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🌦️ Meteorology

Weather is the one part of every flight nobody can fix on the ground. This guide works from the physics up — pressure, temperature and moisture — to the clouds, winds, storms and fog they produce, and then to the codes and charts you will read before and during every flight. Each section gives the mechanism first and the practical rule second, so that you can work an answer out rather than recall it.

29 sections 1,400 linked questions 🧮 Quick tools 📊 Diagrams Aircraft-independent

🧰 Quick tools & cheat sheet play with the numbers

Four calculators for the sums that come up again and again — ISA, pressure and density altitude, cloud base, and the cold-temperature altimeter correction — and a one-screen reference of the figures worth knowing by heart.

🌡️ ISA at a level

Altitude (ft) Actual temperature (°C)

📏 Pressure & density altitude

Aerodrome elevation (ft) QNH (hPa) Outside air temperature (°C)

☁️ Cumulus base from the spread

Surface temperature (°C) Surface dew point (°C) Aerodrome elevation (ft)

🥶 Cold-temperature correction

Indicated height above the setting datum (ft) ISA deviation along the column (°C)

📋 Cheat sheet — figures worth knowing by heart

Item	Value / rule
🌍 ISA at mean sea level	+15°C, 1013.25 hPa, density 1.225 kg/m³
📉 ISA lapse rate	1.98°C per 1000 ft (6.5°C per km) up to 11 km / 36 090 ft, then −56.5°C constant
📏 Height per hectopascal	About 27–30 ft per hPa near the surface (27 ft is the usual planning figure); about 50 ft at 500 hPa
🌀 Lapse rates in a rising parcel	DALR 3°C/1000 ft · SALR about 1.8°C/1000 ft on average (less in warm air, nearer the DALR high up) · dew point 0.5°C/1000 ft
☁️ Cumulus base	(Temperature − dew point) × 400 ft
⚖️ Density altitude	Pressure altitude + 120 ft for every °C above ISA
🥶 Temperature error	4 ft per 1000 ft for each °C off ISA — "cold air, altimeter over-reads"
🧭 Buys Ballot (N hemisphere)	Back to the wind, low pressure on the left
⬇️ "High to low, look out below"	Flying towards lower pressure or colder air, you are lower than indicated
🪁 Surface wind vs gradient wind	N hemisphere, over land: backed ~30°, about half the speed · over sea: backed ~10°, about 70% (veered in the S hemisphere)
🧊 Icing	Supercooled water between 0°C and about −20°C is the main risk; clear ice from large drops near 0°C
⛈️ Storm avoidance	Radar echo: 5 nm below FL200 (10 nm if growing), 20 nm above FL300 · visually 10 nm and 5000 ft · never under the anvil

✈️ How to use this guide Each section stands on its own. When you get a question wrong, jump straight to its section and read the key idea, rather than starting at the top.

🌍 The Atmosphere 61 questions

A thin shell of mixed gases, held down by gravity and heated mostly from below. Nearly all the weather lives in its lowest layer.

🎯 What the air is made of🎯 The layers and their boundaries🎯 Tropopause height by latitude🎯 ISA and ISA deviation

💡 Key idea

The proportions stay the same; the amount does not. Nitrogen (~78%), oxygen (~21%) and argon (just under 1%) stay in the same ratio all the way through the troposphere, because the air keeps being stirred. What falls with height is the total pressure, and with it the partial pressure of oxygen. Water vapour is the small, variable ingredient that matters most for weather, because it changes state and carries latent heat.

The ISA temperature profile. In the real atmosphere the tropopause is higher and colder over the Equator and lower and warmer over the poles.

