DGCA CPL Aviation Meteorology cheat sheet

Dry air by volume: about 78% nitrogen, 21% oxygen, 0.93% argon, 0.04% carbon dioxide; water vapour varies 0-4%.

Standard sea-level pressure is 1013.25 hPa (29.92 inHg); standard sea-level temperature is 15 °C (288.15 K).

Troposphere: temperature falls with height; average lapse rate 1.98 °C/1000 ft (6.5 °C/km); contains ~75% of mass and nearly all water vapour.

Tropopause: isothermal layer separating troposphere from stratosphere; average height 11 km (36,000 ft), higher over equator (~16-18 km) than poles (~8-10 km).

Stratosphere: temperature rises with height due to ozone absorption of UV; ozone concentration peaks around 20-25 km.

Mesosphere: temperature falls again to about -90 °C near 85 km, the coldest part of the atmosphere.

Homosphere (surface to ~80 km) has near-uniform composition; heterosphere above has layers separated by molecular weight.

Pressure decreases roughly exponentially with height: about 50% of surface pressure remains at 18,000 ft and 25% at 34,000 ft.

QNH: altimeter reads elevation above mean sea level when set on the ground; QFE reads height above the aerodrome reference point.

QNE: altimeter set to 1013.25 hPa reads pressure altitude; standard separation is used above the transition altitude.

True altitude = indicated altitude + (QNH - 1013.25) × 30 ft per hPa at low level (approximate).

Pressure altimeter error: fly from high pressure to low pressure without resetting and the aircraft is lower than indicated.

Fly from high temperature to low temperature without resetting and the aircraft is lower than indicated (high to low, look out below).

ISA: sea level 15 °C and 1013.25 hPa, lapse 1.98 °C/1000 ft up to 36,090 ft, then isothermal at -56.5 °C.

Altimeter overreads in cold air: true altitude is lower than indicated; apply cold-temperature correction at low temperatures.

1 hPa pressure change corresponds to about 27 ft at sea level and about 30 ft near the surface in ISA; use 30 ft/hPa for quick estimates.

DALR (dry adiabatic lapse rate) = 3 °C/1000 ft (9.8 °C/km) for unsaturated rising parcels.

SALR (saturated adiabatic lapse rate) varies from about 1.5 °C/1000 ft in warm air to near 3 °C/1000 ft aloft; average ~1.8 °C/1000 ft.

Environmental lapse rate > DALR: absolutely unstable; between SALR and DALR: conditionally unstable; less than SALR: stable.

Relative humidity is the ratio of actual vapour pressure to saturation vapour pressure at the same temperature, expressed as a percentage.

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

Temperature inversion (lapse rate negative) traps moisture and pollutants below it, gives smooth air and often hazy or foggy conditions.

Latent heat: condensation releases about 600 cal/g; evaporation absorbs the same, driving convective development.

Cooling by radiation, advection, or adiabatic ascent produces saturation; saturation at the surface forms fog or dew, aloft forms cloud.

Cloud classification by height: high (cirrus, cirrostratus, cirrocumulus, above ~20,000 ft), middle (altostratus, altocumulus, ~6,500-20,000 ft), low (stratus, stratocumulus, ~surface-6,500 ft).

Cumulus and cumulonimbus have vertical development; cumulonimbus produces thunderstorms, hail, icing and severe turbulence.

Nimbostratus gives continuous steady precipitation; cumulonimbus gives showery precipitation, often with hail.

Stable air produces layered clouds (stratus, altostratus, nimbostratus) with steady precipitation; unstable air produces cumuliform clouds with showery precipitation.

Precipitation types: rain, drizzle (drops < 0.5 mm), snow, sleet, hail; hail forms in cumulonimbus with strong updraughts.

Supercooled water droplets exist in clouds at temperatures between 0 °C and about -20 °C, causing airframe icing.

Cloud base estimate: surface temperature minus dew point (spread) × 400 ft gives approximate convective cloud base in feet.

Cirrus indicates high-level moisture and often precedes a warm front by 12-24 hours.

