DGCA ATPL Aviation Meteorology cheat sheet

Dry air composition by volume: nitrogen ~78.08%, oxygen ~20.95%, argon ~0.93%, carbon dioxide ~0.04%; remaining gases trace.

Homosphere (surface to ~80 km) has near-uniform composition from turbulent mixing; heterosphere above has composition stratified by molecular weight.

Troposphere: average depth 11 km (8 km poles, 16-18 km equator); temperature decreases at average lapse rate 1.98 deg C/1000 ft (6.5 deg C/km).

Tropopause marks the temperature inversion base of the stratosphere; its height is highest over the equator and lowest over the poles, and it breaks (steps) at jet streams.

Stratosphere: temperature is isothermal in its lower part then rises to the stratopause (~50 km) due to ozone absorption of UV.

Mesosphere: temperature falls to about -90 deg C at the mesopause (~85 km); it is the coldest part of the atmosphere.

Standard atmosphere (ISA) sea-level values: temperature 15 deg C, pressure 1013.25 hPa, density 1.225 kg/m3, speed of sound 661 kt.

ISA temperature lapse rate applies only to 11 km (36,090 ft); from 11 to 20 km ISA temperature is constant at -56.5 deg C.

Pressure altitude and density altitude equal indicated altitude only when the aircraft is in ISA conditions at 1013.25 hPa.

Ideal gas law: p = rho * R * T; density is directly proportional to pressure and inversely proportional to absolute temperature.

Charles's law: at constant pressure, volume is proportional to absolute temperature; Guy-Lussac's law: at constant volume, pressure is proportional to absolute temperature.

Boyle's law: at constant temperature, pressure times volume is constant.

Density altitude is pressure altitude corrected for temperature deviation from ISA; high temperature, high humidity and low pressure all increase density altitude.

Density decreases about 1% per 80 m (about 1% per 270 ft) of elevation near sea level in ISA.

Humidity reduces air density because water vapour (molecular mass 18) is lighter than dry air (mean molecular mass ~29).

Diurnal temperature lag: maximum surface temperature occurs about 1400-1500 local, minimum near sunrise.

Temperature inversions are common in winter anticyclones, on clear calm nights (radiation inversion) and in valleys; visibility and icing hazards are often worst within them.

QNH is the altimeter subscale setting that makes the altimeter read elevation above mean sea level at the aerodrome; QFE makes it read height above the aerodrome reference point.

QNE is the standard setting 1013.25 hPa; the altitude indicated on QNE at touchdown is the pressure altitude of the aerodrome, and the difference from elevation is the 'pressure altitude error'.

Transition altitude is the altitude at/below which QNH is used; transition level is the lowest FL available above the transition altitude; the gap between them is the transition layer.

Altimeter overreads when flying from high pressure into low pressure if the subscale is not updated (about 30 ft per 1 hPa).

Altimeter underreads when flying from low pressure into high pressure; 'high to low, look out below'.

1 hPa corresponds to about 27 ft near sea level (8 hPa per 1000 ft in the lower atmosphere).

Low pressure systems (cyclones) have convergence and rising air; high pressure systems (anticyclones) have divergence and subsiding air.

Isobars spaced closely indicate strong pressure gradient and strong wind; pressure gradient force acts perpendicular to isobars from high to low pressure.

Relative humidity = (actual vapour pressure / saturation vapour pressure) x 100%; it changes with temperature even when water vapour content is fixed.

Dew point is the temperature to which air must be cooled at constant pressure to become saturated; dew point depression is the difference between temperature and dew point.

Saturated vapour pressure increases with temperature (roughly doubles for every 10 deg C rise), so warm air holds more moisture.

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

Dry adiabatic lapse rate (DALR) = 3 deg C/1000 ft (9.8 deg C/km); saturated adiabatic lapse rate (SALR) is about 1.5-2 deg C/1000 ft and varies with temperature and pressure.