Layer	Temperature with height	Worth remembering
Troposphere	Normally falls	About ¾ of the mass of the atmosphere, almost all the water vapour and weather
Tropopause	Stops falling	~16–18 km and −75 to −80°C at the Equator; ~11 km in mid-latitudes; ~8 km at the poles. Higher in summer. Breaks near 40° and 60°, where the jet streams sit
Stratosphere	Constant, then rising	Ozone absorbs ultraviolet; very stable and dry, so very little weather. Ends at the stratopause, ~50 km
Mesosphere / thermosphere	Falls, then rises	Above anything an aeroplane reaches

The International Standard Atmosphere

ISA is an agreed, imaginary atmosphere used to calibrate altimeters and to compare aircraft performance. At MSL it is +15°C, 1013.25 hPa and 1.225 kg/m³. Temperature falls 1.98°C per 1000 ft (call it 2°C) to −56.5°C at 36 090 ft, stays constant to 20 km, then rises slowly. Useful pressure levels: 850 hPa ≈ 5000 ft, 700 hPa ≈ 10 000 ft, 500 hPa ≈ 18 000 ft, 300 hPa ≈ FL300, 200 hPa ≈ FL390.

Worked example — ISA deviation

At FL180 the outside air temperature is −27°C. ISA at FL180 is 15 − (2 × 18) = −21°C. The air is 6°C colder than standard, so the deviation is ISA −6. Above 36 000 ft, compare with −56.5°C rather than continuing the lapse rate.

📐 Height per hectopascalH (ft) = 96 T / P, with T in kelvin and P in hPa. At MSL in ISA that gives about 27 ft per hPa; at 300 hPa it is about 70 ft. The thinner the air, the more height for each hectopascal, and a warm column gives more height per hPa than a cold one.

✈️ Pilot tip Above about 50 000 ft, ozone and cosmic radiation become operational issues. During a strong solar flare, the practical answer is to plan a lower level.

🔽 Pressure 44 questions

Pressure is the weight of the air column above you. Everything about altimeter settings follows from how it is measured and then corrected to a reference level.

🎯 Units: hPa, mb, inHg, mmHg🎯 Warm and cold columns🎯 QFE, QNH, QFF🎯 Diurnal variation

💡 Key idea

Pressure falls more slowly with height in warm air. A warm column is "taller", so at any given height above the surface the pressure in it is higher than in a cold column that started with the same surface pressure. That one fact drives the thermal wind, the jet stream and the cold-temperature altimeter error.

Setting	What it is	Altimeter on the ground reads
QFE	Actual pressure at aerodrome level	Zero (height above the aerodrome)
QNH	QFE reduced to MSL using ISA; reported rounded down to a whole hPa	Aerodrome elevation
QFF	QFE reduced to MSL using the actual temperature; used to draw isobars on charts	— (not an altimeter setting)
1013	Standard setting	Pressure altitude, used for flight levels

⚖️ QNH or QFF — which is bigger? They differ only in the temperature assumed for the imaginary column between the aerodrome and MSL. Above MSL and warmer than ISA: the real column is less dense, so QFF < QNH. Above MSL and colder than ISA: QFF > QNH. Below MSL, both answers flip. At an aerodrome exactly at MSL, all three are equal.

1013.25 hPa = 1013.25 mb = 29.92 inHg = 760 mmHg = 101 325 Pa.
Surface pressure normally ranges from about 950 to 1050 hPa. The lowest values occur in the eyes of tropical cyclones, the highest in Siberian winter highs.
The diurnal "pressure wave" has two highs (~1000 and 2200 local) and two lows. It is about 1 hPa in temperate latitudes and up to 3 hPa in the tropics, where it can hide a weather-related change.
A steady fall (a "falling glass") of several hPa in three hours means an approaching or deepening low.

Worked example — QFE to QNH

Elevation 900 ft, QFE 985 hPa, 30 ft per hPa: 900 ÷ 30 = 30 hPa, so QNH ≈ 1015 hPa. Going the other way, subtract.

⚖️ Density 30 questions

Wings and engines respond to air density, not to altitude. Pressure raises density, while heat and moisture lower it.

