Aviation meteorology

In short

Aviation meteorology is the study of the atmosphere as the medium an aircraft flies through: pressure, temperature, humidity, wind, cloud, visibility and the hazards they create for take-off, cruise and landing. It matters because weather, not machinery, is behind most delays, diversions and a large share of weather-related accidents — a density-altitude error on a Leh departure or an unrecognised microburst on short finals is a flight-safety event. In India the Met services for civil aviation are provided by the India Meteorological Department (IMD) under ICAO Annex 3 and WMO practice, with DGCA as the regulator and AAI as the air-navigation service provider; the enabling law is the Bharatiya Vayuyan Adhiniyam, 2024, which replaced the former Aircraft Act, 1934. It is a full paper in the DGCA CPL/ATPL examinations, a core B.Sc Aviation university subject, and is tested in cadet-pilot and airport-operations selections at IndiGo, Air India, Akasa, SpiceJet, AAI, Adani and GMR airports.

Explained

The atmosphere and the International Standard Atmosphere

• Composition (dry air by volume): nitrogen ~78%, oxygen ~21%, argon ~0.9%, carbon dioxide ~0.04%; water vapour is variable (0 to about 4%) and is the only constituent that changes state in the atmosphere.
• Layers: troposphere (all weather, temperature falls with height), tropopause, stratosphere (ozone, temperature steady then rises), mesosphere, thermosphere. Tropopause height is about 11 km (36,090 ft) in the ISA but 16–17 km (up to ~55,000 ft) over tropical India, and lower in winter over north India.
• ISA (ICAO standard atmosphere), the reference for all performance and altimetry: MSL pressure 1013.25 hPa, temperature +15 °C, density 1.225 kg/m³, lapse rate 1.98 °C per 1,000 ft (6.5 °C/km) up to 36,090 ft, then constant −56.5 °C.
• Lapse rates: DALR = 3 °C/1,000 ft (9.8 °C/km) for unsaturated air; SALR ≈ 1.2–1.8 °C/1,000 ft (latent heat release slows cooling); ELR is the actual measured rate from a radiosonde.
• Stability: ELR < SALR → absolutely stable (stratiform cloud, haze, smooth air); ELR > DALR → absolutely unstable (convection, cumuliform cloud, turbulence); between the two → conditionally unstable (needs saturation to become unstable — the normal monsoon situation). Inversion = temperature rising with height; traps moisture and pollution, gives fog, smog and sharp low-level wind shear.
• Humidity terms: absolute humidity (g/m³), mixing ratio (g/kg), relative humidity (actual vapour pressure ÷ saturation vapour pressure), dew point (temperature at which air saturates at constant pressure). A small temperature–dew-point spread means cloud, mist or fog is near.

Pressure, altimetry and density altitude

• Pressure falls roughly 1 hPa per 27 ft near sea level (about 1 hPa per 50 ft at 18,000 ft) — the fall is exponential, not linear.
• Altimeter settings: QNH — set to read aerodrome elevation on the ground, so the altimeter shows altitude AMSL; QFE — set to read zero on the runway, showing height above the aerodrome; QNE / 1013.2 hPa — the standard setting that gives pressure altitude and flight levels. India's transition altitude is commonly 4,000 ft AMSL but is aerodrome-specific — always read AIP India.
• Errors (never corrected by the instrument):
  • *Pressure error:* flying from high to low pressure with an old setting, the altimeter over-reads — "high to low, look out below".
  • *Temperature error:* in air colder than ISA the aircraft is lower than indicated (about 4 ft per 1,000 ft of height per °C of ISA deviation). Critical on cold-weather approaches at Srinagar, Leh and Kathmandu-type terrain.
• Pressure altitude is the altitude in the ISA at which the ambient pressure occurs; density altitude is the altitude in the ISA at which the ambient *density* occurs, and it is the number that governs performance.
• High density altitude (hot, high, humid, low QNH) means less thrust, less lift, longer take-off run, poorer climb gradient — the limiting factor at Leh (elevation ~10,680 ft), Kushinagar-type hot-and-heavy summer departures, and pre-monsoon afternoons at Jaipur, Nagpur and Ahmedabad. Moist air is *lighter* than dry air at the same pressure and temperature (water vapour has lower molecular mass), so humidity worsens it.

