ATPL Meteorology key terms

A defined reference model of how temperature, pressure and density change with height, used to calibrate altimeters and aircraft performance. It assumes sea-level values and fixed lapse rates up to the tropopause.

The difference between the actual temperature at a given level and the standard atmosphere temperature for that level. Positive values mean warmer than standard, which affects density altitude and true altitude indications.

The boundary layer between the troposphere and stratosphere where temperature normally stops decreasing with height. Its height varies with latitude and season and marks the top of most weather and jet streams.

The lowest atmospheric layer, containing most of the air mass, water vapour and all significant weather. Temperature decreases with height within it, driving convection and cloud formation.

The rate at which temperature decreases with increasing height. It is used to assess stability, cloud development and the likelihood of turbulence or icing at different levels.

The actual measured rate of temperature decrease with height in the surrounding atmosphere at a given time and place. Comparing it with adiabatic rates determines stability.

The rate at which an unsaturated parcel of air cools as it rises, approximately 3 degrees Celsius per 1000 feet. It applies until the parcel reaches its condensation level.

The rate at which a saturated rising parcel cools, smaller than the dry rate because latent heat is released as vapour condenses. It varies with temperature and pressure, typically around 1.5 to 2 degrees Celsius per 1000 feet.

A condition where the environmental lapse rate is less than the saturated adiabatic lapse rate, so a displaced parcel returns to its original level. Air is resistant to vertical motion and clouds tend to be layered.

A state where the environmental lapse rate lies between the dry and saturated adiabatic rates. Stable for unsaturated air but unstable once saturation and latent heat release occur, favouring showers and thunderstorms.

Instability produced when a whole layer is lifted and its lower part becomes saturated while the upper part remains dry. The resulting steep lapse rate supports deep convection and cumulonimbus development.

The height at which a rising parcel of air cools to its dew point and water vapour begins to condense. It approximates the base of convective cloud.

The temperature to which air must be cooled at constant pressure for saturation to occur. A small spread between temperature and dew point indicates high humidity, low cloud base and fog risk.

The ratio of the actual water vapour in the air to the maximum it could hold at that temperature, expressed as a percentage. It changes with temperature even when the moisture content stays constant.

The condition in which air holds the maximum water vapour possible at a given temperature and pressure. Further cooling or added moisture then produces condensation into cloud, fog or dew.

Energy absorbed or released when water changes phase, such as during evaporation or condensation. Release on condensation warms rising air and strengthens convective clouds.

The altitude indicated when the altimeter subscale is set to the standard pressure of 1013.25 hectopascals. It is used for flight levels and performance calculations rather than terrain clearance.

The altitude in the standard atmosphere at which air density equals the actual density at the aircraft's position. High temperature, high elevation and low pressure raise it and degrade performance.

The altitude at or below which aircraft set the altimeter to local pressure and above which they use standard pressure. It varies by location and is published on approach and aerodrome charts.

The altimeter setting that, when entered on the subscale, makes the altimeter read aerodrome elevation when the aircraft is on the ground. It provides height above mean sea level.

The altimeter setting that makes the altimeter read zero when the aircraft is at aerodrome level. It is used for height above the aerodrome rather than altitude above sea level.

The leading edge of an advancing colder air mass that undercuts warmer air. It typically brings a narrow band of heavy showers, squalls and a sharp wind veer, often clearing quickly.

The boundary where advancing warm air rides over retreating colder air. It produces extensive layered cloud, steady precipitation and a gradual temperature rise over a wide area.

A front formed when a faster cold front catches a warm front and lifts the warm sector clear of the surface. It combines cloud and precipitation characteristics of both front types.

A large body of air with broadly uniform temperature and humidity acquired over its source region. Its properties and modification during travel determine the weather it brings.

A deep convective cloud reaching the tropopause, producing heavy showers, hail, lightning, severe turbulence and icing. It is a major hazard and must be avoided by a wide margin.

A thick, layered cloud of great vertical extent producing continuous moderate or heavy precipitation. It is typical of warm fronts and often hides the sun completely.

A low, uniform grey layer of cloud with a fairly flat base that may produce drizzle. It often forms in stable air and can lower to become fog.

A cloud at the surface reducing visibility below 1000 metres. It forms by cooling air to its dew point or by adding moisture, and is a major cause of low-visibility approach minima.

The accumulation of ice on aircraft surfaces when flying through supercooled water droplets. It alters aerodynamics and adds weight, and clear ice can be especially hazardous.

Liquid water droplets that remain unfrozen at temperatures below zero degrees Celsius. They freeze rapidly on impact with an aircraft and are the main source of airframe icing.

The height at which the air temperature falls to zero degrees Celsius. It indicates where icing becomes possible and is a key figure in forecasts and flight planning.

A narrow, fast-moving band of strong winds near the tropopause, often associated with strong temperature gradients. It influences flight time, fuel planning and clear air turbulence.

Turbulence encountered in cloud-free air, often near jet streams or mountain waves, which cannot be seen visually. It is a hazard because it is undetectable by weather radar.

An oscillating airflow set up downwind of a mountain barrier in stable, strong winds. It produces severe turbulence, lenticular cloud and sometimes violent downdraughts.

A convective storm producing lightning and thunder, typically requiring moisture, instability and a lifting mechanism. It progresses through cumulus, mature and dissipating stages and poses multiple aviation hazards.

A change in wind speed or direction over a short distance or time, including vertical and horizontal variations. It can cause sudden airspeed changes and is critical during take-off and landing.

A small, intense downdraught that spreads out on reaching the ground, producing strong divergent winds. It can cause severe and sudden loss of airspeed close to the ground.

A routine coded observation of weather at an aerodrome, issued at fixed intervals. It reports wind, visibility, cloud, temperature, dew point and pressure, and may include trend forecasts.

A coded forecast of aerodrome weather covering a stated period, usually with validity of several hours or more. It predicts wind, visibility, weather and cloud, with temporary and probability groups.

A warning message issued for hazardous weather such as severe turbulence, icing, thunderstorms or volcanic ash that may affect flight safety along a route. It is issued for defined areas and periods.

Fine rock particles ejected by a volcano that can damage engines and airframes and reduce visibility. It is tracked by advisory centres and avoided by aircraft.