Geostrophic wind flows parallel to isobars with low pressure to the left in the Northern Hemisphere; speed increases with tighter isobar spacing.

Buys Ballot's law: with back to the wind in the Northern Hemisphere, low pressure lies to the left.

Coriolis force is zero at the equator and maximum at the poles; it acts perpendicular to the wind, to the right in the Northern Hemisphere.

Friction near the surface backs the wind (turns it toward low pressure) and reduces speed; surface wind crosses isobars at about 30-45°.

Sea breeze blows from sea to land by day; land breeze blows from land to sea at night; strongest in the afternoon.

Anabatic (upslope) winds occur by day on sun-facing slopes; katabatic (downslope) winds occur at night, strongest in valleys and over ice.

Föhn or chinook is a warm dry downslope wind on the lee side of mountains, warming by about 3 °C/1000 ft on descent (DALR).

In the Northern Hemisphere, wind veers with height in the friction layer (backing in the Southern Hemisphere).

Air mass source regions: polar (cold, dry), tropical (warm, moist), continental (dry) and maritime (moist); classified by temperature and humidity.

Cold front: steep slope (~1:50), narrow band of heavy showery precipitation, squall lines, rapid clearing and veering wind after passage.

Warm front: shallow slope (~1:150), wide band of stratiform cloud and steady precipitation ahead, gradual clearing after passage.

Occluded front: cold front overtakes warm front; weather resembles a combination, often with extensive cloud and precipitation.

Stationary front: little movement, prolonged cloud and precipitation over the same area.

Frontal passage: temperature change, pressure fall then rise, wind veer, and visibility change are typical indicators.

Warm air overrunning a cold wedge produces a sequence: cirrus, cirrostratus, altostratus, nimbostratus, then rain.

Cold air behind a cold front brings instability, showery precipitation and good visibility between showers.

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

Hazards in a mature thunderstorm: severe turbulence, hail, icing, lightning, microbursts, wind shear, and reduced visibility.

Airframe icing forms in supercooled water between 0 °C and about -20 °C; clear ice from large droplets, rime ice from small droplets.

Icing severity increases with liquid water content, droplet size, and airframe collection efficiency; carburettor icing can occur at temperatures up to 20 °C with high humidity.

Clear air turbulence (CAT) occurs near jet streams, especially at the tropopause and in wind shear zones, without visible cloud.

Low-level wind shear and microbursts are most dangerous below 1000 ft AGL during approach and departure.

Fog types: radiation fog (clear calm nights), advection fog (warm moist air over cold surface), frontal fog, and upslope fog.

Visibility reduction by haze, smoke, and precipitation; fog forms when temperature reaches the dew point and relative humidity approaches 100%.

METAR is a routine aerodrome weather report, usually issued hourly; SPECI is issued when significant changes occur.

TAF is an aerodrome forecast valid typically 9-30 hours; TEMPO indicates temporary fluctuations, BECMG indicates gradual change.

METAR wind group: direction in tens of degrees (e.g., 27015KT = 270° at 15 kt); gusts shown as G, e.g., 27015G25KT.

Visibility in METAR is in metres; RVR reported as Rxx/xxxx; cloud amount codes: FEW (1-2 oktas), SCT (3-4), BKN (5-7), OVC (8).

QNH in METAR is given in hectopascals (Q) or inches of mercury (A); Q1013 means QNH 1013 hPa.

SIGMET warns of significant weather (severe turbulence, icing, thunderstorms, volcanic ash) hazardous to all aircraft.

AIRMET is issued for moderate hazards affecting light aircraft; PIREP is a pilot report of actual conditions encountered.

Significant weather charts (SIGWX) show fronts, pressure systems, turbulence, icing, and jet streams; low-level charts show conditions below 10,000 ft.

Wind shear on a SIGWX chart is depicted by special symbols and shading; jet stream axes are shown as bold arrows with speed.

Volcanic ash and tropical cyclone areas are marked on SIGWX charts and trigger SIGMET issuance.

Isobars on surface charts are drawn at 4 hPa intervals; closer spacing means stronger wind.

