ATPL Meteorology (Sri Lanka CAA Pilot Licence Examinations) cheat sheet

Standard atmosphere (ISA): MSL pressure 1013.25 hPa, temperature +15 °C, density 1.225 kg/m³, lapse rate 1.98 °C/1000 ft up to 36,090 ft (11 km), then isothermal −56.5 °C.

Composition by volume: nitrogen ~78 %, oxygen ~21 %, argon ~0.9 %, CO2 ~0.04 %; water vapour varies 0–4 % and drives most weather.

Troposphere: average depth 8 km at poles, 11 km at mid-latitudes, 16–18 km at equator; contains ~75 % of mass and nearly all water vapour.

Tropopause is defined by the lapse-rate break (isothermal or inversion); it is highest over the equator and lowest over poles, and breaks (folds) near jet streams.

Stratosphere: temperature rises with height due to ozone absorption of UV; ozone layer centred near 20–25 km.

Mesosphere: temperature falls to about −90 °C near 85 km (mesopause), the coldest part of the atmosphere.

Pressure halves roughly every 5500 m (18,000 ft) in the lower atmosphere; at 18,000 ft about 50 % of MSL pressure remains.

Temperature falls about 2 °C per 1000 ft in the troposphere; density falls faster than pressure because both pressure and temperature change.

QNH: altimeter subscale set to station pressure reduced to MSL - altimeter reads elevation AMSL when on the ground.

QFE: subscale set so altimeter reads zero (or field elevation) at the aerodrome reference point; used for circuit/height reference.

QNE: standard setting 1013.25 hPa; used above transition altitude - altimeter reads pressure altitude (flight level).

Pressure altitude = indicated altitude with 1013.25 set; true altitude = pressure altitude corrected for temperature and pressure error.

Rule of thumb: 1 hPa ≈ 27 ft at low level; 1 inHg ≈ 34 ft. Denser (higher) pressure → altimeter under-reads true altitude.

High-to-low pressure or warm-to-cold temperature along track: true altitude lower than indicated - terrain-clearance risk ('high to low, look out below').

Density altitude rises with high elevation, high temperature, high humidity and low pressure; performance charts use pressure altitude and temperature.

Transition altitude/level: climb uses QNH below TA, standard above; descent reverses at transition level; QNH set before descending below TA.

Altimeter error sources: instrument/position error, static-source error, temperature error, pressure-setting error; cold-air correction adds height as terrain rises.

Environmental lapse rate (ELR) is the actual measured rate; ISA is 1.98 °C/1000 ft (~6.5 °C/km).

DALR ≈ 3 °C/1000 ft (1 °C/100 m) for unsaturated air; SALR ≈ 1.5 °C/1000 ft (0.5 °C/100 m), varying with moisture and temperature.

Absolute instability: ELR > DALR. Conditional instability: SALR < ELR < DALR. Absolute stability: ELR < SALR.

Neutral: ELR equals DALR (dry) or SALR (saturated).

Inversions (temperature increasing with height) suppress vertical motion, trap moisture/smoke, and favour fog, haze and low stratus.

Radiation inversion: clear calm night, strongest near dawn over land; valley inversions form by cold-air drainage.

Subsidence inversion: descending air in anticyclones warms adiabatically, producing a dry stable layer often with a marked top to haze/stratus.

Frontal inversion: warm air overrunning cold air creates a stable layer at the frontal surface.

Diurnal variation: surface maximum mid-afternoon, minimum near sunrise; sea surface temperature varies little diurnally.

Relative humidity = (actual vapour pressure / saturation vapour pressure) × 100 %; saturation vapour pressure rises with temperature.

Dew point is the temperature to which air must cool at constant pressure to saturate; dew-point spread indicates how close air is to saturation.

Dew point depression small (< 2–3 °C) implies high RH, fog/stratus risk; large depression implies dry, clear weather.

Cloud base estimate: (temperature − dew point) °C × 400 ft ≈ convective cloud base above the surface.

Condensation requires saturation plus condensation nuclei; in clean air, supersaturation can occur, but in practice cloud forms at ~100 % RH.

Cloud families: high (cirrus, cirrostratus, cirrocumulus), middle (altostratus, altocumulus), low (stratus, stratocumulus, nimbostratus), and vertically developed (cumulus, cumulonimbus).

Continuous precipitation: nimbostratus/frontal stratiform cloud; showery precipitation: cumuliform cloud; hail: cumulonimbus with strong updraughts.

Orographic lifting enhances cloud and precipitation on windward slopes; foehn/rain-shadow drying occurs on lee slopes.

Ice nuclei are scarce; supercooled water droplets persist to about −20 °C, below which ice crystal growth dominates.

