and light wind.
It occurs when the ground cools, so usually happens over night and over land. It requires moist air so that little cooling is needed to reach dew point.
What is Advection Fog?
Advection Fog
Warm moist air moving over a cold surface. Over land occurs when warmer, moist see air meets cooler land. Over sea warm air mvoes from land to cooler sea, the warm sea air flows over cooler currents.
What is needed for the dispersion of Radiation fog?
As the sun heats the air it rises and disperses the fog, clouds may insulate the radiation encouraging dispersal. Increased wind speeds also aids dispersal through lifting it into a low stratus and mixing it with drier air.
What is needed for the dispersion of Advection Fog?
Dispersal of advection fog occurs as a result of drier air moving in the air or by an increase in wind speed which lifts the fog to form stratus cloud.
What is hill fog?
Hill fog
Moist, stable air is forced to rise over high ground. Air cools and condenses forming fog, as with orographic cloud. Vis is usually less than 200m. Hill fog will dissipate over considerable time when upslope wind ceases.
What does the mneumonic WHISTLER refer to?
Thunderstorm Hazards
Weather, heavy precipitation
Hail
Icing
Squall line
Turbulence- downbursts and strong winds
Lightning
Electricity
Rapid pressure changes
What types of cloud are associated with thunderstorms?
TCU, CB, AC Castellanus, Mammatus
How regularly are METARs produced?
Issued at 10 minutes to every hour at military stations.
Issued at 10 minutes to and 20 minutes past each hour at civil stations.
What is the difference between METAR SPEC B, SPEC M and SPEC I?
SPEC B - Improving
SPEC M - Getting worse
SPEC I - No colour state change but change in perhaps pressure.
COR - Correction
How regularly are TAFs issued?
Every 3 hours for miltiry airfields, every 6 hours at civil airfields.
How long is each TAF valid for?
Each TAF is valid for a 30-hour time period, and is updated four times a day.
What is the definition of Dew Point?
The temperature at which the air becomes saturated when at a constant pressure
What is the lowest temperature that water can exist as a liquid in the Earth’s atmosphere?
MS 40 Celsius
If cool air is heated from below what happens to the stability?
It becomes more unstable
If cool air is heated from below what happens to the stability?
It becomes more unstable.
Cloud heights on the Weather Report to ATC are indicated above which datum level?
Ground or Airfield Level
What hazard is usually associated with a thermal low?
Thunderstorms
What hazard is usually associated with a polar low?
Heavy snow
If a warm front has cleared your location, what weather conditions should you expect?
Low cloud, drizzle.
Which type of ice is most adhesive and dense in nature?
Clear Ice
What hazard is usually associated with a high pressure?
Poor visibility (haze, mist, fog)
If the visibility is 1200 m and the RH is 98 %, what weather would be expected?
Mist
What conditions would you expect on the East Coast in Winter with a Pc airmass? (with a long sea track)
Cold with snow showers
In the UK are large TCU and CB liley to be more common around coastal areas at Winter?
Yes
An aircraft is flying with a QNH of 1000 hPa set on the altimeter towards a part of the UK where MSL pressure is 1010 hPa. What is going to happen to the aircraft?
It will ascend by 300 ft.
What are the air cooling (lifting) trigger actions?
Convergence, Convection, Mass Ascent, Orographic Uplift
A cold front has passed over your station. What type of weather would you expect after the front has cleared?
Showers
What is the definition of Dew Point?
The temperature at which the air becomessaturated when cooled at a constant pressure.
What phenomenon is an indicator of thunderstorms?
St Elmo’s fire
For mist (BR) to be reported, what conditions are required in military METARs?
Visibility of 1000m-9999m and RH of at least 95%
When is min vis reported at a Military Airfield?
When it changes the colour state.
What does H stand for in the mneumonic WHISTLE?
Hail
If the ELR was 4 C per 1000 ft what would the stability be?
Unstable
What hazard is usually associated with a thermal low
Thunderstorms
What are the two clouds that can be identified on a METAR or weather report to ATC?
TCU and CB
What conditions would you exoect on the East Coast in winter with a Pc airmass? (with a long sea track)
Cold with snow showers.
An aircraft is flying with a QNH of 1020 hPa set on the altimeter towards a part of the UK where MSL pressure is 1000 hPa. What is going to happen to the aircraft?
It will descend by 600ft
What direction does Coriolis act in the Northern Hemisphere?
Right
With low pressure to the East of the UK, and a high pressure to the West what would the wind direction across the country be?
Northerly
What pressure group is used on a METAR?