🎯 Density vs pressure and temperature🎯 Humid air is lighter🎯 Density altitude🎯 "Hot and high"

💡 Key idea

ρ ∝ P / T. Density rises with pressure and falls with absolute temperature. Water vapour (molecular mass 18) is lighter than nitrogen and oxygen, so moist air is less dense than dry air at the same pressure and temperature. As a rule of thumb, density changes by 1% for every 3°C or every 10 hPa.

📉 With height

Falls, because the fall in pressure outweighs the fall in temperature. About 74% of the sea-level value at 10 000 ft, 50% at 20 000 ft and 25% at 40 000 ft.

🧭 With latitude

At the surface, higher towards the poles (colder air). The two are about equal near 26 000 ft. Higher up, polar air is less dense. The biggest difference is near 50 000 ft.

🛫 Performance

Low density means a longer take-off run, a lower maximum mass and a higher TAS for the same IAS. Denver, Nairobi and Sana'a are classic examples, and so are hot, humid Singapore and Bahrain.

📐 Density altitude

The ISA altitude with the same density as the actual air. It is higher than elevation whenever the air is warmer than ISA.

✈️ Pilot tip High, hot and humid is the worst combination. A cold, high-pressure winter morning gives better-than-book performance.

🌀 Pressure Systems 47 questions

Lows, highs, troughs, ridges and cols. Each has its own vertical motion, and that vertical motion decides the weather.

🎯 Buys Ballot's law🎯 Convergence and divergence🎯 Typical weather of each🎯 Drift and altimeter

💡 Key idea

Low: air converges at the surface and rises, which gives cloud and rain. High: air sinks and diverges at the surface, which gives fine weather under an inversion. In the Northern Hemisphere, stand with your back to the wind and the low is on your left (on your right in the Southern Hemisphere). Around a low the wind blows anticlockwise and slightly inwards; around a high, clockwise and slightly outwards.

System	Chart shape	Typical weather
Depression	Closed isobars, lowest at the centre	Extensive cloud and rain; strong winds where isobars are tight; good visibility outside rain. Polar front lows move west to east and last about 10–14 days
Trough	V-shaped isobars out from a low	A line of enhanced cloud and showers, often a front
Anticyclone	Closed, widely spaced isobars, highest at the centre	Summer: clear, light winds, haze. Winter: low stratus or fog trapped under the subsidence inversion. Slow-moving, sometimes blocking for weeks
Ridge	Rounded isobars out from a high	A short fair spell between two lows, often lasting less than a day
Col	Flat pressure between two highs and two lows	Light, variable winds. Fog in autumn and winter; thunderstorms over land in summer

Deepening means the central pressure of a low is falling; filling means it is rising. A high whose pressure is falling is weakening.
Cold highs sit over the poles and winter continents. Warm highs, such as the Azores high, come from sinking air in the subtropical part of the Hadley–Ferrel–Polar three-cell circulation.
Thermal lows form over hot land in summer, and over relatively warm sea in cold polar air in winter.

⬇️ Drift tells you about pressure In the Northern Hemisphere, drifting right means the wind is from the left, so low pressure lies ahead and to the right and you are flying towards it. On a fixed subscale setting the altimeter will over-read, so you are lower than you think.

🌡️ Temperature 50 questions

The sun heats the ground, and the ground heats the air. That indirect route explains lapse rates, inversions and the daily temperature curve.

🎯 Radiation, conduction, convection🎯 Lapse rates and inversions🎯 Diurnal range🎯 Land vs sea

💡 Key idea

The atmosphere is heated from below. Short-wave solar radiation passes through the air largely unabsorbed and warms the surface. The surface re-radiates long-wave energy, which water vapour and CO₂ absorb. Heat then moves upwards by conduction, convection, turbulence and the release of latent heat. Air itself conducts heat poorly.