Air mass classification: polar (P) or tropical (T) source, plus maritime (m) or continental (c): cP, cT, mP, mT.

Cold front: steep slope (~1:50), rapid passage, squally wind veer, temperature drop, showers/CB, good visibility behind.

Warm front: shallow slope (~1:150–1:200), slow approach, extensive stratiform cloud, steady rain, poor visibility, gradual temperature rise.

Occluded front: cold front overtakes warm front; warm sector lifted off the surface; mixed cloud and precipitation.

Frontal depression life cycle: wave/incipient, young, mature (occluded), decay; deepening when pressure falls rapidly.

In the Northern Hemisphere, air circulates anticlockwise around a low; warm sector lies between warm and cold fronts, typically to the south/east.

Squall line: line of active thunderstorms ahead of or along a cold front, with severe gusts, hail and wind shear.

Trough: elongated low-pressure area; convergence and ascent give cloud and showers; ridge: elongated high, subsidence and fine weather.

Frontal speed: cold fronts typically move faster than warm fronts; occlusion forms where the cold front catches the warm front.

Geostrophic wind is parallel to isobars; near the surface friction causes the wind to cross isobars towards low pressure at 10–45° (typically 30° over land).

Buys Ballot's law (NH): with back to wind, low pressure lies to the left; in the SH, to the right.

Coriolis force increases with latitude and wind speed; it is zero at the equator, so geostrophic balance fails there.

Gradient wind: curved flow around highs and lows; around a low, wind is slower than geostrophic (subgeostrophic); around a high, faster (supergeostrophic).

Thermal wind: the vertical shear of geostrophic wind relates to the horizontal temperature gradient; wind veers with height in warm advection.

Jet stream: narrow band of strong winds near the tropopause (core > 60 kt); polar-front jet at mid-latitudes, subtropical jet nearer the equator.

Jet stream core is typically at 30,000–40,000 ft; strongest in winter; associated with CAT, clear-air turbulence, and rapid frontal development.

Local winds: sea breeze by day (land heats), land breeze by night; anabatic upslope by day, katabatic downslope at night; valley winds follow these patterns.

Föhn/chinook: warm dry downslope wind on lee of mountains; mountain wave turbulence and rotor cloud possible.

Airframe icing types: clear (glaze) from large supercooled droplets, rime from small droplets, mixed; most likely at −2 to −10 °C in stratiform cloud, and in cumuliform cloud up to −20 °C.

Freezing rain/drizzle: supercooled droplets produce rapid severe clear icing; avoid.

Icing intensity depends on liquid water content, droplet size, temperature and airframe exposure time; pitot heat and anti-ice required.

Turbulence: mechanical (terrain/wind), thermal (convection), frontal, and CAT (clear-air turbulence) near jet streams and tropopause breaks.

Thunderstorm ingredients: moisture, instability, lifting trigger; stages: cumulus, mature (updraught + downdraught, lightning, hail, gusts), dissipating.

Microburst/gust front: strong low-level wind shear; avoid take-off/landing near active CB and expect sudden airspeed changes.

Visibility reduction: fog (radiation, advection, frontal, upslope), mist, haze, blowing snow/dust, and precipitation; radiation fog forms with clear skies, light wind, high RH and cooling to dew point.

Volcanic ash and sandstorms: engine and airframe damage; avoid. Hail and icing in CB; lightning strike risk.

Wind shear and downbursts are most dangerous on approach/departure; go-around if airspeed or glidepath deviates significantly.

METAR: routine aerodrome observation; SPECI: special report when conditions change significantly; both use coded groups (wind, visibility, weather, cloud, temperature, QNH).

TAF: aerodrome forecast, valid typically 9–24 h (TAF AMD for amendment); trend forecasts (BECMG, TEMPO) qualify the main forecast.

Wind group: dddffKT, gusts Gxx; VRB for variable; 00000KT calm; 9999 = 10 km or more visibility.

Cloud groups: FEW (1–2 oktas), SCT (3–4), BKN (5–7), OVC (8); vertical visibility VV/// when sky obscured.

Weather codes: RA rain, SN snow, SH showers, TS thunderstorm, GR hail, FZ freezing, BR mist, FG fog, HZ haze; intensity prefix + or −.

SIGMET: warning of significant en-route weather (severe turbulence, icing, CB, volcanic ash, tropical cyclone); AIRMET for less severe phenomena.

GAMET: area forecast for low-level flights; provides en-route weather below a specified level, wind and temperature.

QNH and QFE in METAR/TAF are given in hPa; trend groups describe expected changes within the validity period.

Phenomena codes: CAVOK = visibility ≥10 km, no cloud below 5000 ft or MSA, no significant weather; used in METAR/TAF.