Air is absolutely stable if environmental lapse rate (ELR) < SALR; conditionally unstable if SALR < ELR < DALR; absolutely unstable if ELR > DALR.

Lifting condensation level (LCL) can be estimated as 400 ft per deg C of dew point depression (or 125 m per deg C).

Stability is increased by cooling of the lower layers or warming aloft (e.g. subsidence), and decreased by surface heating or cold advection aloft.

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

Cumuliform clouds indicate instability; stratiform clouds indicate stability; nimbostratus produces continuous steady precipitation.

Cumulonimbus contains strong updraughts and downdraughts, severe turbulence, icing, hail and lightning; it can produce microbursts and gust fronts.

Precipitation types: rain, drizzle (<0.5 mm drops), snow, sleet, hail; showers fall from cumuliform clouds, continuous rain from stratiform clouds.

Radiation fog forms on clear calm nights with high relative humidity; it dissipates after sunrise as the surface warms.

Advection fog forms when warm moist air moves over a cold surface; it is common over the sea and coastal areas and can persist for days.

Frontal fog forms where warm rain falls into cooler air below (warm front) and saturates it; steaming fog forms when cold air moves over warm water.

Orographic fog/cloud forms when moist air is forced to rise over hills; upslope fog is common on windward slopes.

Fog requires relative humidity near 100%, condensation nuclei and cooling to dew point; visibility below 1000 m is fog, 1000 m to 5 km is mist.

Air masses are classified by source region and surface: cP, cT, mP, mT (and mA in some schemes); they modify as they travel over a new surface.

Cold front: steep slope (about 1:50 to 1:100), narrow band of heavy showers, squalls, cumulonimbus, rapid clearing behind; temperature falls after passage.

Warm front: shallow slope (about 1:100 to 1:150), wide band of layered cloud and continuous rain ahead; temperature rises after passage.

Occluded front forms when a cold front overtakes a warm front; the weather depends on whether it is cold or warm occlusion.

Stationary front has little movement; it can persist for days with continuous cloud and rain.

Frontal passage indicators: wind veer/back, pressure trough, temperature change, dew point change, cloud and precipitation sequence.

Polar front is the boundary between polar and tropical air and is the main source of mid-latitude depressions and jet streams.

Squall line is a line of active thunderstorms often ahead of a cold front; it produces severe turbulence, hail and strong gusty winds.

Synoptic charts show isobars (4 hPa intervals), fronts, troughs, ridges, pressure centres and isallobars (pressure tendency).

Mid-latitude depressions form on the polar front and move generally eastward or northeastward; their life cycle is wave, open, occlusion, decay.

Troughs are elongated areas of low pressure; ridges are elongated areas of high pressure; cols are saddle points between two highs and two lows.

Indian monsoon: southwest monsoon (June-September) brings moist onshore winds and heavy rain; northeast monsoon (October-December) affects southeast India.

Monsoon onset is associated with the northward migration of the ITCZ and the establishment of the tropical easterly jet and monsoon trough.

Monsoon depressions form over the Bay of Bengal and move west/northwest across India, producing heavy rain and low-level wind shear.

Break in monsoon occurs when the monsoon trough shifts to the Himalayan foothills, causing dry weather over central India and heavy rain in the north.

Tropical cyclones form over warm oceans (SST > 26.5 deg C) with low vertical wind shear; they do not form within 5 degrees of the equator due to negligible Coriolis force.

Western disturbances are extra-tropical systems that bring winter rain/snow to northwest India and Pakistan.

Thunderstorm life cycle: cumulus (updraught), mature (both up- and downdraughts, hail, lightning), dissipating (downdraught-dominated).

Thunderstorms require conditional instability, high moisture, a lifting trigger and sufficient vertical extent; they can reach tropopause altitude.

Microburst is a small-scale downdraught (<4 km) causing a sudden headwind-to-tailwind shear; it is a major hazard on approach and departure.