Term	Meaning
Environmental lapse rate (ELR)	The actual fall of temperature with height at a place and time. On average about 2°C per 1000 ft, but it varies
Inversion	Temperature rising with height. Very stable; traps haze, fog and pollution. Formed by night radiation, subsidence, fronts or warm air flowing over a cold surface
Isothermal layer	Temperature constant with height, as in the lower stratosphere

What makes the daily range big or small

Large range

Clear skies, light winds, dry air, inland, over bare rock or sand, in the tropics or deserts. Maximum about 1500 local time; minimum about half an hour after sunrise.

Small range

Cloud cover (it blocks heating by day and traps heat at night), strong wind (mixing), high humidity, and over the sea. The sea's daily range is tiny, under about 1°C, because water has a high specific heat and mixes and absorbs heat through depth.

Thermometers are kept in a ventilated screen about 1.2 m above short grass, out of direct sun.
Snow reflects most sunlight, while dark soil and forest absorb it. Wet ground warms more slowly than dry.
Kelvin = °C + 273. To convert, °F = °C × 9/5 + 32.

💧 Humidity 43 questions

Water vapour is invisible fuel. Every change of state takes in or gives out latent heat, and that heat powers clouds and storms.

🎯 Latent heat directions🎯 Relative humidity vs dew point🎯 Saturation over ice🎯 Daily cycle of RH

💡 Key idea

Warm air can hold more water vapour. The saturation vapour content roughly doubles for every 10°C rise. Relative humidity is the ratio of what the air holds to what it could hold at that temperature, so RH rises as air cools even though no water is added. The dew point is the temperature at which the air would become saturated if cooled at constant pressure; the smaller the spread between temperature and dew point, the closer the air is to cloud or fog.

Change of state	Name	Latent heat
Liquid → vapour	Evaporation	Absorbed (cools the surroundings)
Vapour → liquid	Condensation	Released (warms the air, which drives cloud growth)
Solid → liquid / liquid → solid	Melting / freezing	Absorbed / released
Vapour ↔ solid directly	Sublimation / deposition	Absorbed / released (the largest amounts)

Air saturated with respect to water is supersaturated with respect to ice, so ice crystals grow at the expense of supercooled droplets. This is the start of most precipitation.
Humidity is measured with a wet- and dry-bulb psychrometer. The wet bulb is cooled by evaporation, and the bigger the difference between the two readings, the drier the air.
Over a day, absolute humidity changes little, but RH is highest around dawn (coldest) and lowest in mid-afternoon.
Humidity mixing ratio is the grams of vapour per kilogram of dry air. It stays constant in a rising parcel until saturation.

📈 Adiabatics & Stability 57 questions

Lift a parcel of air and compare its temperature with the air around it. Whether it keeps rising or sinks back decides whether you get towering cumulus or flat, layered cloud.

🎯 DALR and SALR🎯 Absolute and conditional instability🎯 Föhn sums🎯 How stability changes

💡 Key idea

A rising parcel cools by expansion with no heat exchange (adiabatically): 3°C per 1000 ft while dry (DALR), and less once saturated (SALR), because condensation releases latent heat: about 1.8°C per 1000 ft on average in temperate latitudes, nearer 1.5°C in warm, moist low-level air. The SALR is smallest in warm air and nears the DALR high up and at high latitudes, where there is little moisture to condense. Compare the parcel with the environmental lapse rate (ELR):

ELR compared with the adiabatic rates	Stability	Typical result
ELR < SALR (including inversions and isothermal layers)	Absolutely stable	Layer cloud, poor visibility in haze, smooth air
SALR < ELR < DALR	Conditionally unstable: stable if dry, unstable once saturated	Cumulus that can grow tall once condensation starts
ELR > DALR	Absolutely unstable (usually only near a heated surface)	Thermals, gusts, showers, good visibility outside them
ELR = DALR or SALR	Neutral (dry or saturated)	The parcel stays where it is put

Föhn: moisture lost as rain on the windward side means the descending air warms at the dry rate for longer than it cooled. It arrives warmer and drier than it started.

