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.