A depression (low) has closed isobars with lowest pressure at centre; an anticyclone (high) has highest pressure at centre.

Col: a saddle point between two highs and two lows, associated with light variable winds and often poor visibility.

Trough: an elongated area of relatively low pressure, associated with cloud and precipitation and wind shifts.

Ridge: an elongated area of relatively high pressure, associated with fair weather.

Upper-air charts use pressure levels (e.g., 500 hPa, 200 hPa) with contour lines of geopotential height and temperature.

Decode METAR trend: NOSIG means no significant change expected; BECMG means gradual change; TEMPO means temporary fluctuations.

TAF validity groups: use the FM group for rapid change at a specific time; BECMG for gradual change over a period; TEMPO for temporary conditions.

Wind direction in METAR is true; in ATIS and reported surface wind it is magnetic; convert as needed for runway use.

CAVOK means visibility 10 km or more, no cloud below 5000 ft or below the highest MSA, and no significant weather.

Cloud base in METAR is height above aerodrome level (QFE); add aerodrome elevation to get altitude above mean sea level.

RVR is reported when visibility is low; it is measured along the runway and is critical for low-visibility operations.

A SPECI is issued for significant changes such as visibility crossing thresholds, thunderstorm onset, or wind shear.

Trend forecasts in METAR apply for the 2 hours following the observation time.

Wind shear and microburst alerts are included in METAR/SPECI remarks and in SIGMET when severe.

Icing forecasts use the freezing level and cloud layers; icing is most likely in stratiform cloud at temperatures between 0 °C and -15 °C.

Turbulence forecasts use jet stream position, wind shear, and mountain wave indicators; CAT is common near the tropopause.

Freezing level is shown on charts as a contour; above it, icing risk exists in cloud.

Thunderstorm forecasts use instability indices (CAPE, lifted index) and moisture; CAPE > 1000 J/kg suggests potential thunderstorms.

Weather radar shows precipitation intensity and movement; echo tops indicate thunderstorm height.

Satellite imagery: visible for cloud, infrared for cloud-top temperature, water vapour for upper-level moisture.

Model output statistics (MOS) and ensemble forecasts provide probabilistic guidance; always cross-check with observations.

METAR example: EGLL 121250Z 27015KT 9999 SCT030 18/12 Q1013 NOSIG - wind 270° at 15 kt, visibility 10 km+, scattered cloud at 3000 ft, temp 18 °C, dew point 12 °C, QNH 1013 hPa.

TAF example: TAF EGLL 121100Z 1212/1318 27015KT 9999 SCT030 TEMPO 1212/1216 4000 SHRA BKN012 - temporary showers and reduced visibility.

QNH is pressure reduced to mean sea level; QFE is pressure at aerodrome level; QNE is 1013.25 hPa.

Altimeter setting: use QNH for terrain clearance; use QNE above transition altitude for separation.

Transition altitude is the altitude at which the altimeter is set to standard pressure; transition level is the lowest flight level above it.

In METAR, the temperature/dew point group is given as TT/TdTd; a small spread indicates high humidity and fog risk.

Wind gust group Gxx indicates maximum gust speed; gust spread greater than 10 kt indicates turbulence.

Decode RVR: R27/0400 means runway 27 RVR 400 m; R27/0400V0600 means variable between 400 and 600 m.

On a surface chart, isobars at 4 hPa spacing; a tight gradient means strong wind; a col means light variable wind.

Fronts on charts: cold front shown with triangles, warm front with semicircles, occluded with alternating symbols.

SIGWX charts show areas of turbulence, icing, and thunderstorms with defined symbols and heights.

Wind barbs: full barb = 10 kt, half barb = 5 kt, pennant = 50 kt; the barb points toward the direction the wind comes from.

Upper-air charts: 500 hPa chart shows 500 hPa height contours; 200 hPa chart shows jet stream and tropopause.

Thickness charts: 1000-500 hPa thickness indicates air mass temperature; low thickness = cold air.