Wind shear is a change of wind speed/direction over short distance; low-level shear is reported in METAR as WS and is critical below 2000 ft.

Clear air turbulence (CAT) occurs in jet streams, especially near the tropopause and on the anticyclonic side of the jet; it is not visible on radar.

Structural icing requires visible moisture and airframe temperature near or below 0 deg C; clear ice forms in large supercooled drops (freezing rain), rime ice in small drops.

Freezing rain is the most dangerous icing condition; it can accumulate rapidly and is associated with warm fronts aloft over cold air below.

Carburettor icing can occur at outside temperatures between -10 deg C and +25 deg C with high humidity; it is most likely at low power settings.

Hail is most likely in the mature stage of a thunderstorm and can be encountered well outside the visible cloud.

METAR is a routine aerodrome report issued hourly (or half-hourly); SPECI is a special report issued when conditions change significantly.

METAR/TAF format order: type, station, time (UTC), wind, visibility, RVR, weather, cloud, temperature/dew point, QNH, remarks.

Wind is given in degrees true (METAR/TAF) and degrees magnetic in ATIS; gusts are shown as G (e.g. 25015G25KT).

Cloud amount codes: FEW (1-2 oktas), SCT (3-4), BKN (5-7), OVC (8); cloud base is given in hundreds of feet AGL.

TAF is a forecast for the aerodrome; validity is typically 9, 24 or 30 hours; change groups are BECMG (gradual), TEMPO (temporary) and FM (rapid change).

SIGMET is issued for significant weather (thunderstorms, icing, turbulence, volcanic ash, tropical cyclones) affecting the safety of aircraft.

AIRMET is issued for moderate hazards (moderate icing, turbulence, mountain obscuration) and low-level wind shear; it is less severe than SIGMET.

GAMET is a low-level area forecast; it covers hazards below FL100 or FL150 depending on the region.

Significant weather charts (SIGWX) show fronts, jet streams, CAT, icing, CBs and tropopause height; low-level charts cover below FL100.

World Area Forecast System (WAFS) provides global gridded forecasts and significant weather charts; WAFCs are in London and Washington.

ICAO Annex 3 specifies meteorological services for international air navigation; it defines METAR, TAF, SIGMET, AIRMET, GAMET and AIREP.

Volcanic ash advisories (VAA) are issued by designated VAACs; they are critical for flight planning and can close airspace.

Tropical cyclone advisories are issued by Tropical Cyclone Warning Centres; SIGMET for TC is issued when the cyclone is within the FIR.

Climatological data are used for aerodrome planning, runway orientation and performance calculations; they include wind roses and temperature extremes.

Jet streams are strong narrow bands of wind near the tropopause; the polar front jet lies near the polar front and the subtropical jet is associated with the Hadley cell.

The ITCZ is a belt of low pressure near the equator with deep convection; it migrates seasonally and affects tropical weather.

El Nino/La Nina (ENSO) affects global weather patterns and monsoon strength; El Nino is associated with weaker Indian monsoon in many years.

Aerodrome warnings are issued for hazardous conditions at the aerodrome such as thunderstorms, strong winds, frost, fog and volcanic ash.

METAR trend forecasts (TTTT) give expected conditions within 2 hours; they use BECMG, TEMPO and NOSIG.

RVR is reported in metres and is significant when visibility is below 1500 m or RVR below 600 m; it is given for the touchdown zone.

Weather codes: RA (rain), DZ (drizzle), SN (snow), SH (showers), TS (thunderstorm), GR (hail), FZ (freezing), BR (mist), FG (fog), HZ (haze).

CAVOK means visibility 10 km or more, no cloud below 5000 ft or below the highest MSA, and no CB/TCU; it replaces visibility, RVR, weather and cloud groups.

Wind direction is given in tens of degrees; VRB indicates variable direction; 00000KT indicates calm; 9999 indicates visibility 10 km or more.

QNH is reported in hPa in METAR; in some countries in inches of mercury (e.g. QNH 2992).

