CPL Meteorology flashcards

32 cards. Read the term and the definition here. The deck at the top of this page is optional.

ISA sea-level conditions

International Standard Atmosphere at mean sea level: temperature 15 °C, pressure 1013.25 hPa, density 1.225 kg/m³, with a lapse rate of 1.98 °C per 1000 ft up to 36,090 ft.

Homosphere and heterosphere

The homosphere, below about 80 km, has well-mixed gases of near-constant proportions. Above the heterosphere boundary, diffusive separation makes the composition vary with height, with lighter gases dominating at great altitude.

ISA tropopause

In the standard atmosphere the tropopause is at 36,090 ft (11 km) with a temperature of -56.5 °C. Above it the temperature is isothermal to 20 km before rising through the stratosphere.

Troposphere characteristics

Lowest atmospheric layer, extending to the tropopause, containing about three-quarters of atmospheric mass and nearly all water vapour. Temperature decreases with height and it is the layer where weather occurs.

Density altitude

Pressure altitude corrected for non-standard temperature. It increases with high elevation, high temperature, high humidity and low pressure, degrading take-off, climb and engine performance.

High-to-low altimeter error

Flying from an area of high pressure into lower pressure without resetting the subscale makes the aircraft fly lower than the altimeter indicates, a hazard near terrain.

Temperature error in altimetry

Flying from warm air into cold air without resetting the altimeter causes true altitude to be lower than indicated, because cold air compresses the pressure levels toward the surface.

QNH, QFE and QNE

QNH set on the subscale reads aerodrome elevation on the ground. QFE reads height above a reference datum. QNE is the standard setting 1013.25 hPa used above the transition level.

Relative humidity

The ratio of actual water vapour pressure to saturation vapour pressure at the same temperature, expressed as a percentage. It decreases as temperature rises if the moisture content is unchanged.

Dry and saturated adiabatic rates

Unsaturated rising air cools at about 3 °C per 1000 ft. Saturated air cools more slowly, about 1.5 °C per 1000 ft, because latent heat is released by condensation.

Absolute and conditional instability

Air is absolutely unstable when the environmental lapse rate exceeds the dry adiabatic rate, absolutely stable when below the saturated rate, and conditionally unstable in between.

Lifted condensation level

The height at which rising air, cooling adiabatically, reaches saturation and cloud begins to form. Convective cumulus cloud bases typically form at this level.

Radiation fog

Fog formed over land on clear, calm nights when rapid radiational cooling lowers the surface air to its dew point. It usually dissipates after sunrise with surface heating.

Advection fog

Fog formed when warm, moist air moves over a colder surface and is cooled below its dew point. It can persist in wind and is common over the sea and coasts.

Cloud families by height

High cloud above about 20,000 ft includes cirrus and cirrostratus; middle cloud between 6,500 and 20,000 ft includes altostratus; low cloud below 6,500 ft includes stratus and stratocumulus.

Cumuliform versus stratiform cloud

Cumuliform clouds, such as cumulus and cumulonimbus, indicate unstable air with vertical development. Stratiform clouds, such as stratus and nimbostratus, indicate stable air with layered, widespread cloud.

Cold front passage

A fast-moving steep front bringing a narrow band of heavy showers, squalls and cumulonimbus, followed by rapid clearing, a temperature drop and a veer of wind.

Warm front passage

A slow-moving shallow front with extensive stratiform cloud and steady rain ahead, poor visibility and low cloud, followed by gradual clearing and a wind veer after passage.

Occluded front

Formed where a faster cold front overtakes a warm front and lifts the warm air entirely off the surface. Weather depends on whether it is a cold or warm occlusion.

Air mass source regions

Regions where air acquires uniform temperature and humidity: maritime tropical warm and moist, continental tropical hot and dry, maritime polar cool and moist, continental polar cold and dry.

Geostrophic wind

Wind flowing parallel to straight isobars when pressure gradient force balances Coriolis force, above the friction layer. In the Southern Hemisphere low pressure lies to its left.

Sea breeze and land breeze

Daytime sea breeze blows from cooler sea to warmer land as land heats faster. At night the land breeze reverses, blowing from cooler land out to sea.

Katabatic and anabatic winds

Anabatic winds are daytime upslope flows on sun-warmed terrain. Katabatic winds are cold, dense downslope flows draining into valleys at night, often producing low-level shear.

Mountain wave indicators

Lenticular cloud, cap cloud and rotor cloud on the lee of a ridge indicate strong mountain waves, with severe turbulence and downdraughts hazardous to light aircraft.

Microburst escape

On encountering a microburst, apply maximum power, pitch to maintain control and accept a temporary airspeed loss. Do not chase the airspeed indicator or try to climb back over the downdraught.

Clear ice versus rime ice

Clear ice forms from large supercooled droplets freezing slowly and spreading, giving heavy, hard, transparent accretion. Rime ice forms from small droplets freezing instantly, giving rough, milky, lighter accretion.

Carburettor icing conditions

Carburettor icing can occur at ambient temperatures between about -10 °C and +30 °C with high relative humidity, and is most severe in humid air at moderate temperatures.

Clear-air turbulence

Turbulence in cloud-free air, most likely near jet streams with strong wind shear and near mountain waves. It is not detectable by airborne weather radar.

METAR cloud and visibility coding

Cloud amount is coded FEW, SCT, BKN or OVC with height in hundreds of feet above aerodrome level. Visibility 9999 means 10 km or more; VRB means variable wind direction.

TAF change groups

A terminal aerodrome forecast uses BECMG for gradual persistent change, TEMPO for temporary fluctuations, and probability groups to describe expected variations within the validity period.

SIGMET and AIRMET

SIGMET advises of significant weather hazardous to all aircraft, such as severe turbulence, icing, thunderstorms and volcanic ash. AIRMET covers less severe phenomena affecting mainly light aircraft.

Surface synoptic chart

A chart of mean sea level pressure with isobars drawn at 4 hPa intervals, pressure centres and fronts. Closely spaced isobars indicate a strong pressure gradient and strong wind.