ATPL Meteorology (050) key terms

A defined reference atmosphere with mean sea level pressure 1013.25 hPa, temperature 15 °C, and a lapse rate of 1.98 °C per 1000 ft up to 36,090 ft. It allows altimeters, aircraft performance, and pressure levels to be compared on a common scale.

The boundary layer between the troposphere and stratosphere where the temperature lapse rate falls to 2 °C per 1000 ft or less. Its height varies with latitude and season and marks the top of most weather and the base of the jet stream region.

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

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 the standard atmosphere.

The altimeter subscale setting that makes the altimeter read zero at the aerodrome reference point, so it indicates height above the aerodrome rather than altitude above mean sea level. Used mainly for aerodrome-level traffic patterns and some approach procedures.

The standard pressure setting of 1013.25 hPa used above the transition altitude, giving flight levels rather than altitudes. The term is also used for the altimeter reading obtained at touchdown with this setting, which varies with actual pressure.

When air is colder than ISA, pressure surfaces are compressed and the altimeter over-reads, so the aircraft is lower than indicated. The error grows with height above the altimeter setting source and is significant for terrain clearance on approach.

When air is warmer than ISA, pressure surfaces are further apart and the altimeter under-reads, so the aircraft is higher than indicated. The error increases with height above the setting source and with the temperature deviation.

The rate at which temperature decreases with increasing height. The environmental lapse rate describes the actual atmosphere, while the dry adiabatic and saturated adiabatic lapse rates describe cooling of rising unsaturated and saturated air respectively.

The actual measured rate of temperature decrease with height in the surrounding atmosphere at a given place and time. Comparing it with the adiabatic lapse rates determines whether the atmosphere is stable, conditionally unstable, or absolutely unstable.

The rate at which rising saturated air cools, roughly 1.5 °C per 1000 ft in warm air and approaching the dry rate in very cold air. Latent heat released by condensation reduces the cooling compared with unsaturated ascent.

A layer in which temperature increases with height, reversing the normal lapse rate. Inversions suppress vertical motion, trap moisture and pollutants beneath them, and are often associated with fog, haze, and low stratus.

A nocturnal inversion formed when the ground cools by outgoing long-wave radiation and chills the air immediately above it. It is strongest on clear, calm nights, is shallow, and disperses after sunrise as the surface warms.

An inversion produced by large-scale descending air in a high-pressure system, which warms adiabatically as it sinks. It typically lies aloft, is marked by a dry layer, and caps convective cloud tops, often producing a sharp top to cumulus fields.

The temperature to which air must be cooled at constant pressure and moisture content for saturation to occur. The spread between temperature and dew point indicates how close the air is to condensation, cloud, or fog.

The ratio of the actual water vapour content of the air to the maximum it could hold at the same temperature, expressed as a percentage. It increases as air cools and decreases as air warms, even if the vapour content is unchanged.

Energy absorbed or released when water changes phase without a temperature change. Evaporation absorbs latent heat and condensation releases it, which is why condensation in rising air warms it and slows its cooling.

The height at which rising air cools to its dew point and condensation begins, forming the base of convective cloud. It is estimated from the surface temperature and dew point spread and lowers as humidity increases.

A condition in which the environmental lapse rate exceeds the dry adiabatic lapse rate, so rising air remains warmer and less dense than its surroundings at every level. It produces vigorous convection, strong gusts, and severe turbulence.

The common state in which the environmental lapse rate lies between the saturated and dry adiabatic rates. Air is stable while unsaturated but becomes unstable once it reaches saturation and releases latent heat, so stability depends on lifting and moisture.

Cloud formed by convection with marked vertical development, such as cumulus and cumulonimbus, consisting mainly of water droplets and often ice at high levels. It is associated with showers, turbulence, icing, and thunderstorm hazards.

Layer cloud formed by large-scale lifting or cooling, such as stratus, altostratus, and cirrostratus. It produces steady precipitation, smooth flying conditions, and reduced visibility, and often indicates a frontal or stable layer.