AIP and NOTAM supplement meteorological information; pilots must check them before flight.

AIREP/AIREP SPECIAL are pilot reports of weather encountered in flight; they are used to update forecasts and warnings.

ICAO meteorological watch offices (MWO) issue SIGMET for their FIR; they monitor and update forecasts.

World Meteorological Organization (WMO) coordinates global observation networks; ICAO sets the aviation-specific standards.

Climatological summaries are used for runway design, take-off performance and fuel planning; they include mean, extreme and frequency data.

Seasonal forecasts (e.g. monsoon outlook) are issued by national meteorological services and are used for planning, not for tactical flight decisions.

Space weather (solar radiation storms) can affect HF communications and navigation; advisories are issued by space weather centres.

Volcanic ash can damage engines and windscreens; avoidance is mandatory when ash is present; VAAC advisories give ash cloud position and forecast.

Tropical cyclone tracks are forecast with increasing error beyond 72 hours; SIGMET for TC is issued when the cyclone is within the FIR and affects aviation.

Aerodrome climatological summaries include wind roses, fog frequency, thunderstorm days and temperature extremes.

SIGMET validity is typically 4 hours (6 hours for tropical cyclones); it is cancelled when the hazard no longer exists.

AIRMET validity is typically 6 hours; it covers moderate hazards and is issued for the FIR.

GAMET is issued for low-level flights and includes section 1 (general) and section 2 (hazards).

METAR wind is reported in degrees true; ATIS wind is reported in degrees magnetic; the difference is magnetic variation.

The trend forecast in a METAR uses the same codes as TAF but covers only 2 hours after the observation time.

TAF change groups: BECMG indicates a gradual change, TEMPO indicates temporary fluctuations, FM indicates a rapid change at a specific time.

SIGMET for volcanic ash is issued when ash is reported or forecast in the FIR; it includes the ash cloud position and forecast.

Pilot reports (AIREP) are the only direct observations of turbulence and icing at flight level; they are used to verify forecasts.

The tropical easterly jet (TEJ) is a summer feature over South Asia associated with the monsoon; it affects flight planning at high levels.

The subtropical jet is strongest in winter and is associated with the Hadley cell; it can produce CAT.

The polar front jet is associated with the polar front and mid-latitude depressions; it is strongest in winter.

Sea breeze forms during the day when land heats faster than sea; it can produce a local wind shift and affect runway choice.

Mountain waves form when stable air flows over a mountain barrier; they produce severe turbulence, downdraughts and lenticular clouds.

Foehn/chinook winds are warm dry downslope winds that can cause rapid temperature rises and turbulence.

Land and sea breezes, valley winds and anabatic/katabatic winds are local circulations that affect take-off and landing.

The diurnal variation of wind is stronger over land than over sea; surface friction reduces wind speed and backs the wind over land.

METAR cloud base is given in hundreds of feet above aerodrome level (AGL); the height of cloud base above MSL is cloud base + aerodrome elevation.

Vertical visibility (VV) is reported when the sky is obscured (e.g. VV002); it is given in hundreds of feet.

Weather intensity is indicated by + (heavy), - (light) and no sign (moderate); proximity is indicated by VC.

The present weather group can include up to three groups; descriptors (MI, BC, PR, DR, BL, SH, TS, FZ) precede precipitation.

TAF is issued at least 3 times per day for major aerodromes; validity is typically 24 or 30 hours for major airports.

The QNH group in METAR is preceded by Q; the temperature/dew point group is separated by a slash (e.g. 12/08).

A SPECI is issued when visibility, RVR, cloud base or weather changes meet specified criteria; it is not issued for routine changes.

The trend forecast in a METAR is not a TAF; it is a short-term forecast valid for 2 hours.

ICAO Annex 3 requires METAR at least hourly and SPECI as needed; TAF at least 3 times daily for major aerodromes.

