NCAA CPL Meteorology key terms

An internationally agreed model of the atmosphere in which temperature falls 6.5 °C per km up to 11 km, sea-level pressure is 1013.25 hPa and sea-level density is 1.225 kg/m³.

The altimeter subscale setting that makes the altimeter indicate elevation above mean sea level at the airfield when the aircraft is on the ground.

The altimeter subscale setting that makes the altimeter read zero, or field elevation, when the aircraft is on the ground at a specific aerodrome.

The altitude in the International Standard Atmosphere at which the air density equals the actual density at a given place and time; high values reduce aircraft and engine performance.

The actual rate at which the surrounding air temperature decreases with height at a particular time and place, measured by radiosonde or aircraft and compared with adiabatic rates.

The temperature to which a parcel of air must be cooled at constant pressure for it to become saturated, with further cooling producing condensation.

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

The height at which a rising parcel of unsaturated air cools to its dew point and cloud begins to form, often taken as the convective cloud base.

Energy released when water vapour changes into liquid water; in rising air this warming slows the cooling rate and can strengthen instability and thunderstorm growth.

The rate at which a rising or sinking parcel of unsaturated air changes temperature by expansion or compression, about 9.8 °C per kilometre, without exchanging heat with its surroundings.

A state in which the atmosphere is stable for unsaturated rising air but unstable once the air becomes saturated, so instability depends on whether condensation and latent heat release occur.

The height above ground of the lowest cloud layer reported as broken or overcast, or of the lowest obscuring phenomenon, used to express aerodrome cloud conditions.

A deep, vertically developed cloud, often with an anvil, producing heavy showers, lightning, hail, strong gusts and severe turbulence; it is the main thunderstorm cloud.

A low, uniform grey cloud layer with a fairly flat base that may produce drizzle, reduce visibility and form at the surface as fog when the base reaches ground.

The theoretical wind that blows parallel to straight, widely spaced isobars when the horizontal pressure-gradient force is exactly balanced by the Coriolis force, above the friction layer.

The wind that flows parallel to curved isobars when pressure-gradient, Coriolis and centrifugal forces balance; it describes flow around highs and lows above friction.

An apparent force caused by Earth's rotation that deflects moving air to the right in the Northern Hemisphere and to the left in the Southern Hemisphere, increasing with latitude and speed.

A change of wind speed or direction between two points in space, or over time, which can cause abrupt changes in aircraft lift and airspeed.

A narrow band of relatively strong wind in the lower atmosphere, often at night over West Africa, which can cause significant low-level wind shear during take-off and landing.

A large body of air with fairly uniform temperature and humidity in its lower layers, acquiring its properties over a source region such as desert, ocean or polar land.

The leading edge of advancing cold air that undercuts warm air, producing a narrow band of showers, thunderstorms, gusty winds and a sharp temperature fall.

The leading edge of advancing warm air that rides over colder air, giving layered cloud, steady precipitation and a gradual temperature rise over a wide area.

The near-equatorial zone where the trade winds of the two hemispheres converge, marked by low pressure, deep convection, heavy rain and seasonal north-south movement.

A dry, dusty north-easterly wind from the Sahara that affects West Africa, mainly during the dry season, reducing visibility and humidity and producing cool nights.

The seasonal inflow of moist south-westerly air from the Atlantic into West Africa, bringing the rainy season when the ITCZ lies north of the coast.

A warm-core, low-pressure system over tropical oceans with organised rotating winds around a centre; it brings destructive winds, storm surge and heavy rain, though it is rare in Nigeria.

A convective storm produced by a cumulonimbus cloud, with lightning and thunder, often accompanied by heavy rain, hail, strong gusts and turbulence.

A small, intense downdraught that spreads out at the surface, producing a sudden, dangerous shift from headwind to tailwind and a sharp loss of airspeed on approach.

Turbulence in cloud-free air, often near jet streams or strong wind shear, that cannot be seen by weather radar and may be encountered unexpectedly during cruise.

A white, rough, brittle ice deposit formed when small supercooled droplets freeze rapidly on an aircraft's leading edges, often in stratiform cloud at temperatures near -10 °C.

A suspension of tiny water droplets near the ground that reduces horizontal visibility to less than 1,000 metres, formed by cooling or by adding moisture to the air.

A routine coded aerodrome weather report issued at fixed intervals, giving wind, visibility, runway visual range, present weather, cloud, temperature, dew point and pressure.

A coded forecast of aerodrome conditions, usually covering 9 to 30 hours, giving expected wind, visibility, weather and cloud, with temporary or becoming changes.

A warning of potentially hazardous en-route weather, such as severe turbulence, icing, thunderstorms or volcanic ash, that may affect the safety of aircraft in flight.

A coded abbreviation meaning visibility is 10 km or more, no cloud below 5,000 feet or below the highest minimum sector altitude, and no cumulonimbus or significant weather.

A forecast chart showing areas of hazardous weather, such as cumulonimbus, turbulence, icing, frontal systems and jet streams, for a specified flight level or layer.

A symbolic diagram around a plotted observation showing temperature, dew point, pressure, wind, cloud cover and present weather at a weather station.

The period, roughly April to October in the south and shorter in the north, when moist monsoon air brings frequent showers, thunderstorms and low cloud over Nigeria.