PPL Meteorology flashcards

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

ISA sea-level temperature

15 °C at mean sea level, with pressure 1013.25 hPa and density 1.225 kg/m³; the standard lapse rate is 1.98 °C per 1000 ft up to 36 090 ft.

QNH

Altimeter setting that makes the instrument read aerodrome elevation when on the ground; it is the pressure reduced to mean sea level using the standard atmosphere.

QFE

Altimeter setting that makes the instrument read zero on the aerodrome; it equals the actual station pressure at the aerodrome reference point elevation.

QNE

Standard altimeter setting of 1013.25 hPa; used at and above the transition level, where the instrument indicates pressure altitude rather than elevation.

Density altitude

Pressure altitude corrected for temperature deviation from ISA; hot, high, humid conditions raise it and degrade takeoff, climb and engine performance.

Relative humidity

Ratio of actual water vapour pressure to the saturation vapour pressure at the same temperature, expressed as a percentage; 100 % means saturation and possible condensation.

Dew point

Temperature to which air must be cooled at constant pressure and moisture content to become saturated; a small temperature–dew point spread means high humidity and likely cloud or fog.

Dry adiabatic lapse rate

Rate at which unsaturated air cools when lifted, about 3 °C per 1000 ft (9.8 °C per km); it applies until the air reaches saturation.

Saturated adiabatic lapse rate

Rate at which saturated air cools on lifting, about 1.5 °C per 1000 ft; it is smaller than the dry rate because latent heat is released by condensation.

Coriolis force

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; zero at the equator.

Geostrophic wind

Theoretical wind blowing parallel to straight isobars, where the pressure-gradient force balances the Coriolis force; it applies in free air above friction layer.

Buys Ballot law

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

Anabatic wind

Daytime upslope wind caused by heating of sun-facing terrain, which warms adjacent air and makes it rise along the slope; opposite of the nocturnal katabatic wind.

Katabatic wind

Downslope wind caused by radiative cooling of air over high terrain, usually at night; cold dense air drains into valleys and can be strong in stable conditions.

Föhn wind

Warm dry downslope wind on the lee side of a mountain: air cools at the dry rate upwind, loses moisture, then warms at the saturated rate descending.

Jet stream

Narrow band of strong upper-tropospheric wind near the tropopause; the polar-front jet can exceed 200 kt and is associated with clear air turbulence.

Radiation fog

Fog forming on clear, calm nights as the ground cools and the air above it reaches saturation; it is shallow and usually disperses after sunrise.

Advection fog

Fog forming when warm moist air moves over a colder surface and is cooled to saturation; it needs wind and can persist for long periods over sea or coasts.

Clear ice

Glaze ice from large supercooled droplets at about 0 °C to −10 °C; it spreads and freezes slowly, giving heavy, hard accretions that seriously degrade performance.

Rime ice

Opaque, rough ice from small supercooled droplets at about −10 °C to −20 °C; it freezes quickly on impact, is lighter than clear ice but still changes aerofoil shape.

Carburettor icing

Ice forming in the carburettor from fuel vaporisation cooling and venturi depression; most likely at ambient temperatures between −7 °C and +21 °C with high humidity.

Cold front

Boundary where cold air advances under warm air; steep slope, fast movement, narrow band of heavy showery rain, squalls, and a wind veer with temperature fall after passage.

Warm front

Boundary where warm air rides over cold air; shallow slope, slow movement, wide band of steady precipitation ahead, and gradual warming with wind veer after passage.

Occluded front

Front formed when a cold front catches a warm front and lifts the warm sector off the ground; it produces complex cloud and continuous precipitation near the surface front.

Tropical revolving storm

Intense low forming over warm ocean water above about 27 °C between 5° and 20° latitude; it does not form at the equator and brings violent wind, rain and squalls.

METAR wind group

Coded surface wind: 27015G25KT means 270° true at 15 kt gusting 25 kt; VRB means variable direction, and a following group such as 240V300 gives the variation range.

METAR cloud groups

Amount codes FEW (1–2 oktas), SCT (3–4), BKN (5–7), OVC (8); height is in hundreds of feet above aerodrome, and VV gives vertical visibility in obscured sky.

TAF TEMPO and BECMG

TEMPO forecasts temporary fluctuations, usually each less than one hour and totalling less than half the period; BECMG forecasts a gradual change over up to two hours.

SIGMET

Warning of weather hazardous to all aircraft: severe turbulence or icing, volcanic ash, tropical cyclones, severe squall lines, and radioactive release; it is issued for a defined area and period.

Microburst

Concentrated downdraft from a thunderstorm reaching the surface in under five minutes and spreading out less than 4 km; causes a sudden headwind then tailwind shear on approach.

Mountain wave

Standing wave in strong stable airflow crossing a mountain barrier; marked by lenticular clouds and rotor turbulence, with severe downdrafts and possible icing on the lee side.

Temperature ISA deviation

Difference between actual temperature at a level and the ISA value; warm air means true altitude is higher than indicated, cold air means it is lower.