Prognostic charts forecast conditions for a valid time; use them for flight planning beyond the observation period.

Significant weather charts are issued for aviation and show hazards at flight levels; check the valid time and coverage.

SIGMET for thunderstorms: indicates severe turbulence, hail, and icing; avoid the area.

SIGMET for volcanic ash: ash can cause engine flameout and windscreen damage; avoid.

SIGMET for tropical cyclone: strong winds, heavy rain, and turbulence; avoid the area.

AIRMET for moderate icing or turbulence: affects light aircraft; plan accordingly.

PIREP: pilot report of actual conditions; valuable for verifying forecasts.

Wind shear alerts: reported in METAR/SPECI and on ATIS; critical during approach and departure.

Microburst: a strong downdraught and outflow; detected by Doppler radar and low-level wind shear alert systems.

Icing and turbulence forecasts are on SIGWX charts and in area forecasts; use them for route planning.

TAF: aerodrome forecast; valid for 9-30 hours; includes wind, visibility, weather, cloud, and temperature.

Area forecast: covers a region; includes cloud, weather, and turbulence/icing for a specified area.

Trend forecast: appended to METAR; valid for 2 hours; indicates expected changes.

Nowcast: short-range forecast (0-6 hours) based on observations and radar.

Ensemble forecast: multiple model runs; provides probability of weather events.

Model output statistics: statistically corrected model output; used for temperature and precipitation forecasts.

Satellite and radar: real-time observations; used for nowcasting and verification.

Weather charts: surface, upper-air, and significant weather; used for flight planning.

Check METAR/TAF for departure, destination, and alternate; ensure weather above minima.

Check SIGMET/AIRMET for hazards along the route; avoid areas of severe weather.

Check wind and temperature aloft for fuel planning and altitude selection.

Check freezing level and icing forecasts; plan de-icing and anti-icing.

Check turbulence forecasts; plan altitude and route to avoid CAT.

Check visibility and cloud base for approach minima; plan alternate.

Check NOTAMs for meteorological equipment outages (e.g., radar, ATIS).

Update weather information en route via ATIS, VOLMET, and ATC.

Take-off minima: visibility/RVR and cloud base; check operator and aerodrome minima.

Approach minima: decision altitude/height (DA/DH) or minimum descent altitude/height (MDA/MDH) plus visibility/RVR.

Alternate minima: check weather at alternate; ensure it meets planning minima.

Low-visibility procedures: CAT I, II, III; require specific RVR and equipment.

Wind limitations: crosswind and tailwind limits; check runway orientation.

Icing and turbulence: check forecasts and PIREPs; plan accordingly.

Thunderstorm avoidance: avoid by at least 20 nm; do not penetrate.

Volcanic ash: avoid; no safe penetration.

METAR/SPECI codes: intensity (+/-), descriptor (MI, BC, PR, DR, BL, SH, TS, FZ), precipitation (DZ, RA, SN, SG, IC, PL, GR, GS), obscuration (BR, FG, FU, VA, DU, SA, HZ, PY), other (PO, SQ, FC, SS, DS).

Cloud codes: FEW, SCT, BKN, OVC; CB and TCU for cumulonimbus and towering cumulus.

Weather codes: SHRA = showers of rain; TSRA = thunderstorm with rain; FZRA = freezing rain; MIFG = shallow fog.

Visibility codes: 9999 = 10 km or more; 0000 = less than 50 m; 4 digits in metres.

Wind codes: 27015KT = 270° at 15 kt; VRB03KT = variable at 3 kt; 27015G25KT = gusting 25 kt.

QNH codes: Q1013 = 1013 hPa; A2992 = 29.92 inHg.

Temperature codes: 18/12 = temperature 18 °C, dew point 12 °C; M05/M08 = -5 °C / -8 °C.

RVR codes: R27/0400 = runway 27 RVR 400 m; R27/0400V0600 = variable 400-600 m.

QNH: altimeter reads elevation above mean sea level when on the ground.

QFE: altimeter reads height above aerodrome reference point.

QNE: standard pressure 1013.25 hPa; used above transition altitude.

