ATPL Meteorology (Sri Lanka CAA Pilot Licence Examinations) key terms

A reference model of the atmosphere in which sea-level pressure is 1013.25 hPa, sea-level temperature is 15 °C, and temperature falls 1.98 °C per 1000 ft up to 36,090 ft. Used to calibrate altimeters and compare aircraft performance.

The boundary layer between the troposphere and stratosphere where the lapse rate becomes nearly isothermal. Its height varies from about 25,000 ft at the poles to 55,000 ft over the equator and is highest over the tropics.

A marked step or discontinuity in tropopause height, typically found where subtropical and polar tropopauses meet above a jet stream. It marks a zone of strong wind shear and possible clear air turbulence.

The rate at which temperature decreases with height in a given layer of air. The environmental lapse rate is measured directly, while dry and saturated adiabatic rates describe rising parcels.

The rate at which an unsaturated parcel cools as it rises, about 3 °C per 1000 ft (1 °C per 100 m). It applies only while the parcel remains unsaturated and is constant with height.

The rate at which a saturated rising parcel cools, about 1.5 °C per 1000 ft near sea level and approaching the dry rate aloft. It is smaller than the dry rate because latent heat is released.

Energy absorbed or released when water changes phase. Evaporation absorbs heat and cools the air, while condensation and freezing release heat and warm it, which drives convective cloud development.

A condition where the environmental lapse rate exceeds both the dry and saturated adiabatic rates, so any rising parcel stays warmer than its surroundings. It favours strong convection and thunderstorm development.

The common state where the environmental lapse rate lies between the dry and saturated adiabatic rates. A parcel is stable while unsaturated but becomes unstable once it reaches saturation and continues rising freely.

The height at which a rising parcel reaches saturation and cloud base forms, found where the dry adiabat through the surface temperature crosses the mixing ratio line. It approximates cumulus cloud base.

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

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

The mass of water vapour contained in a unit mass of dry air, usually expressed in grams per kilogram. It is conserved during adiabatic ascent and descent, making it useful on tephigrams.

The altimeter setting that, when set on the subscale, makes the altimeter read elevation above mean sea level at the aerodrome. It is used below the transition altitude for terrain clearance.

The pressure setting that makes an altimeter read height above a specified reference point, normally the aerodrome. It is used in some countries for traffic pattern and approach work.

The standard setting of 1013.25 hPa used above the transition altitude so all aircraft share a common pressure datum. Altimeters then indicate flight level rather than altitude.

The altitude indicated by an altimeter set to 1013.25 hPa. It differs from true altitude by the pressure error and is used for flight levels and aircraft performance calculations.

The actual vertical distance of an aircraft above mean sea level. It equals indicated altitude corrected for instrument error, pressure error and temperature error.

The altimeter error caused by departure of actual temperature from ISA. In air colder than standard the altimeter over-reads and true altitude is lower than indicated, a serious terrain-clearance hazard.

An adjustment added to published minimum flight altitudes and approach procedure altitudes when operating in temperatures below ISA, so that true terrain clearance is maintained. It applies mainly in mountainous or very cold regions.

The altitude above an aerodrome at which pilots change the altimeter from QNH to standard pressure for flight levels. It varies by country and is published on approach charts.

A surface of constant pressure shown on an altimeter set to 1013.25 hPa, expressed in hundreds of feet. Levels are separated by standard pressure intervals rather than true height.

A line on a weather chart joining places of equal mean sea-level pressure. Close spacing indicates a steep pressure gradient and therefore strong wind.

The force that pushes air from higher toward lower pressure, directed perpendicular to the isobars. Its strength depends on how closely the isobars are spaced.

The theoretical wind that blows parallel to straight isobars when the pressure gradient force is exactly balanced by the Coriolis force. It applies in the free atmosphere above friction layer influence.

The wind that flows parallel to curved isobars when pressure gradient, Coriolis and centrifugal forces balance. Around a low it is weaker than the geostrophic value, around a high it is stronger.