Wind: from global circulation to the runway

• Causes: pressure gradient force drives air from high to low pressure; Coriolis deflects it right in the Northern Hemisphere. Above about 2,000 ft friction is absent and the balance gives the geostrophic wind parallel to straight isobars; around curved isobars the gradient wind applies.
• Buys Ballot's law (NH): stand with your back to the wind and low pressure is on your left.
• Surface friction slows the wind and backs it across the isobars by about 10–20° over sea and 20–40° over land; so with height the wind veers and increases in the Northern Hemisphere — the cause of routine low-level wind shear on climb-out.
• Global pattern: Hadley, Ferrel and polar cells; trade winds, ITCZ, subtropical and polar-front jet streams. Jet streams are narrow cores of wind of 60 kt or more near the tropopause; the subtropical jet sits over north India in winter and the tropical easterly jet over peninsular India in summer. Clear air turbulence lies on the cold (polar) side and below the core.
• Local winds: sea breeze (day, onshore — the afternoon runway change at Chennai and Mumbai) and land breeze (night); anabatic/katabatic valley winds; mountain waves, rotor and lee turbulence downwind of the Himalaya and Western Ghats; loo (hot dry NW India summer wind).
• Indian monsoon, the dominant seasonal driver: SW monsoon normally reaches Kerala about 1 June and covers the country by mid-July, withdrawing from NW India from about mid-September; the NE monsoon (October–December) brings the main rain to Tamil Nadu and coastal Andhra. Winter western disturbances bring cloud, rain, snow and dense fog to north India; pre-monsoon brings *Nor'westers/Kalbaisakhi* over Bengal and Odisha, dust storms (*andhi*) over NW India and *mango showers* over Kerala and Karnataka. Tropical cyclones form over the Bay of Bengal and Arabian Sea mainly in April–June and October–December; IMD names and categorises them (depression, deep depression, cyclonic storm, severe, very severe, extremely severe, super cyclonic storm).

Clouds, air masses, fronts and thunderstorms

• Cloud formation: cooling of air to its dew point by orographic lifting, convection, frontal lifting, convergence or turbulent mixing, on condensation nuclei.
• Ten types in four groups: high — cirrus, cirrocumulus, cirrostratus (ice); middle — altocumulus, altostratus; low — stratus, stratocumulus, cumulus, nimbostratus; vertical — cumulonimbus. Prefix *nimbo-/-nimbus* = precipitating; *alto-* = middle; *strato-* = layered; *cumulo-* = heaped.
• Air masses are named by source: continental/maritime, tropical/polar (cP, mT, etc.). A front is the boundary between two air masses: warm front (shallow slope, extensive stratiform cloud, steady rain, poor visibility ahead), cold front (steep, cumuliform, showers, squalls, clearance behind), occluded front, stationary front.
• Thunderstorm requirements: moisture, instability and a trigger (heating, orography, front or convergence).
• Three stages: (1) *cumulus/developing* — updraughts only; (2) *mature* — updraught and downdraught side by side, heavy rain, lightning, hail, gust front, the most dangerous stage; (3) *dissipating* — downdraughts dominate, anvil spreads.
• Types: air-mass (thermal), orographic, frontal and squall line; a supercell has a single rotating updraught (mesocyclone) and produces the largest hail.
• Hazards in and near a Cb: severe turbulence, hail (possible well outside the cloud under the anvil), lightning, severe icing, microburst, tornado, static, altimeter error, and heavy rain causing hydroplaning. Standard airmanship: avoid by 10 NM, more for severe cells, and never overfly or fly under an active cell.
• Microburst: a concentrated downdraught under 4 km across that spreads outwards. On approach the aircraft first meets increasing headwind (performance increases — tempting the pilot to reduce power), then the downdraught, then a strong tailwind with sudden loss of airspeed and lift. Recovery is full power, pitch to escape, no configuration change — and the real defence is avoidance, aided by Doppler weather radar, predictive wind-shear warning systems and ATC wind-shear warnings.

