ATPL Meteorology (050) cheat sheet

Composition (dry air by volume): nitrogen ~78.09%, oxygen ~20.95%, argon ~0.93%, CO2 ~0.04%; water vapour is variable, 0-4% by volume.

ICAO Standard Atmosphere: MSL temperature 15 °C, pressure 1013.25 hPa, density 1.225 kg/m³; tropospheric lapse rate 6.5 °C/km up to 11 km.

In the ISA, pressure at 11 km (tropopause) is about 226.32 hPa and temperature is -56.5 °C; above 20 km temperature rises isothermally then increases.

Layers by temperature trend: troposphere (decreasing), stratosphere (increasing, ozone heating), mesosphere (decreasing), thermosphere (increasing).

Tropopause height: ~16 km over the equator, ~8-10 km over mid-latitudes, ~8 km or lower over the poles; it is higher in summer and over warm air masses.

Pressure altitude = altitude indicated on 1013.25 hPa; true altitude = actual height above MSL; density altitude = pressure altitude corrected for temperature deviation from ISA.

ISA temperature at a level = 15 °C - (2 °C per 1000 ft) up to 36,090 ft; ISA pressure decreases roughly 1 hPa per 27 ft near MSL (about 1 hPa per 30 ft in standard air).

Height of a pressure level increases with mean virtual temperature of the layer below: warm air = high pressure-level heights, cold air = low heights.

Insolation heats the ground; the ground heats the air by conduction, then convection and turbulence redistribute heat; the atmosphere is heated mainly from below.

Dry adiabatic lapse rate (DALR) = 3 °C/1000 ft (9.8 °C/km); saturated adiabatic lapse rate (SALR) varies from ~1.5 °C/1000 ft in warm air to ~3 °C/1000 ft in very cold air.

Environmental lapse rate (ELR) is the actual measured rate; compare ELR with DALR/SALR to judge stability.

Radiation inversions form on clear, calm nights by rapid radiational cooling of the ground; they are strongest at dawn and dissipate from the ground upward.

Subsidence inversions form under anticyclones where descending air warms adiabatically; they cap pollution and often mark a marked humidity drop.

Frontal inversions occur where warm air overruns cold air; the inversion is at the frontal surface, with a temperature rise and wind veer on passage.

Valley inversions form in calm, clear nights as cold dense air drains downslope (katabatic) and pools in the valley floor; the top of the inversion is often marked by a cloud or haze layer.

Diurnal range is greatest with clear skies, dry air, light winds and inland/continental locations; it is smallest over oceans and under cloud or wind.

QNH is the altimeter setting that makes the altimeter read elevation (AMSL) at the aerodrome; QFE makes it read zero (or field elevation above the reference point) on the ground.

QNE is the standard setting 1013.25 hPa; the altitude indicated with QNE at a pressure level is the pressure altitude, and the transition level uses it.

Altimeter is a pressure instrument: high pressure outside or low setting on the subscale gives a higher reading; fly from high to low pressure (or high to low temperature) without resetting and the true altitude is lower than indicated.

Rule of thumb: 1 hPa pressure change ≈ 30 ft near MSL; 1 °C temperature error ≈ 4 ft per 1000 ft of altitude (approximately 0.4% per °C).

Cold-air correction: when temperature is below ISA, true altitude is lower than indicated; the correction is larger at high altitude and low temperature, and must be applied to procedure/terrain clearance altitudes.

Warm air makes true altitude higher than indicated (the altimeter under-reads height); cold air makes it lower (over-reads height).

Transition altitude is the altitude where the altimeter is set to QNE on climb; transition level is the lowest flight level available above the transition altitude, and it varies with QNH.

Altimeter errors: position/static error, instrument error, temperature error, and pressure-setting error; only the last two are normally correctable by the pilot.

Relative humidity = (actual vapour pressure / saturation vapour pressure at that temperature) × 100%; warm air holds more water vapour.

Dew point is the temperature to which air must be cooled at constant pressure and moisture content to become saturated; dew point depression = temperature - dew point.

Condensation occurs when air is cooled to its dew point or when moisture is added; condensation nuclei are needed for cloud droplets.

Saturation vapour pressure over ice is lower than over water at the same temperature, so ice crystals grow at the expense of supercooled droplets (Bergeron process).

Stability criteria: ELR < SALR = absolutely stable; SALR < ELR < DALR = conditionally unstable; ELR > DALR = absolutely unstable.

