DGCA ATPL Aviation Meteorology key terms

A hypothetical reference atmosphere agreed internationally, with 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. Aircraft performance and altimeter calibration are based on it.

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 the core of jet streams.

The lowest layer of the atmosphere, containing about 75 percent of its mass and nearly all water vapour. Weather, clouds and most turbulence occur here, and temperature falls with height at roughly 2 °C per 1000 ft.

The rate at which temperature decreases with increasing height. The environmental lapse rate describes the actual atmosphere, while dry and saturated adiabatic rates describe rising air parcels and determine stability.

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

The tendency of air, once displaced vertically, to return to its original level (stable) or continue moving away (unstable). Stability governs cloud type, shower activity, turbulence and the likelihood of thunderstorms.

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

The ratio, expressed as a percentage, of the actual water vapour in the air to the maximum it could hold at that temperature. It rises as air cools and falls as air warms, even if moisture content is unchanged.

The state 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.

The change of water vapour into liquid water when air is cooled to saturation, usually onto condensation nuclei. It forms cloud droplets, fog and dew and releases latent heat that can strengthen rising air.

Heat energy absorbed or released when water changes phase, without changing temperature. Released on condensation and freezing, it warms rising air and energises convective clouds and thunderstorms.

The altitude indicated when the altimeter subscale is set to 1013.25 hPa. It is used for flight levels above the transition altitude and for comparing aircraft performance.

The altimeter subscale setting that makes the instrument read aerodrome elevation when the aircraft is on the ground there. It is derived from station pressure reduced to mean sea level using standard atmosphere assumptions.

The altimeter subscale setting that makes the instrument read zero when the aircraft is on the ground at the reference point. It gives height above that point rather than altitude above mean sea level.

The standard pressure setting of 1013.25 hPa used above the transition altitude, so the altimeter reads flight level rather than altitude. It provides a common reference for all aircraft at high level.

The altitude, published for each aerodrome or area, above which pilots set the standard pressure 1013.25 hPa and fly flight levels. Below it the local QNH is used.

A line on a weather chart joining places of equal mean sea level pressure. Spacing indicates pressure gradient and hence wind strength, while shape reveals highs, lows, ridges and troughs.

The rate of change of pressure over horizontal distance, directed from high to low pressure. The steeper the gradient, the stronger the resulting wind, as shown by closely spaced isobars.

A theoretical wind that blows parallel to straight isobars when the pressure gradient force balances the Coriolis force, above friction layer. Real upper winds approximate it, and it strengthens as isobars pack closer.

An apparent force caused by the Earth's rotation that deflects moving air to the right in the Northern Hemisphere and to the left in the Southern. It turns wind along isobars and shapes global circulation.

A rule stating that in the Northern Hemisphere, with your back to the wind, lower pressure lies on your left. In the Southern Hemisphere the reverse applies. It helps locate lows from surface wind.

A region of closed isobars around lower pressure than its surroundings, with air converging and rising. It brings cloud, precipitation and often strong winds, and may deepen or fill over time.

A large body of air with fairly uniform temperature and humidity acquired over a source region such as ocean or desert. Its properties and the weather it brings change as it moves over new surfaces.

The boundary zone between two air masses of different temperature and density. Warm, cold, occluded and stationary fronts each produce characteristic cloud sequences, precipitation and wind shifts.

The leading edge of advancing cold air that undercuts warm air, producing a steep slope, cumuliform cloud, showers and often squally winds. Passage brings a sharp temperature fall and pressure rise.

The boundary where advancing warm air rides over retreating cold air along a shallow slope. It gives a long sequence of layered cloud, steady rain and gradual temperature rise over many hours.

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 rain bands and often marks the mature stage of a depression.

Cumulus and cumulonimbus clouds formed by rising thermals in unstable air. They produce showers, turbulence, icing and sometimes thunderstorms, and are typically embedded in cold air masses.

A cumulonimbus cloud system with lightning and thunder, requiring moisture, instability and a lifting trigger. It brings severe turbulence, icing, hail, microbursts and wind shear hazardous to aircraft.

A small, intense downdraught that spreads out on reaching the surface, producing a sudden headwind then tailwind change. It can cause severe low-level wind shear during approach or departure.

A change of wind speed or direction over a short distance, either horizontally or vertically. Low-level shear on approach can cause sudden airspeed and lift changes and is a major hazard near thunderstorms and fronts.

Irregular eddying air motion that causes aircraft to be buffeted. It may be mechanical from surface friction or terrain, thermal from convection, or clear air turbulence near jet streams.

The build-up of ice on airframe surfaces when flying through supercooled water droplets. It increases weight and drag, reduces lift and can impair controls, with severe risk in freezing rain and cumuliform cloud.

The height at which air temperature is 0 °C, above which icing from supercooled droplets becomes possible. It varies with season, latitude and air mass and is significant for icing and precipitation type.

A suspension of water droplets near the surface reducing visibility below 1000 m. It forms by cooling such as radiation or advection, or by adding moisture, and is a major cause of low-visibility operations.

A narrow, fast-moving band of upper-level wind near the tropopause, often thousands of kilometres long. It affects flight time and fuel, causes clear air turbulence and steers surface pressure systems.

The seasonal reversal of winds over South Asia, bringing moist south-westerlies and heavy rain in summer and dry north-easterlies in winter. It strongly affects aviation weather across the Indian subcontinent.

A routine coded observation of surface weather at an aerodrome, issued usually hourly. It reports wind, visibility, weather, cloud, temperature, dew point and pressure, and is essential for flight planning.

A coded forecast of expected weather at an aerodrome for a specified period, typically 9 to 30 hours. It includes wind, visibility, weather and cloud with temporary and change groups.

A coded advisory warning of significant hazardous weather such as severe turbulence, icing, thunderstorms or volcanic ash affecting a flight information region. It is issued for the safety of en-route aircraft.

A special coded observation issued when specified significant changes in weather occur between routine METAR reports, such as a sudden visibility or cloud base change. It keeps pilots informed of rapid deterioration or improvement.

The greatest distance at which prominent objects can be seen and identified, or measured instrumentally. It is reported in METAR and TAF and is reduced by fog, mist, precipitation, dust and haze.