DGCA CPL Aviation Meteorology key terms

A hypothetical model of the atmosphere used for altimetry and performance calculations, assuming sea-level pressure 1013.25 hPa, temperature 15 degrees C, and a uniform lapse rate of 1.98 degrees C per 1000 ft up to the tropopause.

The altitude indicated on an altimeter when its subscale is set to the standard pressure 1013.25 hPa. It is used for flight levels, performance charts and separation above the transition altitude.

The altimeter subscale setting that, when applied at an aerodrome, causes the altimeter to indicate elevation above mean sea level. It is derived from station pressure reduced to sea level using standard atmosphere assumptions.

The altimeter subscale setting that makes the altimeter read zero on the aerodrome reference point, so it indicates height above the aerodrome rather than altitude above sea level.

The standard pressure setting of 1013.25 hPa used above the transition altitude so that all aircraft share a common datum for flight levels, independent of local pressure changes.

The altitude above an aerodrome at which pilots change the altimeter from the local QNH to the standard pressure setting and begin referring to flight levels instead of altitudes.

The lowest flight level available for use above the transition altitude, chosen so that adequate vertical separation exists between it and the transition altitude for the prevailing pressure.

The boundary between the troposphere and the stratosphere, marked by a sharp change in lapse rate. Its height varies with latitude and season and is where most jet streams and significant turbulence occur.

The lowest layer of the atmosphere, extending from the surface to the tropopause, in which temperature generally decreases with height and where nearly all weather, clouds and water vapour are found.

The rate at which temperature decreases with increasing height in the atmosphere, usually expressed in degrees Celsius per 1000 ft or per kilometre.

The actual measured rate of temperature decrease with height in the surrounding undisturbed air at a given time and place, determined by radiosonde or aircraft sounding.

The rate at which a rising unsaturated parcel of air cools by expansion, approximately 3 degrees C per 1000 ft, independent of the surrounding air temperature.

The rate at which a rising saturated parcel cools, roughly 1.5 to 2 degrees C per 1000 ft, smaller than the dry rate because latent heat is released as water vapour condenses.

Heat energy absorbed or released when water changes state, such as during evaporation or condensation. Release of latent heat on condensation warms rising air and drives convective cloud development.

The tendency of a parcel of air to return to its original level, continue rising, or remain in place after displacement. It depends on the comparison between the environmental lapse rate and the parcel's adiabatic lapse rate.

A layer in which temperature increases with height instead of decreasing. It is a very stable layer that suppresses vertical motion, traps moisture and pollutants below it, and can cause poor visibility.

The temperature to which air must be cooled at constant pressure and moisture content for it to become saturated. A small spread between temperature and dew point indicates high humidity and likely cloud or fog.

The ratio, expressed as a percentage, of the actual water vapour content of the air to the maximum it could hold at that temperature and pressure.

The condition in which air holds the maximum water vapour possible at a given temperature, so that further cooling or addition of moisture causes condensation into cloud droplets or fog.

Cloud formed by rising warm air currents that cool adiabatically and condense, producing cumulus-type clouds with vertical development such as cumulus and cumulonimbus.

A heavy, dense cloud with great vertical extent, often with an anvil top, producing thunderstorms, heavy rain, hail, lightning, severe turbulence and icing. It is the most hazardous cloud to aviation.

A low, uniform grey layer of cloud with a fairly flat base, often producing drizzle or snow grains. When it reaches the ground it becomes fog and can reduce visibility significantly.

A suspension of water droplets in the air at the surface that reduces horizontal visibility below 1000 metres. It forms when air near the ground is cooled to its dew point or gains moisture.

Fog formed on clear, calm nights when the ground cools rapidly by radiation, chilling the air above it to saturation. It is shallow, typically forms over land and usually clears after sunrise.

Fog formed when warm, moist air moves over a colder surface and is cooled from below to its dew point. It can be persistent, occur at sea, and is often associated with warm fronts.

A narrow, fast-moving band of strong winds in the upper troposphere near the tropopause, associated with large temperature gradients and capable of causing severe clear-air turbulence and wind shear.

A change in wind speed or direction over a short distance, either horizontally or vertically. Low-level wind shear near the ground is a serious hazard during approach and departure.

A local daytime wind that blows from the cooler sea towards the warmer land because the land heats more quickly. It reverses at night to become a land breeze.

A local daytime upslope wind caused by air over a heated hillside becoming warmer and less dense than air at the same level over the valley, so it rises up the slope.

A local downslope wind caused by cold, dense air draining down a hillside or valley under gravity, typically at night. It can produce significant low-level wind shear and turbulence.

A large body of air with fairly uniform temperature and humidity characteristics acquired over a source region, such as polar land or tropical ocean, and later modified as it moves.

The boundary zone between two air masses of different temperature and density. Its passage brings characteristic changes in temperature, pressure, wind and weather.

The leading edge of advancing cold air undercutting warmer air. It produces rapid lifting, a narrow band of heavy showers or thunderstorms, gusty winds and a sharp temperature fall after passage.

The leading edge of advancing warm air riding over colder air, producing a broad belt of layered cloud and steady precipitation ahead of the surface front, often followed by poor visibility.

A front formed when a faster-moving cold front catches up with a warm front and lifts the warm air between them completely off the ground, producing complex cloud and precipitation.

A local storm produced by a cumulonimbus cloud, with lightning and thunder and usually heavy rain, hail, gusty winds and severe turbulence. It has cumulus, mature and dissipating stages.

A small, intense downdraught from a thunderstorm that spreads out on hitting the ground, producing a sudden, strong, divergent wind shift that is extremely dangerous during approach and departure.

The accumulation of ice on aircraft surfaces when flying through supercooled water droplets. It adds weight, disrupts airflow and reduces lift, and is most likely in stratiform cloud between 0 and minus 20 degrees C.

Liquid water droplets that remain unfrozen at temperatures below 0 degrees C. When an aircraft strikes them they freeze on impact, causing airframe icing.

Turbulence encountered in cloud-free air, often near jet streams or mountain waves, which is difficult to detect visually or on weather radar and can occur without warning.

An oscillating flow pattern set up downwind of a mountain range when stable air crosses it at speed, producing strong up- and downdraughts, rotor turbulence and lenticular clouds.

A routine coded observation of surface weather at an aerodrome, issued typically every half hour or hour, reporting wind, visibility, weather, cloud and temperature and the QNH.

A special coded aerodrome weather report issued when significant changes in conditions occur between routine METAR observations, such as a sudden drop in visibility or onset of a thunderstorm.

A Terminal Aerodrome Forecast giving the expected weather at an aerodrome over a stated validity period, including wind, visibility, weather, cloud and significant changes with time.

A warning message issued for weather conditions potentially hazardous to all aircraft, such as severe turbulence, severe icing, volcanic ash or severe thunderstorms, over a defined area.

An information message issued for weather phenomena of moderate significance to light aircraft, such as moderate turbulence, moderate icing or widespread low visibility, below SIGMET thresholds.