Brunei DCA CPL(A) Meteorology key terms

An agreed model of the atmosphere in which sea-level pressure is 1013.25 hPa, sea-level temperature is 15 degrees Celsius, and temperature falls at 1.98 degrees Celsius per 1000 feet up to 36,090 feet.

The boundary layer between the troposphere and stratosphere where the lapse rate becomes nearly zero; its height varies from about 55,000 feet near the equator to 35,000 feet or lower near the poles.

The rate at which air temperature decreases with increasing height. The standard average is about 2 degrees Celsius per 1000 feet, but actual rates vary with air mass and local conditions.

The actual measured rate of temperature decrease with height in the surrounding undisturbed air at a given place and time, used to judge whether that air is stable or unstable.

The altitude in the standard atmosphere at which a given air density would occur. High temperature, high humidity and low pressure all raise density altitude and degrade aircraft and engine performance.

The altitude indicated on an altimeter set to 1013.25 hPa. It is used above the transition altitude and for flight levels, and differs from true altitude when pressure differs from standard.

The altimeter setting that, when set on the subscale, causes the altimeter to indicate elevation above mean sea level at the aerodrome reference point. It is used below the transition altitude.

The altimeter setting that causes the altimeter to read zero at the aerodrome reference point, so it indicates height above that point rather than altitude above mean sea level.

The temperature to which air must be cooled at constant pressure and moisture content for saturation to occur. A small gap between temperature and dew point indicates high relative humidity.

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

Air holding the maximum water vapour possible at its current temperature and pressure. Further cooling then produces condensation into cloud, fog or dew.

The rate at which a rising or sinking parcel of unsaturated air changes temperature, about 3 degrees Celsius per 1000 feet, caused solely by expansion or compression.

The rate at which a rising parcel of saturated air cools, roughly 1.5 degrees Celsius per 1000 feet near the surface and less at higher levels, because latent heat is released as vapour condenses.

Energy absorbed or released when water changes state. Condensation releases heat that warms rising cloud air and helps drive deep convection in the tropics.

A condition in which the environmental lapse rate exceeds the dry adiabatic lapse rate, so a displaced parcel keeps accelerating away from its original level whether or not it is saturated.

The common state in which the environmental lapse rate lies between the saturated and dry adiabatic rates, so air is stable while unsaturated but becomes unstable once lifted to saturation.

The height at which rising air cools to its dew point and cloud begins to form. It is estimated from the surface temperature and dew point spread.

Cloud formed by rising thermals in an unstable layer, appearing as cumulus and growing into cumulonimbus when the unstable layer is deep and moist.

A deep, vertically developed cloud with a heavy shower or thunderstorm, often an anvil top, severe turbulence, icing and strong gusty surface winds beneath it.

Air in which a lifted parcel cools faster than its surroundings and tends to sink back, giving layered cloud, smooth conditions and restricted vertical mixing.

A low, uniform grey cloud layer with a flat base, often producing drizzle or light rain and reduced visibility when it reaches the surface as fog.

Cloud in contact with the ground that reduces visibility below 1000 metres. It forms mainly by cooling air to its dew point or by adding moisture to it.

A large body of air with fairly uniform temperature and humidity acquired over a source region, such as polar continental or maritime tropical, that later affects the weather it crosses.

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

The near-equatorial belt where the northeast and southeast trade winds meet, marked by deep convection, heavy showers and frequent thunderstorms, shifting with the seasons.

Persistent easterly surface winds in the tropics, blowing from subtropical high pressure toward the equatorial trough and deflected by the Coriolis effect.

A local daytime circulation in which cooler air flows inland from the sea toward heated land, often bringing a wind shift and a shallow inland convergence line.

A local night-time circulation in which cooler air flows from land out over the warmer sea, typically weaker than the daytime sea breeze.

A narrow, fast-moving band of upper-tropospheric wind near the tropopause, associated with strong wind shear, clear-air turbulence and rapid changes in flight time.

A change of wind speed or direction over a short distance or time, vertically or horizontally. It is a major hazard at low level during take-off and landing.

A small, intense downdraught from a thunderstorm that spreads out at the surface, producing sudden headwind then tailwind changes and severe low-level wind shear.

A line of active thunderstorms, often ahead of a cold front, producing severe turbulence, heavy rain, hail and strong gusty winds along a narrow band.

Turbulence in cloud-free air, usually linked to strong wind shear near jet streams or mountain waves, that is hard to detect visually and can be severe.

Ice accumulation on aircraft surfaces when flying through supercooled water droplets, degrading lift, increasing drag and weight and affecting instruments and engine intakes.

Liquid water droplets that remain unfrozen below 0 degrees Celsius. They freeze on impact with an aircraft and are the main cause of airframe icing.

The height at which air temperature is 0 degrees Celsius. Above it, icing risk from supercooled droplets increases, and it is reported in aviation forecasts.

A routine coded observation of surface weather at an aerodrome, giving wind, visibility, weather, cloud, temperature, dew point and pressure, issued usually hourly.

A coded forecast of expected conditions at an aerodrome over a stated period, including wind, visibility, weather and cloud, with temporary and probability groups.

A coded warning of hazardous en-route weather such as severe turbulence, severe icing, volcanic ash, tropical cyclone or severe squall line, issued for a defined flight information region.