CPL Meteorology key terms

A set of agreed average values of temperature, pressure and density at each height, used to calibrate altimeters and compare aircraft performance. It assumes sea-level values of 15 degrees Celsius and 1013.25 hPa.

The boundary layer between the troposphere and the stratosphere where temperature stops falling with height. Its height varies with latitude and season and it marks the top of most weather and the core of jet streams.

The rate at which temperature falls with increasing height in a layer of air. The average environmental value in the lower atmosphere is about 2 degrees Celsius per 1000 feet, but it varies greatly between air masses.

The actual measured rate at which the surrounding air cools with height at a particular place and time. Comparing it with the adiabatic rates shows whether the air is stable, neutral or unstable.

The rate at which a rising parcel of unsaturated air cools, about 3 degrees Celsius per 1000 feet. It applies only while the parcel remains unsaturated and is used to judge initial stability.

The rate at which a rising parcel of saturated air cools, roughly 1.5 to 2 degrees Celsius per 1000 feet. It is smaller than the dry rate because latent heat released by condensation partly offsets cooling.

A condition in which the environmental lapse rate is smaller than the saturated adiabatic rate, so a displaced parcel returns to its original level whether or not it is saturated. Vertical motion and cloud growth are suppressed.

A condition in which the environmental lapse rate exceeds the dry adiabatic rate, so a displaced parcel keeps accelerating away whether saturated or not. It favours strong updraughts, towering cloud and severe turbulence.

The common state in which the environmental lapse rate lies between the dry and saturated adiabatic rates. Air is stable while unsaturated but becomes unstable once lifted to saturation, so cloud depth depends on the trigger.

The temperature to which air must be cooled at constant pressure and moisture content for saturation to occur. A small gap between temperature and dew point means high relative humidity, mist or fog risk and a low cloud base.

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

The mass of water vapour contained in a unit mass of dry air, usually given in grams per kilogram. Unlike relative humidity it does not change when the air is heated or cooled without condensation.

Energy absorbed or released when water changes state, without any change in temperature. Evaporation absorbs heat and condensation releases it, which is why saturated rising air cools more slowly and why thunderstorms are so energetic.

The height to which surface air must rise by convection before it becomes saturated and cloud forms. It gives a fair estimate of the base of cumulus and cumulonimbus when the surface dew point is representative.

A layer in which temperature increases with height instead of falling. It strongly suppresses vertical mixing, traps haze, smoke and low cloud beneath it, and can produce marked wind shear at its top.

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 the standard atmosphere.

The altimeter subscale setting that makes the instrument read height above a specified reference point, normally the aerodrome. It is used in some circuit and approach procedures but gives no terrain clearance away from that point.

The standard setting of 1013.25 hPa used above the transition altitude so that all aircraft share a common pressure datum. With QNE set the altimeter indicates pressure altitude, forming the basis of flight levels.

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

The pressure altitude corrected for the effect of temperature on air density. High density altitude on hot days or at high elevations lengthens take-off runs and reduces climb performance and engine power.

The actual vertical distance of the aircraft above mean sea level. It differs from indicated altitude because of pressure and temperature differences from the standard atmosphere, and errors grow in very cold air.

The height above which pilots set the standard pressure setting and fly by flight levels. Below it they use the aerodrome or area QNH, and the change is made at a published level to keep all aircraft on one datum.

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

The rate at which pressure changes over horizontal distance, directed from high towards low pressure. It is the main driving force of wind, and the tighter the gradient the stronger the resulting wind.

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 wind speed and latitude and is zero at the equator.

The theoretical wind that blows parallel to straight isobars when the pressure gradient force balances the Coriolis force, above the friction layer. Real upper winds approximate it, and it is slower near the equator.

The balanced wind along curved isobars, where pressure gradient, Coriolis and centripetal forces act together. It is stronger than geostrophic wind around a high and weaker around a low at the same gradient.

A rule stating that in the Northern Hemisphere, with your back to the wind, low pressure lies on your left and high pressure on your right. It helps locate pressure systems from a single surface wind observation.

A daytime coastal wind blowing from sea to land because the land heats faster and pressure falls over it. It forms a shallow circulation with a returning upper flow and often brings a sharp temperature drop and a sea-breeze front.