A deep convective cloud reaching the tropopause, with a glaciated anvil, heavy showers, lightning, hail, and strong up- and downdrafts. It is a major hazard to aviation and should be avoided by a wide margin.

A large body of air with relatively uniform temperature and humidity characteristics acquired over a source region. Its classification as polar, tropical, maritime, or continental, and its modification along its track, determines the weather it brings.

The boundary zone between two air masses of different density, marked by a temperature and humidity change, a pressure trough, and characteristic cloud and precipitation. Frontal weather depends on the front's type, slope, and speed of movement.

The leading edge of advancing warm air overriding colder air, with a shallow slope producing extensive layered cloud and steady precipitation ahead of the surface position. Visibility is often poor and the passage brings a gradual wind veer and pressure fall then rise.

The leading edge of advancing cold air undercutting warmer air, with a steep slope producing a narrow band of cumuliform cloud, showers, and possible thunderstorms. Passage brings a sharp wind veer, temperature fall, pressure rise, and rapid clearing.

A high-pressure system with descending air, diverging surface winds, and generally fine weather. Subsidence aloft creates inversions, low-level haze, and stratus, and light winds in winter favour radiation fog.

A low-pressure system with converging surface winds and ascending air, producing cloud, precipitation, and often strong winds. Frontal depressions form and deepen along the polar front and move with the upper flow.

An elongated area of relatively low pressure between two highs, shown as a kink in the isobars. It is associated with convergence, ascending air, cloud, and showers, and often extends from a depression centre.

An elongated area of relatively high pressure between two lows, shown as a kink in the isobars. It brings descending air, generally settled weather, and a wind veer as it passes.

A daytime circulation in which air over the warmer land rises and cooler air flows inland from the sea. It forms a shallow front near the coast, may trigger showers, and reverses at night as a land breeze.

A warm, dry downslope wind on the lee side of a mountain range, produced when moist air is forced over the barrier, loses moisture on the windward side, and warms adiabatically during descent. It brings turbulence, wind shear, and rapid temperature rises.

A narrow band of strong upper-tropospheric winds near the tropopause, associated with strong temperature gradients. It steers frontal systems, causes clear air turbulence at its edges, and provides significant tailwind or headwind benefits.

Turbulence occurring in cloud-free air, typically in strong wind shear zones near the jet stream, at the tropopause, or in mountain wave flow. It is invisible to weather radar and is a hazard to aircraft in cruise.

The accumulation of ice on aircraft surfaces from supercooled water droplets in cloud or precipitation. It degrades lift, adds weight, and can block pitot or static sources, with clear ice and freezing rain being especially hazardous.

A localized, intense downdraft from a thunderstorm that spreads out near the surface, producing a headwind then tailwind shear over a short distance. It is extremely hazardous on approach and departure and may occur in apparently dry conditions.

A rapid change of wind speed or direction over a short distance at low altitude, caused by fronts, thunderstorms, inversions, or buildings. It can cause sudden airspeed loss close to the ground and is reported in METAR and SIGMET as a warning.

Fine abrasive particles ejected by an eruption and carried downwind in invisible or hazy clouds. It can cause engine flameout, windscreen and pitot damage, and is reported by SIGMET and volcanic ash advisory centres.

A routine coded observation of surface weather at an aerodrome, including wind, visibility, cloud, temperature, dew point, and pressure, issued typically at half-hourly or hourly intervals. It may include a trend forecast for the following two hours.

A coded forecast of aerodrome weather covering a stated period, typically 9 to 30 hours, giving expected wind, visibility, cloud, and significant weather with temporary or probability groups. It is used for flight planning and alternate selection.

A warning message of significant meteorological conditions affecting flight safety, such as severe turbulence, icing, thunderstorms, volcanic ash, or tropical cyclones, over a large area. It is issued for the safety of all aircraft in the area.

An upper-air chart depicting fronts, pressure systems, jet streams, tropopause height, turbulence, icing, and significant cloud for a stated level and validity time. It is used to plan routes and levels and to anticipate hazards.