Worked example — Föhn temperature

Air at +20°C, dew point +12°C, at sea level crosses a 10 000 ft ridge and descends to sea level. Cloud base = (20 − 12) × 400 = 3200 ft. To the base it cools at 3°C/1000 ft: 20 − 9.6 = 10.4°C. From 3200 to 10 000 ft (6800 ft) it cools at an SALR of, say, 1.5°C/1000 ft: 10.4 − 10.2 = 0.2°C at the summit. It then descends 10 000 ft dry, warming by 30°C, to about +30°C on the lee side, 10°C warmer than it started.

What changes stability

Becomes more unstable

Heating from below (a sunny afternoon, cold air over warm sea), cooling aloft, adding moisture low down, or lifting a whole layer that is moist at the bottom and dry at the top.

Becomes more stable

Cooling from below (clear nights, warm air over cold sea), warming aloft, subsidence under a high, which builds an inversion.

🌪️ Turbulence & Windshear 55 questions

Bumps come from three places: the friction layer near the ground, in and around cloud, and in clear air near jet streams and mountains. Windshear close to the ground is the one that can actually put an aircraft on the ground.

🎯 Thermal vs mechanical🎯 Mountain waves and rotors🎯 CAT near jets🎯 Microburst sequence

💡 Key idea

Windshear is any change of wind velocity over a short distance, vertical currents included. Vertical shear is quoted in knots per 100 ft, horizontal shear in knots per 1000 ft. Turbulence is what an aircraft feels when it flies through shear that changes on the scale of its own size.

☀️ Thermal turbulence

Convection over land heated by the sun. Worst around 1500 local on clear days; almost absent over the open sea.

🏢 Mechanical turbulence

Wind over rough ground, hills, buildings and hangars near a threshold. Grows with wind speed and roughness. The friction layer is about 2000–3000 ft deep by day and shallower at night.

🏔️ Mountain waves

Need a strong wind within about 30° of perpendicular to the ridge, increasing with height, and a stable layer near ridge top. Lenticular clouds sit on the wave crests; the rotor under the first crest is the worst place to be. Waves can exist in dry air with no cloud at all.

🌬️ Clear air turbulence

Strongest on the cold (polar) side of a jet, just below the core, where shear is greatest. Worse in sharply curved troughs, in developing or fast-moving jets, and over mountains.

Reporting intensity

Intensity	What it feels like
Light	Slight, erratic changes in attitude or altitude; seat belts felt slightly
Moderate	Changes in attitude or altitude, but always under control; about 0.5–1.0 g changes at the CG; unsecured objects move
Severe	Large, abrupt changes; aircraft may be momentarily out of control; occupants forced against the belts
Chop	Rapid, rhythmic bumps with little change of altitude or attitude

"Occasional" means less than a third of the time, "intermittent" between a third and two thirds, and "continuous" more than two thirds.

A microburst lasts only a few minutes. The initial headwind tempts a crew to reduce power just before the downdraught and tailwind arrive, which is the dangerous part.

✈️ Pilot tip In mountain waves, cross the ridge at right angles, at turbulence penetration speed, with plenty of height. A useful guide is clearance at least equal to the ridge's height above the surrounding terrain. At high altitude, reduce to penetration speed and consider descending to widen the buffet margin.

📏 Altimetry 63 questions

An altimeter is a barometer with a scale in feet, calibrated for an atmosphere that never quite exists. Every error comes from the real air not matching ISA.

🎯 High to low, look out below🎯 Transition altitude and level🎯 27 ft per hPa sums🎯 Cold-temperature error

💡 Key idea

Flying towards lower pressure or colder air at a constant indicated altitude, the aircraft descends. Both errors make the altimeter over-read, so true altitude is below indicated altitude. That is why regional QNH is the lowest forecast value, why settings are rounded down, and why minimum altitudes are increased in very cold weather.

