ATPL Meteorology (050) flashcards

32 cards. Read the term and the definition here. The deck at the top of this page is optional.

ICAO standard sea-level pressure

1013.25 hPa, with sea-level temperature 15 °C and density 1.225 kg/m³; these values define the International Standard Atmosphere used for pressure altitude and altimeter calibration.

Standard temperature lapse rate

In the ICAO Standard Atmosphere the temperature decreases by 6.5 °C per kilometre (about 2 °C per 1000 ft) from sea level up to 11 km, where it becomes isothermal.

Tropopause height variation

The tropopause is about 16 km over the equator and 8 km over the poles; it is higher over warm air and in summer, and lower over cold air and in winter.

Density altitude

Pressure altitude corrected for the difference between actual temperature and ISA temperature. High temperature, high humidity or low pressure all increase density altitude and degrade aircraft performance.

Radiation inversion

A ground-based inversion formed on clear, calm nights when the surface cools rapidly by radiation. It is strongest around dawn and dissipates from the ground upward after sunrise.

Dry adiabatic lapse rate

The rate at which unsaturated air cools when lifted, about 3 °C per 1000 ft or 9.8 °C per km. It applies until the air reaches saturation.

QNH versus QFE

QNH is the altimeter setting that makes the altimeter indicate elevation above mean sea level on the ground; QFE makes it indicate zero or height above the aerodrome reference point.

Cold-air true altitude error

When air is colder than ISA, the altimeter over-reads and the aircraft's true altitude is lower than indicated. The error increases with altitude and with the temperature deviation.

Transition altitude and level

The transition altitude is where the altimeter is set to standard pressure on climb; the transition level is the lowest usable flight level above it, and it varies with QNH.

Dew point depression

The difference between air temperature and dew point. A small depression means air close to saturation, so fog, low cloud and precipitation are more likely.

Conditional instability

A layer is conditionally unstable when its environmental lapse rate lies between the saturated and dry adiabatic rates; it becomes unstable only if the air is saturated and lifted.

Bergeron process

Ice crystals grow at the expense of supercooled water droplets because saturation vapour pressure over ice is lower than over water at the same sub-freezing temperature.

Convective cloud base estimate

For convective cloud over land by day, the base in feet is about 400 times the surface temperature-dew point spread in degrees Celsius.

Freezing rain formation

Snow falls through a warm layer aloft, melts, then falls through a sub-freezing layer near the surface. The drops remain liquid and freeze on impact with the aircraft or ground.

Nimbostratus characteristics

A thick, grey, layered cloud producing continuous steady precipitation. It is associated with slow widespread lifting along warm fronts or in frontal troughs, with low bases and poor visibility.

Warm front cloud sequence

Approaching a warm front, cloud appears in order: cirrus, cirrostratus, altostratus, then nimbostratus. Precipitation starts ahead of the surface front, and the wind veers.

Cold front passage signs

A narrow band of cumuliform cloud and showers, a sharp wind veer, a temperature fall, a pressure rise after a trough, and improved visibility behind the front.

Air mass classification

Air masses are labelled by source region and temperature: cA, cP, mP, cT and mT. Continental types are dry, maritime types moist, and polar types cold.

Buys Ballot's law

In the Northern Hemisphere, with your back to the wind, low pressure lies on your left. In the Southern Hemisphere, low pressure lies on your right.

Anticyclone weather

High pressure with descending air, light winds, clear skies and a subsidence inversion. Radiation fog and haze are common in winter, and the centre may be calm.

Sea breeze mechanism

By day the land heats faster than the sea, pressure falls over land, and cooler air flows from sea to land. The breeze is strongest in the afternoon and may reach 20 kt.

Föhn wind

Air that descends the lee side of a mountain warms at the dry adiabatic rate after losing moisture on the windward side, producing warm, dry, gusty downslope winds and turbulence.

Geostrophic wind

The wind that blows parallel to straight isobars above the friction layer, where the pressure gradient force balances the Coriolis force; it is stronger when isobars are close together.

Jet stream

A narrow band of strong winds near the tropopause, with a core exceeding 60 kt. The polar front jet is linked to the polar front and to cyclogenesis.

Clear air turbulence

Turbulence in cloud-free air, most likely near jet streams, the tropopause, strong wind shear and mountain waves. It is invisible to weather radar and cannot be detected visually.

Jet streak quadrants

In the Northern Hemisphere, ascent and cyclogenesis are favoured in the left exit and right entrance quadrants of a jet streak, where ageostrophic circulation produces divergence aloft.

Clear ice

A dense, transparent, heavy icing type formed by large supercooled droplets, often in cumuliform cloud or freezing rain. It is the most hazardous icing type.

Microburst

A concentrated downdraught less than 4 km in diameter producing a strong divergent outflow. An aircraft first meets a headwind, then a tailwind, causing a sudden loss of airspeed.

Volcanic ash hazard

Ash is abrasive and melts in the engine hot section, damaging windscreens and pitot tubes. Avoid the ash cloud, report it, and check SIGMET and NOTAM information.

METAR trend group

A short forecast appended to a METAR or SPECI covering the next two hours at that aerodrome; NOSIG means no significant change is expected.

SIGMET versus AIRMET

SIGMET warns of significant en-route weather such as severe turbulence, icing, thunderstorms, volcanic ash and tropical cyclones. AIRMET covers less severe conditions relevant to lighter aircraft.

TAF change groups

BECMG indicates a gradual change, TEMPO a temporary fluctuation, and FM a rapid change. TAF validity is typically 9, 24 or 30 hours.