ATPL Meteorology (Sri Lanka CAA Pilot Licence Examinations) flashcards

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

ISA sea-level values

Standard atmosphere at mean sea level: pressure 1013.25 hPa, temperature +15 °C, density 1.225 kg/m³. Used as the reference for altimetry and aircraft performance calculations.

Tropopause definition

The boundary between troposphere and stratosphere marked by a lapse-rate break where temperature stops decreasing with height. It is higher over the equator than over the poles and can fold near jet streams.

Troposphere composition

About 78 % nitrogen, 21 % oxygen, 0.9 % argon and 0.04 % carbon dioxide by volume, with water vapour varying from near zero to 4 %. Water vapour drives cloud and precipitation.

Pressure lapse rate

In the lower atmosphere pressure falls roughly by half every 5500 m (18,000 ft). At 18,000 ft about 50 % of sea-level pressure remains, which is why altimeters need a subscale setting.

QNH meaning

Altimeter subscale setting that makes the instrument read aerodrome elevation above mean sea level when on the ground. It is station pressure reduced to sea level using ISA conditions.

QFE meaning

Altimeter subscale setting that makes the instrument read zero, or field elevation, at the aerodrome reference point. Used for circuit heights and approach rather than en-route altitude.

QNE and flight levels

Subscale setting of 1013.25 hPa used above the transition altitude. The altimeter then displays pressure altitude, expressed as flight levels, so all aircraft share a common reference.

High-to-low pressure error

Flying from high to low pressure without resetting the subscale makes the altimeter over-read, so true altitude is lower than indicated. Terrain clearance is reduced and 'look out below' applies.

Density altitude

Pressure altitude corrected for non-standard temperature. It increases with high elevation, high temperature, high humidity and low pressure, reducing engine power and lift performance.

Environmental lapse rate

The actual measured rate of temperature decrease with height at a given time and place. It is compared with the dry and saturated adiabatic rates to determine stability.

Dry adiabatic lapse rate

Rate at which unsaturated air cools when lifted, about 3 °C per 1000 ft or 1 °C per 100 m. Used to assess stability and to compute convective cloud base.

Saturated adiabatic lapse rate

Rate at which saturated air cools on lifting, about 1.5 °C per 1000 ft, varying with temperature and moisture. Latent heat release makes it smaller than the dry rate.

Conditional instability

A layer is conditionally unstable when the environmental lapse rate lies between the saturated and dry adiabatic rates. Air is stable if unsaturated but becomes unstable once saturation and cloud form.

Radiation inversion

Temperature increase with height formed on clear, calm nights as the surface cools by radiation. Strongest near dawn; traps moisture and favours fog, mist and low stratus.

Subsidence inversion

Warming by adiabatic compression of descending air in an anticyclone creates a stable layer aloft. It often caps haze or stratocumulus and produces a marked top to the moist layer.

Dew point

Temperature to which air must be cooled at constant pressure to become saturated. A small dew-point spread indicates high relative humidity and risk of fog or low cloud.

Cloud base estimate

Convective cloud base above the surface can be estimated as (temperature minus dew point) in °C multiplied by 400 ft. A spread of 5 °C gives roughly 2000 ft.

Relative humidity

Ratio of actual water vapour pressure to saturation vapour pressure at the same temperature, expressed as a percentage. Warm air can hold more vapour, so RH changes with temperature.

Nimbostratus characteristics

Deep, grey, layered cloud producing continuous steady rain or snow. Typically associated with warm fronts and frontal depressions; base is low and visibility poor beneath it.

Cold front passage

Steep frontal surface moving quickly, giving a sudden temperature drop, wind veer and squally gusts, heavy showers or thunderstorms, then rapid clearing with good visibility.

Warm front passage

Shallow frontal surface approaching slowly with extensive stratiform cloud, steady precipitation and poor visibility. Temperature rises gradually and wind veers after passage.

Occluded front

Front formed when a faster cold front overtakes a warm front and lifts the warm sector off the surface. Produces mixed cloud and precipitation, often with a temperature fall.

Air mass classification

Source-region labelling: polar or tropical, maritime or continental - cP, cT, mP, mT. Maritime air is moist and unstable at low levels; continental air is drier with larger diurnal range.

Buys Ballot's law

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

Geostrophic wind

Wind blowing parallel to straight isobars aloft where Coriolis force balances the pressure-gradient force. It is stronger with tight isobars and at higher latitudes.

Gradient wind

Wind following curved isobars. Around a low it is subgeostrophic because centrifugal force adds to the inward pull; around a high it is supergeostrophic.

Jet stream

Narrow band of strong winds near the tropopause, core often above 60 kt at 30,000–40,000 ft. Strongest in winter; linked to clear-air turbulence and frontal development.

Clear-air turbulence

Turbulence in cloud-free air, often near jet streams, tropopause breaks or mountain waves. It is not visible on radar and can be severe, so forecast charts and PIREPs are used.

Airframe icing risk

Most likely at −2 to −10 °C in stratiform cloud and up to −20 °C in cumuliform cloud. Large supercooled droplets give clear ice; small droplets give rime.

Thunderstorm stages

Cumulus stage with strong updraught; mature stage with both updraught and downdraught, lightning, hail and gusts; dissipating stage dominated by downdraught and precipitation.

Radiation fog formation

Forms over land on clear, calm nights with high relative humidity as the surface cools to dew point. Dissipates after sunrise; most common in autumn and winter in valleys.

METAR and SPECI

METAR is a routine aerodrome observation; SPECI is issued when conditions change significantly, such as visibility, cloud or wind. Both use coded groups for wind, weather, cloud and QNH.

SIGMET purpose

Warning of significant en-route weather such as severe turbulence, severe icing, cumulonimbus, volcanic ash or tropical cyclones. Issued for the safety of aircraft in flight.

CAVOK meaning

Coded in METAR or TAF when visibility is 10 km or more, no cloud below 5000 ft or the minimum sector altitude, and no significant weather such as CB or TCU.

TAF trend groups

BECMG indicates a gradual, permanent change within a period; TEMPO indicates temporary fluctuations; both qualify the main forecast and are not the prevailing conditions throughout.