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

A hypothetical reference atmosphere assuming 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. It lets pilots compare aircraft and altimeter performance on a common baseline.

The boundary layer between the troposphere and stratosphere where temperature stops decreasing with height. Its height varies from roughly 25,000 ft at the poles to 55,000 ft in the tropics and marks the top of most weather.

The rate at which temperature falls with increasing height. The standard environmental lapse rate averages about 2 °C per 1000 ft, but local values vary and determine atmospheric stability.

A layer in which temperature increases with height instead of decreasing. Inversions trap moisture and pollutants beneath them, produce smooth air above, and are common in clear calm nights and behind warm fronts.

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

The pressure altitude corrected for non-standard temperature. High density altitude reduces engine power, propeller thrust and wing lift while increasing take-off and landing distances, so it is a key hot-and-high performance factor.

The pressure value dialled into an altimeter subscale so the instrument reads elevation above a reference datum such as mean sea level. QNH refers to sea level, QFE to aerodrome level and QNE is standard pressure 1013.25 hPa.

The altitude above which a pilot changes the altimeter from local QNH to standard pressure 1013.25 hPa and flies flight levels. Its value is published for each aerodrome or area and is typically a few thousand feet.

The ratio, expressed as a percentage, of the actual water vapour in the air to the maximum the air could hold at that temperature. It rises as air cools and reaches 100 percent at saturation.

The temperature to which air must be cooled at constant pressure for its water vapour to reach saturation. A small spread between temperature and dew point signals high humidity and a likelihood of cloud, fog or precipitation.

The difference between the actual air temperature and the dew point. A small spread means the air is close to saturation, so cloud bases are low and fog or drizzle becomes likely with slight cooling or lifting.

The tendency of a displaced parcel of air to return to its original level, continue rising, or stay put. Stable air resists vertical motion and gives layered cloud, while unstable air supports towering cloud and showers.

Cumulus and cumulonimbus clouds formed by rising thermals of warm, unstable air. They show vertical development, sharp edges and often produce showers, turbulence and icing in their upper parts.

Layered cloud such as stratus, altostratus and cirrostratus, formed where air ascends gently in a stable layer. It produces widespread steady precipitation, poor visibility and smooth but persistent icing.

A deep convective cloud with a towering vertical extent, often an anvil-shaped top and severe embedded hazards. It contains strong updraughts and downdraughts, heavy rain, hail, lightning, turbulence and airframe icing.

A cloud at the surface reducing horizontal visibility below 1000 m. It forms by cooling air to its dew point or by adding moisture, and is classified by cause as radiation, advection, frontal, upslope or steam fog.

Fog formed on clear, calm nights when the ground cools rapidly by radiation and chills the air above it to saturation. It is shallow, patchy, burns off after sunrise and is common in valleys in autumn and winter.

Fog produced when warm, moist air moves over a colder surface and is cooled from below to its dew point. It can be deep, widespread and persistent, and is typical of warm air flowing over cold sea or snow.

A large body of air with fairly uniform temperature and humidity acquired over a source region such as tropical ocean or polar land. Its properties and the surface it crosses determine the weather it brings.

The boundary zone between two air masses of different temperature and density. Fronts are associated with cloud, precipitation, wind shifts and pressure troughs, and are classified as cold, warm, occluded or stationary.

The leading edge of advancing cold air that undercuts warm air, producing a steep frontal surface. It brings a narrow band of heavy showers, squalls, turbulence and a sharp veer of wind as it passes.

The boundary where advancing warm air rides over retreating cold air along a shallow slope. It produces a wide sequence of cloud from cirrus through altostratus to nimbostratus with steady rain and a gradual wind veer.

A front formed when a faster cold front catches a warm front and lifts the warm air between them clear of the surface. It combines cloud and precipitation from both fronts and marks a mature low-pressure system.

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

A weather map showing pressure systems, fronts, isobars and significant weather over a large area at a stated time. Pilots use it to anticipate wind, cloud and frontal passage along a route.

A region where pressure is lower than its surroundings, with winds circulating cyclonically and converging inward. Air rises in the centre, producing cloud, precipitation and often strong or gusty winds.

A region where pressure is higher than its surroundings, with winds circulating anticyclonically and diverging outward. Subsiding air aloft suppresses cloud, giving clear skies, light winds and often overnight fog or haze.

The rate of change of pressure over horizontal distance, shown by isobar spacing. The steeper the gradient, the stronger the resulting wind, all else being equal.

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 grows with latitude and with wind speed.

The theoretical wind that blows parallel to straight isobars when the pressure gradient force balances the Coriolis force. It approximates the free-atmosphere wind away from surface friction and curvature effects.

The balanced wind along curved isobars, accounting for centrifugal force as well as pressure gradient and Coriolis effects. Around a low it is weaker than the geostrophic value, and around a high it is stronger.

A narrow, fast-moving band of upper-level wind near the tropopause, typically 100 kt or more. It steers surface weather systems, produces strong wind shear at its edges and gives significant tailwind or headwind benefits.

A change in wind speed or direction over a short distance, either horizontally or vertically. Low-level shear on approach or departure can cause sudden airspeed loss and is a major cause of accidents.

A strong, small-scale downdraught that spreads out horizontally on hitting the ground, producing a sudden headwind then tailwind change. It can cause severe airspeed loss close to the ground and is often linked to thunderstorms.

Turbulence occurring in cloud-free air, most often near jet streams, mountain waves or sharp tropopause folds. It is invisible to weather radar and is best avoided by studying wind shear forecasts and upper-air charts.

A cumulonimbus system with lightning and thunder, developing through cumulus, mature and dissipating stages. Hazards include hail, strong gusts, microbursts, icing, lightning and heavy rain, all best circumnavigated by a wide margin.

A line of active thunderstorms, often ahead of a cold front, producing a continuous band of heavy showers and severe gusty winds. It can extend for hundreds of miles and move rapidly across a route.

The build-up of ice on aircraft surfaces when flying through supercooled water droplets. It adds weight and drag, reduces lift and can jam controls, with the greatest risk in stratiform cloud between 0 °C and about −20 °C.

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

Rain that falls as liquid but freezes on contact with a surface at or below 0 °C. For aircraft it indicates severe icing risk, because large supercooled drops freeze rapidly on impact and can defeat de-icing systems.

A standing oscillation in the airflow downwind of a mountain range in a stable, strong cross-ridge wind. It produces severe turbulence, lenticular cloud, strong downdraughts and dangerous lee-side rotor zones.

An intense low-pressure system over warm tropical oceans with organised convection and maximum sustained winds of at least 34 kt. It brings destructive winds, torrential rain, storm surge and embedded severe turbulence.

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

A terminal aerodrome forecast giving expected conditions at an aerodrome, usually valid for 9 to 30 hours. It uses the same code as METAR plus change groups for expected timing of significant variations.

A warning message of hazardous weather that may affect the safety of all aircraft, such as severe turbulence, icing, volcanic ash or tropical cyclones. It is issued for a defined area and is valid for up to four hours.

A special aerodrome observation issued when significant weather changes occur between routine METAR reports, such as visibility or cloud base crossing operational thresholds. It uses the same format as a METAR.