PPL Meteorology key terms

A theoretical atmosphere assuming sea-level pressure of 1013.25 hPa, temperature 15 °C, and a standard lapse rate, used as the reference for calibrating altimeters and computing pressure and density values.

The altimeter subscale setting that makes the instrument indicate elevation above mean sea level when the aircraft is on the ground at that location; derived from station pressure reduced to sea level.

The altimeter subscale setting that makes the altimeter read zero when the aircraft is on the ground at the reference point, so it indicates height above that point rather than altitude.

The standard pressure setting of 1013.25 hPa used above the transition altitude, giving pressure altitude and flight levels rather than height above sea level.

The altitude indicated when the altimeter is set to 1013.25 hPa; it is the height above the standard pressure datum and is used for flight levels and aircraft performance calculations.

Pressure altitude corrected for non-standard temperature; it represents the altitude in the standard atmosphere at which the air density would equal the actual density, and it drives engine and aerodynamic performance.

The altitude, published for each location, above which pilots set the standard pressure setting and fly flight levels; below it they use the local QNH and report altitude.

The lowest flight level available for use above the transition altitude; it is chosen so that adequate vertical separation exists between aircraft using standard pressure and those using local QNH.

The rate at which temperature decreases with increasing height in the atmosphere; the standard average is about 2 °C per 1000 ft, though actual rates vary with humidity and stability.

A layer in which temperature increases with height instead of decreasing; it suppresses vertical mixing, traps moisture and pollutants below it, and often produces smooth air and poor visibility.

The tendency of a displaced parcel of air to return to or move away from its original level; stable air resists vertical motion and favours smooth flying, while unstable air favours convection and cumuliform cloud.

Vertical motion of air driven by heating from below, producing rising thermals, cumuliform cloud, turbulence, and often showers or thunderstorms when sufficient moisture is present.

A change in temperature caused by expansion or compression of a parcel of air without exchange of heat with its surroundings; rising air cools and sinking air warms at the corresponding dry or saturated rate.

The temperature to which air must be cooled at constant pressure and moisture content for saturation to occur; a small spread between temperature and dew point indicates high humidity and likely cloud 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; cooling the air raises it toward saturation.

The condition in which air holds the maximum water vapour possible at a given temperature and pressure; further cooling or added moisture results in condensation into cloud, fog, or dew.

Fog formed overnight when the ground cools by outgoing radiation and chills the adjacent moist air to its dew point; it is favoured by clear skies, light wind, and high humidity.

Fog produced when warm, moist air moves over a colder surface and is cooled below its dew point; it can persist in moderate wind and is common over cold sea areas.

Warming and drying of air as it descends the leeward side of a mountain range after losing moisture on the windward side; it can produce strong, gusty winds and very dry conditions.

An oscillating airflow downwind of a mountain barrier, often marked by lenticular cloud, severe turbulence, and strong downdrafts; it can extend many kilometres downwind of the ridge.

A change in wind speed or direction over a short distance, either horizontally or vertically; it can cause sudden airspeed changes and is hazardous during takeoff, approach, and landing.

A small, intense downdraft that spreads out at the surface, producing a sudden headwind then tailwind change along a flight path; it is a serious low-level hazard, especially in thunderstorm environments.

The leading edge of cool outflow air spreading from a thunderstorm, marked by a wind shift and temperature drop; it can trigger new storms and create low-level wind shear.

A deep convective cloud reaching the tropopause with a towering or anvil-shaped top; it produces thunderstorms, heavy rain or hail, lightning, severe turbulence, icing, and strong gusty winds.

A line of active thunderstorms, often ahead of or along a cold front, producing a continuous or nearly continuous band of hazardous weather along its length.

The leading edge of an advancing colder air mass; it typically brings a rapid wind shift, temperature drop, pressure rise, and a band of showers or thunderstorms at or just behind the surface position.

The boundary where advancing warm air rides over colder air; it produces a broad area of layered cloud, steady precipitation ahead of the surface front, and gradual changes in wind and temperature.

A front formed when a faster cold front overtakes a warm front and lifts the warm sector clear of the surface; it brings complex weather, often with cloud and precipitation near the surface front.

A line on a weather chart joining points of equal mean sea-level pressure; the spacing indicates pressure gradient, with closely spaced isobars meaning stronger winds.

The rate of change of pressure over horizontal distance; the pressure gradient force drives air from higher to lower pressure, and steeper gradients produce stronger winds.

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 wind speed.

The theoretical wind that results when the pressure gradient force balances the Coriolis force in straight, frictionless flow aloft; it blows parallel to the isobars.

In the Northern Hemisphere, with your back to the wind, lower pressure lies to your left; the rule helps pilots locate pressure systems and anticipate wind changes.

Irregular, gusty motion of air that causes bumpy flight; it may arise from surface heating, mechanical obstruction, wind shear, or mountain waves, and varies in intensity and extent.

Accumulation of ice on aircraft surfaces when flying through supercooled water droplets; it adds weight, reduces lift, and disrupts airflow, with severe risk in freezing rain or deep stratiform cloud.

The altitude at which the air temperature is 0 °C; above it, supercooled water and airframe icing become possible, and it is a key planning figure for flights through cloud or precipitation.

Liquid water droplets that remain unfrozen at temperatures below 0 °C; on contact with an aircraft they freeze rapidly, making them the primary cause of airframe icing.

The greatest distance at which prominent objects can be seen and identified; it is reduced by fog, mist, precipitation, haze, and blowing dust or snow, and is reported in metres or kilometres.

A routine coded observation of surface weather at an aerodrome, issued at fixed intervals and reporting wind, visibility, cloud, temperature, dew point, pressure, and significant weather.

A coded forecast of expected weather at an aerodrome for a stated period, including wind, visibility, cloud, and significant weather, with change groups for anticipated temporary or persistent variations.

A warning message issued for weather hazardous to all aircraft, such as severe turbulence, severe icing, volcanic ash, or severe sandstorms, covering a specified area and validity period.