Transport Canada Flight Dispatcher Meteorology Exam key terms

A defined reference atmosphere assuming sea-level pressure of 1013.25 hPa, temperature of 15 °C, and a lapse rate of 1.98 °C per 1000 ft, used to calibrate altimeters and aircraft performance data.

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

The boundary layer between the troposphere and stratosphere where temperature normally stops decreasing with height; its height varies with latitude and season and marks the top of most weather.

The altitude indicated when an altimeter is set to the standard datum of 1013.25 hPa; it is used for flight levels and performance calculations rather than terrain clearance.

The sea-level pressure value, derived from station pressure, that is entered into a pressure altimeter so it indicates elevation above mean sea level at the reporting station.

The altitude in the standard atmosphere at which air density equals the actual density at a given location; high temperature, high humidity, and low pressure all raise it and degrade aircraft performance.

An altimeter setting that, when applied to an altimeter on the ground at a station, causes it to read the station's elevation above mean sea level; it is the standard setting for terminal and low-level operations.

An altimeter setting that makes the altimeter read zero when the aircraft is on the ground at the reference station; it indicates height above that station rather than altitude above sea level.

The ratio, expressed as a percentage, of the actual water vapour content of the air to the maximum amount the air could hold at that temperature and pressure before saturation occurs.

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 dewpoint indicates high humidity and likely cloud or fog.

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

The change of water vapour into liquid water, occurring when air is cooled to saturation in the presence of condensation nuclei; it forms cloud, fog, and visible moisture on aircraft surfaces.

Microscopic particles such as dust, salt, and combustion products on which water vapour condenses to form droplets; their abundance affects droplet size, visibility, and cloud development.

The tendency of a displaced parcel of air to return to its original level (stable) or continue rising (unstable); it governs whether clouds spread out in layers or build vertically.

A condition in which a layer of air becomes unstable when lifted as a whole, typically because dry air overlies moist air; it favours rapid development of showers and thunderstorms when triggered.

A layer in which temperature increases with height, reversing the normal lapse rate; it suppresses vertical mixing, traps moisture and pollutants, and can produce persistent low cloud or fog.

A visible aggregate of water droplets or ice crystals suspended in the atmosphere, formed by cooling to saturation; cloud type and height indicate the stability and moisture of the air mass.

A deep, vertically developed cloud with a towering or anvil-shaped top, capable of producing thunderstorms, heavy showers, hail, severe turbulence, icing, and strong gusty winds at the surface.

Precipitation that falls as liquid water and freezes on contact with surfaces at or below 0 °C, producing severe airframe icing; it typically occurs when a warm layer overlies subfreezing air.

A large body of air with relatively uniform temperature and moisture characteristics, acquired over a source region, whose properties determine the weather it brings when it moves over an area.

The transition zone between two air masses of different temperature and density; its passage brings characteristic changes in temperature, wind, pressure, and precipitation.

A front formed when a faster-moving cold front overtakes a warm front and lifts the warm air entirely off the surface; it often produces extensive cloud and steady precipitation.

A large low-pressure system that forms along the polar front in middle latitudes, typically with warm and cold fronts, and moves eastward bringing successive weather changes over a few days.

An elongated area of relatively low pressure or a dip in the upper-level flow; it is associated with rising motion, cloud, and precipitation, and often steers surface systems eastward.

An elongated area of relatively high pressure or a crest in the upper-level flow; it favours subsidence, clearing, and stable conditions, and can block or slow the movement of weather systems.

A convective storm produced by a cumulonimbus cloud, with lightning, thunder, and often heavy rain, hail, and strong gusty winds; it poses serious hazards to aircraft in all phases of flight.

A change in wind speed or direction over a short distance or time, including low-level and microburst shear; it can cause sudden, severe airspeed and flight-path changes near the ground.

A localized, intense downdraft from a thunderstorm that spreads out on reaching the ground, producing a sharp and potentially fatal wind shear hazard during takeoff and landing.

The accumulation of ice on airframe surfaces when flying through supercooled water droplets; it adds weight, reduces lift, and disrupts airflow, with severity depending on droplet size and temperature.

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

An oscillating airflow set up downwind of a mountain barrier when stable air crosses it; it produces severe turbulence, strong downdrafts, and lenticular clouds, and can extend many kilometres downwind.

A warm, dry wind descending the lee side of a mountain range after the air has lost moisture on ascent; it can cause rapid temperature rises and hazardous gusty winds.

A local daytime wind that blows from a cooler sea toward warmer land as the land heats; it can produce a shallow marine air layer, low cloud, and wind shifts near the coast.

A cloud at the surface that reduces visibility below 1 km; it forms by cooling to saturation or by adding moisture, and is a major cause of low visibility for terminal operations.

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

A special coded observation issued when significant weather changes occur between routine reports, such as rapid visibility, cloud, or wind changes that affect aircraft operations.

A report of actual weather conditions aloft, such as turbulence, icing, and cloud tops, provided by pilots; it is valuable because it describes conditions that surface observations cannot show.

A coded aerodrome forecast of expected wind, visibility, cloud, and weather for a specified period, often with temporary and probability groups indicating short-term variations.

A graphical forecast product depicting expected weather such as cloud, precipitation, turbulence, and icing over a region, used by dispatchers and pilots for flight planning.

A warning of significant en-route weather hazardous to all aircraft, such as severe turbulence, severe icing, volcanic ash, or tropical cyclones, issued for a defined area and period.

An advisory of en-route weather that may affect aircraft safety but is less severe than SIGMET criteria, covering phenomena such as moderate turbulence, moderate icing, and extensive low cloud.

An analysed or forecast chart showing fronts, pressure centres, and areas of significant weather such as thunderstorms and turbulence, used for synoptic-scale flight planning.