Stable air: stratiform cloud, steady precipitation, smooth air, restricted visibility in haze; unstable air: cumuliform cloud, showers, turbulence, good visibility between showers.

Convective condensation level (CCL) is the height where rising surface air reaches saturation; the lifting condensation level (LCL) is the height where a lifted parcel first becomes saturated.

Latent heat: condensation releases ~2,260 J/g, freezing releases ~334 J/g; this warms the air and drives the SALR below the DALR.

Cloud genera: cirrus, cirrocumulus, cirrostratus (high, above ~20,000 ft); altocumulus, altostratus (middle, ~6,500-20,000 ft); stratus, stratocumulus, nimbostratus (low); cumulus, cumulonimbus (vertical).

Stratus forms by cooling of a layer to saturation (radiation, advection over a cold surface, or lifting); drizzle and poor visibility are typical.

Cumulus forms by convection of unstable air; cumulonimbus requires deep instability, high moisture and a lifting trigger, and produces showers, hail and lightning.

Nimbostratus is associated with slow, widespread lifting along a warm front or in a frontal trough; continuous steady precipitation, often with low cloud base.

Precipitation types: drizzle (droplets < 0.5 mm), rain, snow, snow grains, snow pellets, ice pellets, hail, and freezing rain/drizzle when supercooled drops freeze on impact.

Freezing rain forms when a warm layer aloft melts snow and the drops then fall through a sub-freezing layer near the surface without freezing before impact.

Hail forms in cumulonimbus where strong updraughts recycle graupel through the supercooled water region; large hail indicates severe updraughts.

Cloud base (convective) ≈ 400 ft per °C of surface dew point depression (spread), valid for convective cloud over land in daytime.

Air mass classification: cA, cP, mP, cT, mT; source regions are cold continents, warm oceans, tropical deserts and polar oceans; classification depends on temperature and moisture relative to the surface.

Warm front: warm air overrunning cold air, slope ~1:150, cloud sequence Ci → Cs → As → Ns, precipitation ahead of the surface front, wind veers, pressure falls then levels.

Cold front: cold air undercutting warm air, slope steeper (~1:50), narrow band of cumuliform cloud and showers, wind veers sharply, pressure rises after passage, temperature falls.

Occluded front: cold front catches warm front; warm sector lifted off the ground; complex cloud and precipitation, often with a temperature trough and wind veer.

Stationary front: little movement, often parallel to the upper flow; persistent cloud and precipitation can last for days.

Frontal passage signs: pressure trough (minimum at passage), wind veer, temperature change, dew point change, visibility change, and cloud/precipitation sequence.

In the Northern Hemisphere, fronts lie in troughs of low pressure and move with the thermal wind; cold fronts generally move faster than warm fronts.

Polar front jet and baroclinic zones are associated with the polar front; frontogenesis occurs where temperature gradients tighten, frontolysis where they relax.

Anticyclone: high pressure, descending air, divergence at the surface, subsidence inversion, light winds, clear skies, radiation fog and haze in winter; in the Northern Hemisphere wind is clockwise and outward.

Depression (low): ascending air, convergence at the surface, pressure falls, cloud and precipitation; in the Northern Hemisphere wind is anticlockwise and inward.

Trough: elongated area of low pressure; associated with fronts and showery weather; ridge: elongated area of high pressure, associated with fair weather and subsidence.

Col: saddle point between two highs and two lows; light variable winds and poor visibility are typical.

Isobars close together = strong pressure gradient = strong wind; the geostrophic wind is inversely proportional to latitude and directly proportional to the pressure gradient.

Buys Ballot's law (Northern Hemisphere): with your back to the wind, low pressure is on your left; in the Southern Hemisphere, low pressure is on your right.

Thermal lows form over hot land in summer (e.g., over deserts) and are shallow; they produce dust, haze and convective cloud.

Synoptic charts: surface chart shows isobars, fronts and pressure centres; significant weather charts (SIGWX) show fronts, jet streams, CAT, icing, turbulence and volcanic ash.

Global circulation: trade winds (easterlies) from the subtropical highs to the equatorial trough (ITCZ); westerlies in mid-latitudes; polar easterlies.

In the Northern Hemisphere, surface wind crosses isobars towards low pressure at ~30° over land and ~10-20° over sea; the geostrophic wind blows parallel to the isobars above the friction layer.