An upslope wind that develops on sun-facing slopes as the ground heats the adjacent air. It can trigger cumulus cloud over ridges and is often accompanied by mechanical turbulence and reduced climb performance.

A downslope wind caused by cold, dense air draining under gravity, typically at night on sloping terrain or from an ice sheet. It produces low-level wind shear, cold pools and can feed fog in valleys.

A narrow, fast band of upper-level wind near the tropopause, often thousands of kilometres long and a few hundred kilometres wide. It steers frontal systems and gives strong tailwinds or headwinds and clear-air turbulence.

The leading edge of advancing cold air that undercuts and lifts warm air. It brings a fairly rapid passage with cumuliform cloud, showers, gusty veering winds, a temperature fall and often a narrow band of heavy rain.

The leading edge of advancing warm air riding up over colder air ahead. It gives a gradual sequence of increasing cloud, lowering base and steady precipitation, followed by a rise in temperature and backing wind after passage.

A front formed when a faster cold front catches up with a warm front and lifts the warm sector clear of the surface. It typically brings complex cloud and prolonged precipitation and marks a mature, weakening depression.

A line of active thunderstorms arranged along or ahead of a front, often producing severe gusts, heavy rain and hail. It can move quickly and affect an aerodrome with little warning from surface observations alone.

A measure of the energy available to rising air, calculated from the area between the parcel and environmental temperature curves. Large values indicate strong updraught potential, deep convection and possible severe thunderstorms.

A change in wind speed or direction over a short distance, either horizontally or vertically. It can cause sudden airspeed changes on approach and departure and is especially dangerous close to the ground in convective conditions.

A small, intense downdraught that spreads out on hitting the ground, producing a sharp and short-lived reversal of wind. An aircraft flying through it first gains then loses airspeed and lift, and it is a major low-level hazard.

Turbulence encountered in cloud-free air, most often near jet streams, mountain waves or sharp upper-level wind shear. It is invisible to weather radar and is best avoided by studying wind and temperature patterns aloft.

An oscillating airflow set up downwind of a mountain range when stable wind crosses the ridge. It produces severe turbulence, strong up- and downdraughts, lenticular cloud and sometimes a dangerous rotor zone below.

The accumulation of ice on an airframe when flying through supercooled water droplets or wet snow. It adds weight, spoils aerofoil shape and can block pitot or static sources, with severity depending on droplet size and temperature.

Rain that falls as liquid but freezes on contact with a surface at or below zero degrees Celsius. It indicates a warm layer aloft above cold air and is one of the most severe icing hazards for aircraft.

Fog formed overnight when clear skies and light wind allow the ground and adjacent air to cool to saturation. It is common in autumn and winter, thickest around dawn, and usually clears after sunrise.

Fog formed when relatively warm, moist air moves over a colder surface and is cooled to its dew point. It can persist day or night, is common over cold sea areas, and may cover large areas with strong wind.

A routine coded observation of surface weather at an aerodrome, issued at fixed intervals and including wind, visibility, weather, cloud and temperature. It is the basic evidence of current conditions for departure and arrival planning.

A coded forecast of expected conditions at an aerodrome, covering wind, visibility, weather and cloud for a stated period. It may include temporary or becoming groups and is a primary document for flight planning.

A special aerodrome observation issued when conditions change significantly between routine reports, for example a sudden drop in visibility or the onset of a thunderstorm. It warns of deterioration not yet captured in the routine sequence.

A warning message about weather that is significant to all aircraft, such as severe turbulence, icing, volcanic ash or a severe squall line. It is issued for a defined area and period and must be checked before flight.

An advisory of weather that may affect light aircraft, such as moderate turbulence, moderate icing or extensive low cloud. It sits below SIGMET in severity and helps light aircraft plan around less extreme but still relevant hazards.

A chart depicting fronts, pressure centres, and areas of significant cloud, turbulence, icing and jet streams for a given level and validity time. It gives a broad picture of hazards and is used with area forecasts for route planning.

A weather map showing mean sea level pressure, fronts and pressure systems at a stated time. It helps identify air masses, frontal timing and the likely trend of wind and weather along a route.