PCAA Meteorology key terms

A defined reference model of the atmosphere in which sea-level pressure is 1013.25 hPa, sea-level temperature is 15 °C, and temperature falls at 6.5 °C per km up to 11 km. It lets altimeters and performance tables be compared consistently.

The boundary layer between the troposphere and stratosphere where temperature normally stops decreasing with height. Its height varies with latitude and season, being high over the tropics and low over polar regions, and it marks the top of most weather.

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

The actual measured rate of temperature decrease with height in the surrounding air at a given place and time, obtained from soundings or forecasts. Comparing it with adiabatic rates shows whether the atmosphere is stable, neutral or unstable.

The temperature to which air must be cooled at constant pressure and moisture content for it to become saturated. A small spread between temperature and dew point indicates high humidity and a greater likelihood of cloud, fog or precipitation.

The ratio, expressed as a percentage, of the actual water vapour in the air to the maximum amount it could hold at that temperature. It changes with temperature even when the moisture content stays the same.

The rate at which an unsaturated parcel of rising air cools, about 3 °C per 1000 ft or 9.8 °C per km. It applies only while the parcel remains unsaturated and is used with the environmental lapse rate to judge stability.

The rate at which a saturated rising parcel cools, roughly 1.5 to 2 °C per 1000 ft, because latent heat released by condensation partly offsets expansion cooling. It is smaller than the dry rate and varies with temperature and pressure.

Energy absorbed or released when water changes phase, such as during evaporation and condensation. Release of latent heat in rising saturated air warms the parcel and is a major energy source for thunderstorms and tropical systems.

A state in which the environmental lapse rate lies between the dry and saturated adiabatic rates. Air is stable while unsaturated but becomes unstable once it rises enough to saturate, which favours showers and thunderstorms when lifting occurs.

A measure of the potential buoyant energy available to a rising parcel, calculated from the area between the parcel path and the environmental sounding. Larger values indicate stronger possible updraughts and more vigorous thunderstorm development.

The height at which a lifted parcel becomes warmer than its surroundings and therefore rises freely without further forcing. It lies above the lifting condensation level and marks the base of the positively buoyant layer.

The height at which a rising parcel of air cools to its dew point and condensation begins. It approximates the base of cumulus clouds and is estimated from the surface temperature and dew point spread.

A layer in which temperature increases with height instead of decreasing. Inversions are very stable, trap moisture and pollutants beneath them, and are often associated with fog, haze and low-level wind shear.

The altimeter subscale setting that, when set on the ground at an aerodrome, makes the altimeter read the aerodrome elevation. It is the standard setting for en-route and terminal altimetry below the transition altitude.

The altimeter subscale setting that makes the altimeter read zero on the ground at a specified reference point, usually the aerodrome or runway threshold. It is used mainly for traffic patterns and approach height references.

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

The pressure altitude corrected for non-standard temperature, representing the altitude at which the air density would match current conditions in the standard atmosphere. High density altitude reduces engine power, lift and propeller efficiency.

The altitude below which the altimeter is set to local pressure settings such as QNH and above which flight levels with 1013.25 hPa are used. Its value is published for each state or region and varies between countries.

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 Hemisphere. It grows with latitude and wind speed but is zero at the equator.

The theoretical wind that results when the pressure gradient force balances the Coriolis force, blowing parallel to straight isobars above the friction layer. It approximates upper winds and is used to read wind from pressure charts.

The force produced by differences in atmospheric pressure between two places, acting from high toward low pressure. Its strength depends on the spacing of isobars, and it is the primary driver of wind.

A line on a weather chart joining places of equal mean sea-level pressure. Closely spaced isobars indicate a strong pressure gradient and stronger winds, while widely spaced isobars indicate light winds.

A narrow, fast-moving band of upper-tropospheric wind near the tropopause, typically thousands of kilometres long and a few kilometres deep. It steers weather systems, causes clear air turbulence and offers strong tailwinds or headwinds.

A local daytime wind that blows from the sea toward the warmer land because land heats faster and forms relatively lower pressure. It can reach tens of kilometres inland and often brings a coastal cloud line or temperature drop.

An upslope wind that develops on sun-facing slopes during daytime as the air next to the heated ground becomes buoyant. It can trigger cumulus formation over ridges and is often accompanied by turbulence in mountainous terrain.

A downslope wind driven by gravity when air adjacent to a cooled slope becomes denser than the surrounding air. It commonly occurs at night and in the early morning and can produce cold, gusty drainage flows into valleys and airports.

A smooth, lens-shaped cloud that forms in the crests of standing mountain waves where stable moist air is forced upward over a ridge. It signals strong winds aloft and possible severe turbulence and downdraughts on the lee side.

A deep, vertically developed convective cloud with a flat, often anvil-shaped top reaching the tropopause. It produces heavy showers, hail, lightning, strong gusty surface winds and severe turbulence, and should be avoided by aircraft.

Rain that falls as liquid but freezes on contact with surfaces at or below 0 °C, producing rapid airframe ice accumulation. It occurs when a warm layer aloft melts snow that then falls into a subfreezing layer near the surface.

A cloud at the Earth's surface that reduces horizontal visibility below 1000 m. It forms when air is cooled to its dew point or moisture is added, and it is a major cause of low-visibility approach and landing disruptions.