Term	Definition
Altitude	Vertical distance above MSL (QNH set)
Height	Vertical distance above a specified datum, such as the aerodrome (QFE set)
Elevation	Height above MSL of a point on, or fixed to, the ground
Flight level	A surface of constant pressure referenced to 1013.25 hPa, in hundreds of feet
Transition altitude	At or below it, vertical position is an altitude on QNH
Transition level	The lowest usable flight level above the transition altitude
Transition layer	The airspace between the two

Worked example — wrong setting

Aerodrome elevation 300 ft, 1020 hPa set but the actual QNH is 1005 hPa. The setting is 15 hPa too high, and each hPa is worth 27 ft, so the altimeter reads 15 × 27 = 405 ft too high: 705 ft. Setting a higher number on the subscale always makes the altimeter read higher.

Worked example — temperature error

Indicating 3000 ft on QFE with the air below at ISA −20. The correction is 4 ft × 20 × 3 = 240 ft, so true height ≈ 2760 ft. The error is proportional to the ISA deviation and to the height above the setting datum. It is zero on the ground at the datum and largest at the start of an approach.

🧮 Safe flight levels To find the lowest usable flight level for a safety altitude, correct the safety altitude to pressure altitude using the lowest forecast QNH (QNH below 1013 means add 27 ft per hPa), then allow for temperature below ISA, and round up to the next available level.

Pressure altitude = elevation + (1013 − QNH) × 27 ft.
Density altitude ≈ pressure altitude + 120 ft per °C above ISA.
Two aircraft at the same flight level stay separated even in different temperatures: they both fly the same pressure surface, whatever their true altitudes.

🧭 Winds 71 questions

Air starts to move from high to low pressure; the Earth's rotation turns it until it blows along the isobars; friction near the ground turns it back towards the low.

🎯 Pressure gradient + Coriolis🎯 Geostrophic and gradient wind🎯 Surface wind rules🎯 Sea, land, katabatic, anabatic

💡 Key idea

Above the friction layer the wind is geostrophic: the pressure gradient force (towards low) is balanced by the Coriolis force (to the right of the motion in the north, to the left in the south). The wind then blows parallel to straight isobars. Coriolis = 2Ωρ V sin(latitude): zero at the Equator, greatest at the poles, and negligible within about 15° of the Equator. So the same isobar spacing gives a stronger wind at low latitude and in thinner air.

Wind	Balance	Note
Geostrophic	PGF = Coriolis	Straight, parallel isobars; above the friction layer; not near the Equator
Gradient	PGF, Coriolis and centrifugal	Curved isobars. For the same spacing it is slower around a low and faster around a high than the geostrophic value
Antitriptic	PGF and friction	Very low latitudes, where Coriolis is negligible
Surface wind	Friction reduces speed, so Coriolis weakens	Backed and slower in the north (veered in the south), blowing across the isobars towards low pressure

Diurnal variation over land

By day

Convection mixes faster air down from above, so the surface wind veers and strengthens, reaching a maximum near 1500 local. The 1500 ft wind becomes lighter and backed.

By night

A surface inversion decouples the layers. The surface wind backs and drops (helping fog form), while the wind just above the inversion veers and strengthens towards the gradient wind, which gives shear at the inversion.

🏖️ Sea breeze

Land heats faster than sea on sunny days with a slack gradient. About 10 kt, reaching 8–15 nm inland; veers through the day (north). A line of cumulus marks its front, and it can carry sea fog inland.

🌙 Land breeze

The reverse at night: about 5 kt offshore, within about 5 nm of the coast.

⛷️ Katabatic

Cold, dense air sliding down slopes on clear nights, strongest over snow. Brings frost, fog and inversions to valley floors.

🌄 Anabatic

A light wind (~5 kt) up a sun-heated slope, usually weaker than a katabatic wind because gravity works against it.

Wind direction is always where the wind blows from: true in reports and forecasts, magnetic from ATC for take-off and landing.
A veer is a clockwise change in direction and a back is anticlockwise, in both hemispheres.
A gust is reported when it is 10 kt or more above the mean. A squall lasts a minute or more. Gale: a mean above 33 kt or gusts above 42 kt; hurricane force: above 63 kt.
Surface wind is measured at 10 m. 10 m/s ≈ 20 kt.