Altimeter error: high to low pressure or high to low temperature means true altitude is lower than indicated.

Cold temperature correction: apply when temperature is below ISA; true altitude is lower than indicated.

Pressure altitude: altitude when altimeter set to 1013.25 hPa.

Density altitude: pressure altitude corrected for temperature; high density altitude reduces performance.

Standard atmosphere: 15 °C, 1013.25 hPa at sea level; lapse rate 1.98 °C/1000 ft to 36,090 ft.

Stable air: stratiform clouds, steady precipitation, smooth air, good visibility outside precipitation.

Unstable air: cumuliform clouds, showery precipitation, turbulence, good visibility between showers.

Conditionally unstable: stable if unsaturated, unstable if saturated; common in mid-latitudes.

Inversion: temperature increases with height; traps moisture and pollutants; smooth air.

Cloud base: estimate by (temperature - dew point) × 400 ft for convective clouds.

Fog: radiation fog on clear calm nights; advection fog when warm moist air moves over cold surface.

Thunderstorm: requires moisture, instability, and lifting mechanism; three stages: cumulus, mature, dissipating.

Icing: supercooled water between 0 °C and -20 °C; clear ice from large droplets, rime from small droplets.

Geostrophic wind: parallel to isobars, low pressure to left in Northern Hemisphere.

Friction layer: wind backs and slows near surface; crosses isobars at 30-45°.

Sea breeze: sea to land by day; land breeze: land to sea at night.

Anabatic: upslope by day; katabatic: downslope at night.

Föhn: warm dry downslope wind; warms at DALR on descent.

Jet stream: strong upper-level wind; CAT near jet stream.

Wind shear: change in wind speed/direction with height; hazardous at low levels.

Microburst: strong downdraught and outflow; hazardous during approach and departure.

Air mass: large body of air with uniform temperature and humidity; source regions polar, tropical, continental, maritime.

Cold front: steep slope, narrow band of heavy showery precipitation, rapid clearing.

Warm front: shallow slope, wide band of stratiform cloud and steady precipitation.

Occluded front: cold front overtakes warm front; complex weather.

Stationary front: little movement; prolonged precipitation.

Frontal passage: temperature change, pressure fall then rise, wind veer, visibility change.

Warm air overrunning cold air: cirrus, cirrostratus, altostratus, nimbostratus, rain.

Cold air behind cold front: instability, showery precipitation, good visibility between showers.

Thunderstorm hazards: severe turbulence, hail, icing, lightning, microbursts, wind shear.

Icing: supercooled water; clear ice, rime ice; carburettor icing at temperatures up to 20 °C with high humidity.

CAT: near jet streams, tropopause, wind shear zones; no visible cloud.

Low-level wind shear: most dangerous below 1000 ft AGL.

Fog: radiation, advection, frontal, upslope; reduces visibility.

Volcanic ash: engine flameout, windscreen damage; avoid.

Tropical cyclone: strong winds, heavy rain, turbulence; avoid.

Microburst: strong downdraught and outflow; detected by Doppler radar.

METAR: routine aerodrome weather report; hourly.

SPECI: special report for significant changes.

TAF: aerodrome forecast; 9-30 hours.

SIGMET: significant weather warning; severe turbulence, icing, thunderstorms, volcanic ash.

AIRMET: moderate hazards affecting light aircraft.

PIREP: pilot report of actual conditions.

Trend forecast: appended to METAR; valid 2 hours.

Area forecast: covers a region; cloud, weather, turbulence/icing.

Surface chart: isobars at 4 hPa; fronts; pressure systems.

SIGWX chart: turbulence, icing, thunderstorms, jet streams.

Upper-air chart: pressure levels (500 hPa, 200 hPa); height contours; temperature.

Wind barbs: full barb 10 kt, half 5 kt, pennant 50 kt.

Thickness chart: 1000-500 hPa thickness; low thickness = cold air.

Prognostic chart: forecast for valid time.

Significant weather chart: hazards at flight levels.

Check valid time and coverage on all charts.