Coriolis force is proportional to wind speed and the sine of latitude; it is zero at the equator, which is why tropical cyclones do not form within ~5° of the equator.

Sea breeze: daytime, land heats faster, pressure falls over land, wind blows from sea to land; land breeze: night, land cools faster, wind blows from land to sea.

Anabatic (upslope) winds occur on sunny slopes by day; katabatic (downslope) winds occur at night as cold dense air drains into valleys.

Föhn (chinook) wind: air descends the lee side of a mountain, warms at the DALR after losing moisture on the windward side; it produces warm, dry, gusty conditions and downslope turbulence.

Mountain wave and rotor turbulence occur downwind of ridges in strong stable flow; lenticular clouds and cap clouds mark the wave.

Wind shear is a change of wind speed and/or direction over a short distance; low-level wind shear is most hazardous near the ground during takeoff and landing.

Jet stream: a narrow band of strong winds (> 60 kt) near the tropopause; the polar front jet is associated with the polar front, the subtropical jet with the Hadley cell.

Jet stream core is usually at or just below the tropopause; wind speed increases with horizontal temperature gradient (thermal wind relationship).

Jet streaks: entrance and exit regions have ageostrophic circulation; the left exit and right entrance quadrants favour ascent and cyclogenesis (Northern Hemisphere).

Clear air turbulence (CAT) is most likely near jet streams, in the vicinity of the tropopause, in strong wind shear, and in mountain wave regions; it is invisible to radar.

Tropopause folding brings stratospheric air with high ozone and low humidity down into the troposphere; it is associated with CAT and rapid cyclogenesis.

Upper winds veer with height in the Northern Hemisphere in warm advection and back with height in cold advection.

Upper-air charts: 500 hPa (~18,000 ft) and 300/250 hPa (~30,000-34,000 ft) show geopotential height, temperature, wind and humidity; contour spacing indicates wind speed.

The tropopause is a stable layer; it limits the vertical development of most cloud, but overshooting tops of cumulonimbus can penetrate it.

Aircraft icing: clear ice from large supercooled droplets (SLD) in cumuliform cloud or freezing rain, rime ice from small droplets in stratiform cloud; most frequent at -2 °C to -15 °C.

Icing severity increases with liquid water content, droplet size and time in cloud; freezing rain and SLD are severe hazards and require immediate exit.

Thunderstorm life cycle: cumulus (updraughts), mature (updraughts and downdraughts, lightning, hail, gust front), dissipating (downdraughts dominate).

Microburst: a concentrated downdraught (< 4 km diameter) producing a strong divergent outflow; headwind then tailwind shear can cause a sudden loss of airspeed and performance.

Low-level wind shear and gust fronts are most hazardous on approach and departure; escape technique is to go around and avoid the shear.

Hail, tornadoes and lightning are associated with severe cumulonimbus; a tornado is a violently rotating column of air extending from the cloud base to the ground.

Visibility reduction: fog, mist, haze, smoke, dust, sand, precipitation and blowing snow; fog forms when the temperature-dew point spread is small and the air is cooled or moistened.

Volcanic ash: abrasive, melts in the engine hot section, damages windscreens and pitot tubes; avoid the ash cloud and report it; SIGMET and NOTAM are issued for ash.

METAR: routine aerodrome weather report issued at half-hourly or hourly intervals; contains wind, visibility, RVR, weather, cloud, temperature, dew point, QNH and trend.

SPECI: special aerodrome report issued when significant changes occur (e.g., visibility, cloud base, wind, weather) between routine reports.

TAF: aerodrome forecast for a specified period, with BECMG (gradual change), TEMPO (temporary fluctuation) and FM (rapid change) groups; validity typically 9, 24 or 30 hours.

TREND: a short forecast appended to METAR/SPECI covering the next 2 hours at that aerodrome (NOSIG = no significant change expected).

SIGMET: information on significant en-route weather (thunderstorms, severe turbulence, icing, volcanic ash, tropical cyclones) affecting all aircraft; AIRMET is for lighter aircraft and less severe conditions.

GAMET: area forecast for low-level flights (below FL100/FL150), issued in plain language with sections for wind, visibility, weather, cloud and turbulence/icing.

PIREP: pilot report of actual conditions encountered; it is the primary source of real-time information on turbulence, icing and wind shear.

Weather charts and NOTAM: surface and upper-air charts, SIGWX charts, and NOTAMs for aerodrome/airspace changes; check the validity time and the units (hPa, ft, kt, °C).