CAA NZ PPL Meteorology key terms

The altimeter subscale setting that makes the instrument read aerodrome elevation when the aircraft is on the ground. It is derived from the local pressure reduced to mean sea level using the standard atmosphere.

The altimeter subscale setting that makes the instrument read zero (or aerodrome elevation reference height) when the aircraft is on the ground at that aerodrome. Used mainly for circuit and approach height references.

The standard pressure setting of 1013.2 hPa used above the transition level so all aircraft share a common datum. With QNE set, the altimeter indicates pressure altitude, not height above ground.

A surface of constant atmospheric pressure related to the standard datum of 1013.2 hPa, shown as three digits (e.g. FL150 means 15,000 ft pressure altitude). Aircraft fly at flight levels above the transition altitude.

The altitude at or below which an aircraft's altimeter is set to the aerodrome QNH. Above it, pilots change to the standard pressure setting; the value is published for each aerodrome.

The airspace between the transition altitude and the transition level within which aircraft may be climbing or descending through the changeover. Its depth varies with pressure so that adequate separation is maintained.

The lowest flight level available for use above the transition altitude. It is determined from the transition altitude and current QNH so that it always lies above the transition altitude.

A designated area, often around an aerodrome, within which a specified QNH is used below the transition altitude. Pilots must obtain the current setting before entering and while operating inside it.

The altitude indicated when the altimeter is set to 1013.2 hPa. It is used in performance calculations and is the basis for flight levels and density altitude computations.

Pressure altitude corrected for non-standard temperature. High density altitude reduces engine power, propeller thrust and wing lift, increasing take-off and landing distances and reducing climb performance.

International Standard Atmosphere: a defined model with sea-level temperature 15 degrees C, pressure 1013.2 hPa, and a lapse rate of about 2 degrees C per 1,000 ft in the troposphere. Used as a reference for performance and altimetry.

The rate at which temperature decreases with increasing height. The average environmental tropospheric rate is about 2 degrees C per 1,000 ft; actual rates vary with air mass and stability.

A layer in which temperature increases with height, reversing the normal lapse rate. Inversions trap moisture and pollutants below them, often producing haze, fog or low cloud and smooth air.

The tendency of a displaced parcel of air to return to (stable) or continue away from (unstable) its original level. Stability governs cloud type, turbulence, shower activity and the likelihood of thunderstorms.

Cloud formed by rising thermals in an unstable air mass, ranging from fair-weather cumulus to towering cumulus and cumulonimbus. Associated with turbulence, showers and, in mature cells, thunderstorms.

A deep, vertically developed cloud with a towering or anvil-shaped top, producing heavy showers, hail, lightning and strong gusty winds. It presents severe hazards to light aircraft and should be avoided by a wide margin.

A storm cell with a cumulonimbus cloud containing lightning and thunder, typically in three stages: cumulus, mature and dissipating. Hazards include hail, strong up- and downdrafts, microbursts, icing and wind shear.

A small, intense downdraft that spreads out on reaching the ground, producing a sudden headwind then tailwind shift along the flight path. It can cause severe loss of airspeed and altitude on approach or take-off.

A change in wind speed or direction over a short distance, either horizontally or vertically. Low-level shear on approach or departure can cause abrupt airspeed and lift changes and is a major hazard near thunderstorms and fronts.

The leading edge of cool outflow air spreading from a thunderstorm, often marked by a roll cloud and a sudden wind shift. It brings sharp gusts, temperature drop and turbulence at the surface.

A line of active thunderstorms, often ahead of or along a cold front, extending many kilometres. It produces strong gusty winds, heavy rain, hail and severe turbulence along its length.

The boundary where advancing cold air undercuts warmer air, forcing rapid uplift. It typically brings a narrow band of showers, thunderstorms, gusty wind shifts and a marked temperature drop as it passes.

The boundary where advancing warm air rides over cooler air, producing extensive layered cloud and steady precipitation ahead of the surface front. Slopes are shallow, so weather changes are gradual.

