ATPL Meteorology flashcards
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32 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. These values are the reference for pressure altitude, density altitude and altimeter calibration.
- Tropopause height variation
- The tropopause is about 16 km over the equator and about 8-10 km over the poles. It is higher in summer and lower in winter. It marks the top of the weather-bearing troposphere.
- QNH versus QFE
- QNH set on the altimeter subscale gives elevation above mean sea level. QFE gives height above the aerodrome reference point. QNE (1013.25 hPa) gives pressure altitude or flight level.
- Cold air altimeter error
- In air colder than ISA, the altimeter over-reads: true altitude is lower than indicated altitude. This reduces terrain clearance, so minimum safe altitudes must be increased in cold weather.
- Density altitude
- Pressure altitude corrected for temperature deviation from ISA. High density altitude reduces engine power, propeller thrust and lift and increases take-off and landing distances.
- Relative humidity
- The ratio of actual vapour pressure to saturation vapour pressure at the same temperature, expressed as a percentage. It rises as air cools even if the water vapour content is unchanged.
- Dew point spread
- Temperature minus dew point. A small spread (about 2 °C or less) indicates air close to saturation, so fog, low stratus or mist is likely if the air cools further.
- Latent heat release
- Condensation releases latent heat of vaporisation (about 2.5 × 10^6 J/kg), warming the air. This is why the saturated adiabatic lapse rate is less than the dry adiabatic lapse rate.
- Conditional instability
- A layer is conditionally unstable when the environmental lapse rate lies between the saturated and dry adiabatic lapse rates. Unsaturated parcels are stable; saturated rising parcels become unstable.
- Dry adiabatic lapse rate
- The rate at which an unsaturated rising parcel cools: about 9.8 °C per km, or 3 °C per 1000 ft. It applies until the parcel reaches saturation and condensation begins.
- Stable air characteristics
- Stable air suppresses vertical motion and produces stratiform cloud, steady precipitation, haze and poor visibility. Turbulence is usually limited and showers are absent.
- Radiation fog
- Fog forming over land on clear, calm nights when the surface cools by radiation and the air reaches saturation. It is usually shallow and dissipates after sunrise.
- Advection fog
- Fog forming when warm, moist air moves over a colder surface and is cooled to saturation. It can be persistent, may occur with wind and is common over the sea and coasts.
- Freezing rain hazard
- Rain falling through a sub-freezing layer becomes supercooled and freezes on impact, producing clear ice on the airframe. It is one of the most severe icing hazards for aircraft.
- Cold front passage
- A cold front has a steep slope and brings a narrow band of heavy showers or thunderstorms. Passage causes a sharp temperature fall, wind veer, pressure rise and improved visibility.
- Warm front passage
- A warm front has a shallow slope and brings widespread stratiform cloud and steady rain ahead of the surface front. Passage causes gradual warming, wind veer and improving visibility.
- Occluded front
- Formed when a cold front overtakes a warm front and lifts the warm sector off the ground. Weather is mixed, with cloud and precipitation near the triple point.
- 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.
- Sea breeze
- A local circulation blowing from sea to land by day, driven by differential heating. The land warms faster, creating lower pressure over land and onshore flow near the coast.
- Geostrophic wind
- Wind blowing parallel to straight isobars above the friction layer, where the pressure gradient force balances the Coriolis force. Near the surface, friction causes the wind to cross isobars.
- Clear air turbulence
- Turbulence occurring in cloud-free air, often near jet streams, in mountain waves or at the boundaries of air masses with strong wind shear. It is invisible to the eye and to weather radar.
- Clear ice
- Heavy, uneven ice formed by large supercooled drops that spread before freezing. It is the most dangerous structural icing because of its weight and severe aerodynamic effects.
- Microburst
- A strong, localized downdraught producing a damaging outflow at the surface. An aircraft first meets a headwind, then a tailwind, causing a sudden loss of airspeed and lift.
- Thunderstorm mature stage
- The stage with both updraughts and downdraughts, lightning, hail, heavy rain and gusty surface winds. It is the most hazardous stage for aircraft and lasts about 20-30 minutes.
- METAR report
- A routine aerodrome weather report issued every 30 or 60 minutes. It gives wind, visibility, weather, cloud, temperature, dew point, QNH and a short trend forecast.
- TAF forecast
- An aerodrome forecast valid typically for 9, 24 or 30 hours. It gives expected wind, visibility, weather and cloud, with change groups BECMG, TEMPO and FM.
- SIGMET
- A warning of significant weather hazardous to aircraft, such as severe turbulence, icing, thunderstorms, volcanic ash, tropical cyclones or a severe squall line.
- Cloud amount codes
- In METAR and TAF: FEW means 1-2 oktas, SCT 3-4, BKN 5-7 and OVC 8 oktas. Heights are given in hundreds of feet above aerodrome level.
- ITCZ
- The intertropical convergence zone is a belt of low pressure near the equator where the trade winds converge. It brings heavy convective showers and thunderstorms, often with extensive cloud.
- Tropical cyclone formation
- Tropical cyclones form over warm oceans with sea surface temperatures above about 26.5 °C, usually between 5° and 20° latitude. They do not form at the equator because Coriolis force is negligible.
- Monsoon circulation
- A seasonal reversal of wind: summer onshore winds bring heavy rain, while winter offshore winds bring dry conditions. It affects regions such as South Asia and West Africa.
- Foehn wind
- A warm, dry wind on the leeward side of mountains, caused by air descending after crossing the ridge. It can produce rapid temperature rises and clear skies.