Fog that forms overnight over land when clear skies and light winds allow the surface to cool the air above it to saturation. It is usually shallow, burns off after sunrise, and is common in valleys and irrigated plains.

Fog that forms when warm, moist air moves over a colder surface and is cooled from below to saturation. It can be deep and persistent, is common over cold sea areas, and may drift over coastal aerodromes with onshore winds.

A large body of air with relatively uniform temperature and humidity characteristics acquired over a source region such as a desert, ocean or polar area. Its properties and the boundaries between air masses largely determine day-to-day weather.

The boundary zone between two air masses of different temperature and density. Fronts are associated with cloud, precipitation, pressure changes and wind shifts, and their passage usually brings a marked change in flying conditions.

The leading edge of advancing cold air that undercuts warmer air, producing a steep slope and fast-moving narrow bands of showers or thunderstorms. Passage brings a sharp temperature drop, wind veer and pressure rise.

The leading edge of advancing warm air that rides over cooler air along a shallow slope, producing extensive layered cloud and steady precipitation ahead of the surface position. Visibility is often poor in the frontal zone.

A front formed when a faster-moving cold front catches up with a warm front and lifts the warm sector clear of the surface. Weather is often complex, with mixed cloud types and precipitation on both sides of the front.

A line of active thunderstorms arranged along or ahead of a cold front, often producing strong gusty winds, heavy rain and severe turbulence over a narrow band. It can move rapidly and affect an aerodrome within minutes.

Turbulence encountered in cloud-free air, typically near jet streams, mountain waves or sharp wind shears, and invisible to weather radar. It is a hazard to aircraft in cruise and is forecast using wind and temperature gradient data.

An oscillating flow that develops downwind of a mountain barrier when stable air crosses the ridge at speed. It produces severe turbulence, strong downdraughts, lenticular clouds and dangerous lee-side wind shear.

A small, intense downdraught from a thunderstorm that spreads out on the surface, causing a sudden headwind-to-tailwind change over a short distance. It is a severe low-level wind shear hazard, especially during approach and departure.

A rapid change in wind speed or direction over a short distance at low altitude, often caused by fronts, thunderstorms, inversions or terrain. It can abruptly alter airspeed and glidepath and has caused serious accidents.

A heavy, transparent, glossy deposit of airframe ice formed when large supercooled water droplets freeze slowly and spread over the surface. It is dense, hard to remove and can seriously degrade lift and increase weight and drag.

A rough, opaque, milky deposit of airframe ice formed when small supercooled droplets freeze almost instantly on impact. It is less dense than clear ice but still disturbs airflow and can block pitot heads and intakes.

A routine coded observation of surface weather at an aerodrome, issued at fixed intervals and including wind, visibility, weather, cloud, temperature, dew point and pressure. Pilots use it to check current conditions before flight.

A special coded aerodrome weather report issued when conditions change significantly between routine reports, such as a sudden drop in visibility or a change in cloud base. It uses the same format as a METAR.

A coded forecast of weather conditions expected at an aerodrome over a stated period, including wind, visibility, weather and cloud, with temporary and becoming groups. It is used for flight planning and alternate selection.

A coded warning of hazardous en-route weather such as severe turbulence, severe icing, volcanic ash or tropical cyclone activity that may affect flight safety. It is issued for a defined area and period and must be considered in planning.

A coded advisory of moderate en-route hazards such as moderate turbulence, moderate icing or widespread low visibility that may affect flights. It complements SIGMET information by covering less severe but still significant conditions.

A report of actual weather conditions encountered in flight, such as turbulence, icing, cloud tops or wind at altitude, submitted by a pilot. It provides real-time information that ground-based systems cannot observe.

A weather map showing pressure systems, fronts and isobars at a given time over a large area. It is used to locate highs, lows and frontal zones and to anticipate wind, cloud and precipitation along a route.

A chart depicting en-route hazards such as cumulonimbus, turbulence, icing, jet streams and tropical cyclones at a specified flight level. It is a primary planning document for medium and high-level flights.

A forecast chart giving wind direction, speed and temperature at specified pressure levels and grid points. It is used to calculate upper winds, drift, ground speed and fuel burn and to identify jet stream strength.

A seasonal reversal of prevailing winds caused by differential heating of land and sea, bringing a moist onshore flow and heavy rain in summer. Over South Asia it dominates the summer weather and strongly affects aviation in Pakistan.

A trough of low pressure in the upper westerlies that moves from the eastern Mediterranean and Iran toward South Asia, bringing cloud, rain, snow over mountains and thunderstorms in winter. It is a main winter weather producer in Pakistan.

A region of strong, turbulent wind lifting large amounts of dry soil and sand into the air and reducing visibility severely. It is common in arid regions of Pakistan before monsoon rain and is a serious hazard to low-level flight.

A downward intrusion of stratospheric air into the troposphere along a fold in the tropopause, often near jet streams. It brings very dry, stable air aloft and is associated with clear air turbulence and rapid cyclogenesis.

Drizzle consisting of very small supercooled droplets that freeze on impact with surfaces at or below 0 °C. It can produce rapid, hard-to-see ice accumulation and is treated as a serious icing hazard.