Icing, turbulence and visibility hazards

• Airframe icing needs visible moisture and a surface at 0 °C or below; most likely between 0 and −20 °C. Types: clear/glaze ice (large supercooled drops, freeze slowly, hard, heavy, spreads back over the wing — the worst), rime ice (small drops freeze instantly, opaque, brittle, on leading edges), mixed, and hoar frost (sublimation on a cold parked aircraft; must be removed before flight). Freezing rain and supercooled large droplets give the fastest accretion and may exceed the certified protection.
• Effects: lift loss and drag rise, weight increase, stall at a lower angle of attack, blocked pitot/static giving false readings, propeller imbalance, aerial and control damage. Engine icing: carburettor icing in piston aircraft (can occur up to +25 °C in humid air because of fuel-vaporisation cooling), impact and intake icing in jets. Protection: hot bleed air, electro-thermal or pneumatic boots, de-icing and anti-icing fluids before departure.
• Turbulence types: mechanical (surface friction, buildings, terrain), thermal/convective, orographic and mountain-wave, frontal, clear air turbulence near the jet stream (shear-driven, no cloud cue), and wake turbulence from a preceding aircraft (hence ICAO/CAT separation minima behind Heavy and Super aircraft).
• Reduced visibility: fog (visibility below 1,000 m), mist (1,000–5,000 m with RH high), haze (dry particles), smoke, blowing dust and sand, heavy precipitation.
• Fog types: radiation (clear sky, light wind 2–8 kt, moist air, long night — the dense December–January fog at Delhi, Lucknow, Amritsar and Varanasi), advection (warm moist air over a cold surface), steam, frontal, upslope and freezing fog.
• RVR is the instrumentally measured (transmissometer/forward-scatter) distance along the runway, used for low-visibility operations; CAT I/II/III approach minima and CAT III-B-capable runways and crew/aircraft approvals at Delhi IGI and other fog-prone airports keep operations going in Indian winter — specific minima are published in AIP India and DGCA requirements.

Observations, codes and the Indian met service

• Measurement: mercury/digital barometers and barographs, dry- and wet-bulb thermometers and Stevenson screen, hair/electronic hygrometers, anemometer and wind vane, rain gauge, ceilometer and cloud searchlight, transmissometer, radiosonde/pilot balloon for upper air, Doppler Weather Radar network, and INSAT-3D / 3DR / 3DS satellites (INSAT-3DS launched February 2024).
• METAR — routine aerodrome report, normally half-hourly; SPECI — special report on significant change; TAF — aerodrome forecast, typically 9, 24 or 30 hours of validity; TREND — 2-hour landing forecast appended to a METAR.
• SIGMET — warning of en-route hazardous weather (thunderstorms, severe turbulence, severe icing, mountain wave, dust/sandstorm, volcanic ash, tropical cyclone) issued by the Meteorological Watch Office for its FIR, valid up to 4 hours (6 hours for volcanic ash and tropical cyclones); AIRMET/GAMET — low-level information for light aircraft; aerodrome warnings and wind-shear warnings; VOLMET and ATIS for broadcast; WAFS significant-weather and upper-wind charts from the World Area Forecast Centres (London and Washington).
• In India: IMD is the designated meteorological authority; Aerodrome Meteorological Offices (AMO/AMS) at airports, Meteorological Watch Offices for the Indian FIRs (Delhi, Mumbai, Kolkata, Chennai and Guwahati), briefing through IMD's aviation portals and the automated message switching system; DGCA's Civil Aviation Requirements and AIP India govern how the information is used. Organisational details and office lists change — check the current AIP and IMD publications.
• Advantages of the coded system: compact, language-independent, directly machine-readable for flight planning. Limitations: a METAR describes a point at a time; convective weather in India can change within minutes, TAFs have poor skill for fog onset timing, and radar has terrain-shadow gaps — hence nowcasts and pilot reports (AIREP) remain essential.