QNH
How often are TAFs issued at a military airfield?
Every 3 hours
How long is a METAR trend valid for?
2 hours
In TAFs, what does the change group BECMG indicate?
A gradual but permanent change in one or more elements of the forecast, during the time period shown.
What is an inversion?
A layer where temperature increases with height.
What is another term for a high pressure system?
Anti-cyclone.
What type of wings catch more super cooled water droplets and are therefore more susceptible to icing?
Thin Wings.
A change in state from a solid to a gas is called what?
Sublimation
What type of icing is frost like and is brittle?
Rime Ice
What is the average SALR?
1.5 C per 1000 ft
How does the stability of Tropical Maritime air mass change enroute to the UK?
It becomes stable.
What type of occlusion occurs when cool Pm air behind a cold front catches up with a warm front moving into cold Pc air?
Warm Occlusion
What does ITCZ stand for?
Inter Tropical Convergence Zone
What is the main ocean current that affects the UK?
Gulf stream
What direction are the trade winds in the Northern Hemisphere?
North Easterly.
What conditions would you expect in NW Scotland in a Polar Maritime airmass in Winter?
Unstable, SCT/BKN CU TCU CB, SHRA and TSRA
What is the minimum amount of cloud used to determine an airfield’s colour state?
Scattered SCT
How long is a METAR trend valid for?
2 hours
What are the requirements for CAVOK?
Greater than 10000m vis, NSW, no CB or TCU, no cloud below 5000 ft
What direction does Coriolis act to in the Southern Hemisphere?
Left
If the visibility is 1200 m and the RH is 90% what weather would be reported?
Haze
A cold front has passed over your station, what weather would you expect after the front has cleared?
Showers
The transition in state from solid to a gas is called what?
Sublimation
What type of occlusion occurs when cold Pm air behind a cold front catches up with a warm front moving into not as cold rPm air?
Cold Occlusion
When does Advection Fog occur?
Warm, moist air moving across a cold surface
What does it mean if cloud is coloured green on a cross section?
Temperature in cloud is above freezing
What is the average height of the tropopause at the equator?
55 000 ft
Is latent heat released when water changes state from a liquid to a gas?
No
What is the DALR?
3 C per 1000 ft
What is the definition of Relative Humidity?
The ratio of the water vapour content of the air compared to the maximum that could be held.
Which direction does Coriolis act in the Northern Hemisphere?
Right
What is the name of the wind that blows down into Valleys at night?
Katabatic wind
A katabatic wind is a drainage wind that carries high-density air from a higher elevation down a slope under the force of gravity.
With low pressure to the East of the UK, and high pressure to the West, what would the general wind direction across the country be?
Northerly
Is latent heat released when water changes state from a liquid to a gas?
No
What does 05/05 mean in a METAR?
Temperature 05, Dew Point 05, RH 100%
What is the DALR?
3 C per 1000 ft
What are the air cooling (lifting) trigger actions?
Convergence, Convection, Mass Ascent, Orographic Uplift
A tropical maritime air mass is likely to affect the UK when a low pressure is over the… ?
A low pressure is over the Atlantic
How does the stability of a Polar Maritime airmass change as it comes towards the UK?
It becomes more unstable.
What conditions would you expect in NW Scotland in a Polar Maritime airmass in Winter?
Unstable
SCT/BKN CU TCU CB, SHRA TSRA
What does BKN070 stand for in a METAR or TAF?
5-7 Oktas at 7000 ft
What type of occlusion occurs when cold Pm air behind a cold front, catches up with a warm front moving into warmer rPm air?
Cold occlusion
When is a tropical maritime air mass likely to affect the UK?
When a low pressure is over the Atlantic.
What type of occlusion occurs when cool Pm air behind a cold front catches up with a warm front moving into cold Pc air?
Warm occlusion.
When does advection fog arise?
When warm, moist air moves over a cold surface.
What conditions are required for Radiation Fog to form?
Clear skies, light winds, moist air
What is another term for a high pressure system?
Anti-cyclone
What weather conditions would you expect if a Tropical Maritime airmass was affecting South-West England?
Low cloud, mist, drizzle
What is an extension of low pressure called?
Trough
What does BKN070 stand for in a METAR or TAF?
5-7 Oktas at 7000 ft
What type of precipitation is only associated with CBs?
Hail
What conditions are required for Radiation Fog to form?
Clear skies, light winds, moist air
What conditions are needed for Advection Fog to form?
Warm, moist air moving across a cold surface.
What is the name of the wind that blows up Valleys during the day?
Anabatic Wind