A front formed when a cold front overtakes a warm front, lifting the warm air completely off the surface. It brings complex cloud and precipitation, often with a mix of frontal characteristics.

The boundary between two air masses of different temperature and density. Fronts concentrate weather such as cloud, precipitation, wind shifts and temperature change, and are shown on synoptic charts.

A large body of air with broadly uniform temperature and moisture characteristics acquired over its source region. Its properties and the fronts between air masses determine much of the weather a flight encounters.

A line on a weather chart joining places of equal mean sea level pressure. Closely spaced isobars indicate a steep pressure gradient and therefore stronger surface winds.

The rate of change of atmospheric pressure over horizontal distance. A steep gradient produces strong winds; the gradient force is balanced by Coriolis and friction effects near the surface.

An apparent force caused by the Earth's rotation that deflects moving air to the left in the Southern Hemisphere. It acts perpendicular to the wind and increases with wind speed and latitude.

The theoretical wind that blows parallel to straight isobars when the pressure gradient force balances the Coriolis force, in free air above friction. Surface winds are slower and cross isobars towards low pressure.

A rule stating that in the Southern Hemisphere, with your back to the wind, low pressure lies on your right. It helps locate pressure systems and assess wind direction around them.

A cloud at ground level reducing horizontal visibility below 1,000 m. It forms when air is cooled to its dew point or moisture is added, and is a major hazard for take-off, landing and low-level flight.

The temperature to which air must be cooled at constant pressure to become saturated. A small difference between temperature and dew point indicates high humidity and a greater likelihood of fog or low cloud.

The ratio, expressed as a percentage, of the actual water vapour in the air to the maximum it could hold at that temperature. Reaching 100 percent means saturation and possible condensation into cloud or fog.

The condition in which air holds the maximum water vapour possible at a given temperature and pressure. Further cooling or added moisture causes condensation into cloud, fog or precipitation.

The change of water vapour into liquid droplets, occurring when air is cooled below its dew point, usually on condensation nuclei. It is the process that forms cloud, fog and visible moisture.

The accumulation of ice on an airframe when flying through supercooled water droplets in cloud or precipitation. It adds weight, disrupts airflow and reduces lift, and can impair controls and pitot or static systems.

The height at which the air temperature is 0 degrees C. Above it, supercooled water and airframe icing become more likely; the level is forecast and published for flight planning.

Ice forming in a carburettor from fuel vaporisation cooling and pressure drop, which can occur at outside temperatures well above freezing in humid air. It causes rough running and power loss unless carburettor heat is applied.

The greatest horizontal distance at which prominent objects can be seen and identified. It is reported in metres or kilometres and is reduced by fog, haze, precipitation, blowing dust or smoke.

Runway Visual Range: the distance over which a pilot on the runway centreline can see markings or lights. It is measured instrumentally and reported in metres, often used for low-visibility approach minima.

A routine coded observation of surface weather at an aerodrome, issued at fixed intervals. It includes wind, visibility, cloud, temperature, dew point, QNH and significant weather, and is used for pre-flight and in-flight planning.

Terminal Aerodrome Forecast: a coded forecast of expected conditions at an aerodrome, usually valid for 24 to 30 hours. It covers wind, visibility, cloud and significant weather changes and their timing.

A special aerodrome observation issued when conditions change significantly between routine METAR reports, such as a marked deterioration or improvement in visibility or cloud. It uses the same code format as a METAR.

A short forecast appended to a METAR or SPECI indicating expected changes within the next few hours, such as temporary or becoming conditions. It helps pilots judge imminent weather at the aerodrome.

A weather map showing pressure systems, fronts, isobars and significant weather over a large area at a given time. It is used to understand the broad weather pattern affecting a flight.

A change in wind speed or direction over a short distance, either horizontally or vertically. Low-level shear can cause abrupt airspeed and lift changes and is a major hazard near thunderstorms and fronts.