Key points

• ISA – 1013.25 hPa, +15 °C, 1.225 kg/m³, 1.98 °C/1,000 ft to 36,090 ft, then −56.5 °C.
• Tropopause – top of the weather; ~36,090 ft in ISA but 50,000–55,000 ft over tropical India.
• QNH / QFE / QNE – altitude AMSL / height above aerodrome / pressure altitude on 1013.2.
• 1 hPa ≈ 27 ft near MSL; the relation weakens with height.
• Cold or low pressure – altimeter over-reads, aircraft is lower than indicated.
• Density altitude – the performance altitude; rises with heat, elevation, humidity and low QNH.
• DALR 3 °C/1,000 ft, SALR ≈1.5 °C/1,000 ft; comparing ELR with them gives stability.
• Inversion – warm over cold; fog, haze, smooth air, sharp shear at its top.
• Buys Ballot's law – back to the wind in the NH, low pressure on the left.
• Wind veers and strengthens with height in the NH as friction ends.
• Cb – the single most dangerous cloud; mature stage carries updraught, downdraught, hail and microburst.
• Avoid thunderstorms by 10 NM, never fly under the anvil or through a squall line.
• Microburst – headwind gain, then downdraught, then tailwind loss; apply full power and avoid.
• Clear ice worse than rime; most icing risk between 0 and −20 °C in visible moisture.
• CAT – turbulence near the jet stream with no cloud warning.
• Fog below 1,000 m visibility; radiation fog needs clear skies, light wind and moist air.
• RVR – measured runway visibility for CAT I/II/III low-visibility operations.
• METAR/SPECI/TAF/SIGMET – observation / special / forecast / en-route hazard warning.

Formulas and examples

1. Density altitude at Leh (elevation 10,680 ft), QNH 1013, OAT +20 °C

• PA = 10,680 + 27 × (1013 − 1013) = 10,680 ft.
• ISA temperature at 10,680 ft = 15 − 1.98 × 10.68 ≈ −6.1 °C.
• ΔISA = 20 − (−6.1) = +26.1 °C.
• DA = 10,680 + 120 × 26.1 = 10,680 + 3,132 ≈ 13,810 ft — the aircraft performs as if at nearly 14,000 ft, which is why Leh operations are restricted to early morning with reduced payload.

2. Pressure altitude and cold-weather altimeter error

• Delhi, elevation 777 ft, QNH 1002 hPa: PA = 777 + 27 × (1013 − 1002) = 777 + 297 = 1,074 ft.
• Srinagar, elevation 5,300 ft, ΔISA = −20 °C, indicated altitude 10,000 ft AMSL: height above station = 4,700 ft.
• Error = 4 × 20 × 4.7 = 376 ft → true altitude ≈ 10,000 − 376 = 9,624 ft, i.e. 376 ft lower than indicated — significant against 9,000 ft ridges.

3. Stability and cloud base

• Surface T = 30 °C, Td = 18 °C, ELR measured at 1.0 °C/1,000 ft.
• Base ≈ 400 × (30 − 18) = 4,800 ft AGL.
• Lift a dry parcel 3,000 ft: parcel = 30 − 3 × 3 = 21 °C; environment = 30 − 1 × 3 = 27 °C. The parcel is colder and denser, so it sinks back → stable (ELR < SALR): expect haze and shallow cloud, not thunderstorms.

4. Crosswind check

• Runway 27 (270°), wind reported 210/25 kt → θ = 60°.
• Crosswind = 25 × sin 60° = 25 × 0.866 = 21.7 kt; headwind = 25 × cos 60° = 12.5 kt.
• A Cessna 172 with 15 kt maximum demonstrated crosswind cannot accept it; request the reciprocal or another runway.

5. Density and true airspeed at the 850 hPa level, OAT +10 °C

• ρ = 85,000 / (287.05 × 283.15) = 85,000 / 81,278 = 1.046 kg/m³.
• TAS/EAS = √(1.225 / 1.046) = √1.171 = 1.082 → TAS is about 8% above EAS.

METAR decode: VIDP 051230Z 28008KT 4000 HZ FEW030 SCT100 32/22 Q1004 NOSIG

Where it is used

• Airline flight dispatch and operations control: IndiGo, Air India, SpiceJet and Akasa dispatchers read TAF/SIGMET/WAFS charts to choose fuel, alternates and payload, and to issue monsoon and fog-season advisories.
• Flight crew: pre-flight briefing, altimeter setting and temperature corrections, density-altitude performance at Leh, Kullu, Shimla and Dehradun, thunderstorm avoidance using onboard weather radar.
• Airport operations and ATC (AAI, Adani, GMR, Bangalore International Airport): low-visibility procedures, runway changes with the sea breeze, RVR monitoring, wind-shear and aerodrome warnings, snow and water on runway reports.
• IMD aviation meteorology: AMOs and MWOs at Indian airports, DWR and satellite nowcasting, cyclone warnings that trigger airport closure (Chennai, Visakhapatnam, Bhubaneswar, Kolkata).
• Flying training and examinations: DGCA CPL/ATPL Aviation Meteorology paper, flying-club decision-making, instructor briefings.
• Drones, UAS and airport design: BVLOS wind limits, wind-rose data for runway orientation, and insurance/accident investigation (AAIB) where weather is a factor.