The 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 increases with latitude and wind speed.

In the Northern Hemisphere, with your back to the wind, low pressure lies to your left and high pressure to your right. In the Southern Hemisphere the relationship is reversed.

The lowest part of the atmosphere, roughly the first 2000 to 3000 ft, where surface friction slows the wind and turns it across the isobars toward low pressure. Winds here are weaker than aloft.

The wind reported at an aerodrome, measured at about 10 m above ground. It is backed and reduced compared with the free-atmosphere wind because of friction and boundary-layer effects.

A change of wind speed or direction over a short distance, either horizontally or vertically. Strong low-level shear can be hazardous on approach and may indicate microburst or frontal activity.

A narrow, fast-moving band of wind near the tropopause, typically exceeding 60 kt, associated with strong temperature gradients. It steers frontal systems and offers favourable tailwinds or severe headwinds.

Turbulence occurring in cloud-free air, often at jet-stream level near strong wind shear or tropopause breaks. It cannot be seen on radar and is a hazard to high-altitude flight.

Ice accumulation on aircraft surfaces when flying through supercooled water droplets at temperatures between 0 °C and about −20 °C. It adds weight, reduces lift and may disrupt control surfaces.

Liquid water droplets that remain unfrozen below 0 °C. They freeze on impact with an aircraft, making them the primary cause of airframe icing in stratiform and convective cloud.

Rain that falls as liquid but freezes on contact with surfaces at or below 0 °C. It produces rapid severe icing and signals a warm layer aloft above a sub-freezing surface layer.

A localized downdraft from a thunderstorm that spreads out at the surface over a small area, causing sudden strong wind shifts. It is a severe low-level wind shear hazard on approach and departure.

A strong downdraft from a convective storm that spreads outward on reaching the ground. It produces damaging surface winds and hazardous wind shear, and may occur in dry or wet form.

The leading edge of a cooler air mass advancing under warmer air. It produces a narrow band of showery cloud and rain, often with thunderstorms, followed by a rapid drop in temperature and pressure rise.

The leading edge of a warm air mass overriding cooler air. It brings a broad, shallow band of layered cloud and steady precipitation, with a gradual fall in pressure and rise in temperature.

A front formed when a cold front overtakes a warm front and lifts the warm air completely off the surface. It produces complex cloud and precipitation, often heaviest near the occlusion point.

A low-pressure system that develops along a frontal boundary in mid-latitudes, with a warm and cold front meeting at a low centre. It moves generally eastward with the upper flow.

A large body of air with broadly uniform temperature and humidity characteristics acquired over a source region. Its classification, such as polar maritime or tropical continental, predicts the weather it brings.

The area over which an air mass acquires its temperature and moisture characteristics, such as a cold continent or a warm ocean. Movement away from it gradually modifies the air mass.

A deep convective cloud reaching great vertical extent, often with an anvil top. It produces thunderstorms, heavy showers, hail, turbulence and icing, and is a major aviation hazard.

A low, uniform grey layer of cloud that may produce drizzle or snow grains and often reduces visibility. It forms in stable, moist air and may cover hills as fog.

A suspension of water droplets reducing horizontal visibility below 1 km at ground level. Radiation fog forms by nocturnal cooling, while advection fog forms when moist air moves over a colder surface.

Fog formed on clear, calm nights when the ground cools rapidly and chills the air above to its dew point. It is shallow, forms near dawn and usually disperses after sunrise.

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

A coded forecast of expected surface weather at an aerodrome, valid typically for 9 to 30 hours. It uses similar codes to METAR and includes temporary and probability change groups.

A special coded observation issued when significant weather changes occur between routine reports, such as a sudden drop in visibility or onset of a thunderstorm. It is important for operational decisions.

A forecast chart showing hazardous weather for aviation, including fronts, jet streams, turbulence, icing and cumulonimbus. It is used for pre-flight planning at upper levels.