The WAFS provides global forecasts of wind, temperature, humidity, geopotential height and significant weather.

The WAFC significant weather chart (SIGWX) covers the globe and shows jet streams, CAT, icing, CBs and tropopause height.

Low-level significant weather charts cover below FL100 (or FL150) and show fronts, cloud, turbulence and icing.

Aerodrome warnings are issued for thunderstorms, strong winds, frost, fog, volcanic ash and other hazards affecting the aerodrome.

Wind shear warnings are issued for the aerodrome when wind shear is reported or forecast below 1600 ft.

The meteorological watch office (MWO) issues SIGMET and AIRMET for its FIR and monitors the weather.

Pilot reports (AIREP) are used to verify and update forecasts; they are mandatory for certain hazards.

The ITCZ is a belt of low pressure near the equator with deep convection and heavy rain; it migrates with the seasons.

The monsoon is a seasonal reversal of wind; the southwest monsoon brings moist air and heavy rain to India from June to September.

The northeast monsoon brings rain to southeast India from October to December.

Western disturbances bring winter rain and snow to northwest India and Pakistan.

Tropical cyclones form over warm oceans and can affect coastal areas; they are most common in the Bay of Bengal and Arabian Sea.

El Nino is associated with weaker Indian monsoon rainfall in many years; La Nina is associated with stronger monsoon.

The subtropical high (e.g. Azores high) affects trade winds and weather; it is associated with subsidence and fine weather.

The polar front is the boundary between polar and tropical air; it is the main source of mid-latitude depressions.

ICAO Annex 3 defines the meteorological services for international air navigation; it includes METAR, TAF, SIGMET, AIRMET, GAMET and AIREP.

WMO coordinates global observation networks; ICAO sets aviation-specific standards.

Volcanic ash advisories are issued by VAACs; they are critical for flight planning and can close airspace.

Tropical cyclone advisories are issued by TCWCs; SIGMET for TC is issued when the cyclone is within the FIR.

Space weather advisories are issued for solar radiation storms that can affect HF communications and navigation.

Aerodrome climatological summaries include wind roses, fog frequency, thunderstorm days and temperature extremes.

Seasonal forecasts are used for planning, not for tactical flight decisions.

Climatological data are used for runway design, take-off performance and fuel planning.

METAR wind is reported in degrees true; ATIS wind is reported in degrees magnetic; the difference is magnetic variation.

The trend forecast in a METAR uses the same codes as TAF but covers only 2 hours after the observation time.

TAF change groups: BECMG indicates a gradual change, TEMPO indicates temporary fluctuations, FM indicates a rapid change at a specific time.

SIGMET for volcanic ash is issued when ash is reported or forecast in the FIR; it includes the ash cloud position and forecast.

Pilot reports (AIREP) are the only direct observations of turbulence and icing at flight level; they are used to verify forecasts.

SIGMET validity is typically 4 hours (6 hours for tropical cyclones); it is cancelled when the hazard no longer exists.

AIRMET validity is typically 6 hours; it covers moderate hazards and is issued for the FIR.

GAMET is issued for low-level flights and includes section 1 (general) and section 2 (hazards).

The tropical easterly jet (TEJ) is a summer feature over South Asia associated with the monsoon; it affects flight planning at high levels.

The subtropical jet is strongest in winter and is associated with the Hadley cell; it can produce CAT.

The polar front jet is associated with the polar front and mid-latitude depressions; it is strongest in winter.

Sea breeze forms during the day when land heats faster than sea; it can produce a local wind shift and affect runway choice.

Mountain waves form when stable air flows over a mountain barrier; they produce severe turbulence, downdraughts and lenticular clouds.

Foehn/chinook winds are warm dry downslope winds that can cause rapid temperature rises and turbulence.

Land and sea breezes, valley winds and anabatic/katabatic winds are local circulations that affect take-off and landing.

The diurnal variation of wind is stronger over land than over sea; surface friction reduces wind speed and backs the wind over land.