Interview questions

• Q: What is the ISA and why is it needed? A: A standard reference atmosphere (1013.25 hPa, +15 °C, 1.225 kg/m³, lapse 1.98 °C/1,000 ft) that lets altimeters be calibrated and aircraft performance be compared on a common basis.
• Q: Difference between QNH, QFE and QNE? A: QNH makes the altimeter read altitude above mean sea level, QFE reads height above the aerodrome, and QNE is the standard 1013.2 hPa setting that gives pressure altitude and flight levels.
• Q: What is density altitude and why does it matter? A: The ISA altitude corresponding to the actual air density; the higher it is, the less lift and thrust available, so take-off run lengthens and climb gradient falls.
• Q: Does humidity increase or reduce air density? A: It reduces it — water vapour is lighter than the nitrogen and oxygen it displaces — so humid air worsens performance.
• Q: "High to low, look out below" — explain. A: Flying into lower pressure or colder air without resetting the altimeter makes it over-read, so the aircraft is actually lower than indicated.
• Q: How do you estimate the base of convective cloud? A: Multiply the surface temperature–dew-point spread in °C by about 400 to get feet above ground, because the spread closes about 2.5 °C per 1,000 ft.
• Q: Which stage of a thunderstorm is most dangerous and why? A: The mature stage, because updraught and downdraught coexist with heavy rain, hail, lightning, a gust front and possible microburst.
• Q: How does a microburst affect an aircraft on approach? A: Initial headwind increase makes performance look good, then the downdraught and a tailwind cause a sudden loss of airspeed and height close to the ground.
• Q: Which ice type is more dangerous, clear or rime, and why? A: Clear ice — large supercooled drops freeze slowly and run back over the wing, adding more weight and deforming more of the aerofoil than rime on the leading edge.
• Q: Can carburettor icing occur in warm weather? A: Yes — fuel vaporisation and pressure drop can cool the venturi below freezing in humid air even around +20 to +25 °C.
• Q: What is CAT and where do you expect it? A: Clear air turbulence from wind shear near the jet stream, usually on the cold side of and just below the core, with no cloud to warn of it.
• Q: Difference between a METAR, a TAF and a SIGMET? A: A METAR is an observed aerodrome report, a TAF is a forecast for an aerodrome over a stated validity period, and a SIGMET is a warning of hazardous en-route weather for an FIR issued by the Meteorological Watch Office.
• Q: Why is fog worst at Delhi in December and January? A: Long clear nights, light winds, high moisture after western disturbances and a strong surface inversion with abundant pollution nuclei produce dense radiation fog, mitigated by CAT III operations.

Common mistakes

• Confusing pressure altitude with density altitude, or forgetting the 120 ft per °C correction for ISA deviation.
• Using 1 hPa = 27 ft at all altitudes — the pressure–height relation is exponential and roughly 50 ft/hPa near 18,000 ft.
• Getting the altimeter temperature error backwards: in cold air the aircraft is lower than indicated, not higher.
• Applying DALR to saturated air (or SALR to dry air) when judging stability, and comparing lapse rates without stating which is the ELR.
• Taking dew point as a measure of how much vapour air can hold rather than the temperature at which it saturates.
• Assuming hail and turbulence exist only inside the visible cloud — both occur well outside and under the anvil of a Cb.
• Treating a METAR as a forecast, quoting cloud heights AMSL instead of AGL, or reading METAR times as IST when all aviation codes use UTC.

Related topics

Air navigation: charts, dead reckoning and radio aids · Flight planning, weight and balance · Aircraft performance and limitations · Air traffic control and airspace procedures · Aviation safety, human factors and CRM · Airport operations and low-visibility procedures.