🛫 Upper Winds & Jet Streams 53 questions

Upper winds follow temperature. Because the poles are colder than the tropics, the winds aloft are mostly westerly, and where the temperature contrast is sharpest they form jet streams.

🎯 Contour charts🎯 Thermal wind🎯 Jet types and locations🎯 CAT and crossing a jet

💡 Key idea

The thermal wind is the vector difference between the winds at two levels. It blows parallel to the mean isotherms (thickness lines) of the layer between them, with cold air on the left in the north. Stand with your back to the upper wind and low pressure, low temperature and low contour heights are all on your left. A wind that veers with height means warm advection; one that backs with height means cold advection.

Chart	≈ Flight level
850 · 700 · 500 hPa	FL050 · FL100 · FL180
400 · 300 · 250 hPa	FL240 · FL300 · FL340
200 · 150 hPa	FL390 · FL450

A contour chart shows the height of one pressure surface. Contours (isohypses) behave like isobars, so the wind blows along them and is stronger where they are close together. Isotachs join points of equal wind speed. High 1000–500 hPa thickness means a warm column.

Jet streams

A jet stream is a narrow, fast current, at least 60 kt, typically about 2000 nm long, 200 nm wide and 2 nm deep. Its core lies just below the tropopause, near a break in it. Speeds above 300 kt have been measured.

Jet	Where and when
Polar front jet	Over the polar front depressions, parallel to their fronts in the warm air above the frontal surfaces (~300 hPa). Variable in position; stronger and further south in winter
Subtropical jet	Poleward side of the subtropical highs, ~200 hPa, about 25–40°N in winter and 40–45°N in summer; steadier
Tropical easterly jet	Northern summer, ~10–20°N, from the South China Sea across India to Africa, ~150 hPa; easterly because the heated Asian landmass reverses the temperature gradient
Polar night jet	Winter, high in the stratosphere (~50 hPa), in high latitudes
Arctic jet	Winter, between Arctic and polar air, ~400 hPa
Somali (Findlater) jet	A low-level jet off East Africa in the summer monsoon

✈️ Pilot tip In the Northern Hemisphere, looking downstream along a westerly jet, the cold air is on the left. Rapid temperature change, changing drift and growing wind speed say you are near the core. If the jet is turbulent, change level or move laterally away from the core, especially out of the cold side below it, rather than staying in it.

☁️ Clouds 51 questions

Ten cloud types in three families and three height bands. Learn what each looks like and what it does to an aeroplane.

🎯 Oktas and ceiling🎯 Low, medium, high bands🎯 Which cloud gives what🎯 Warning clouds

💡 Key idea

The shape of a cloud is a picture of the stability that made it. Flat, layered (stratiform) cloud means stable air lifted gently. Heaped (cumuliform) cloud means unstable air rising in convection. Fibrous (cirriform) cloud is ice, high up. "Nimbo-" or "-nimbus" means the cloud is producing precipitation.

Band (temperate)	Clouds	What to expect
High: ~16 500 ft to the tropopause	Cirrus (Ci), cirrostratus (Cs, the halo cloud), cirrocumulus (Cc)	Ice crystals; no precipitation reaches the ground. Ci thickening into Cs often heralds a warm front
Medium: ~6500–23 000 ft	Altostratus (As, a "ground-glass" sun), altocumulus (Ac)	Icing possible. Ac lenticularis = mountain waves; Ac castellanus = mid-level instability, often before thunder
Low: surface–6500 ft	Stratus (St), stratocumulus (Sc), nimbostratus (Ns)	St: hill fog and drizzle. Ns: thick, continuous rain or snow with icing
Through all levels	Large cumulus (Cu congestus / TCU) and cumulonimbus (Cb)	Severe turbulence and icing, hail, lightning, heavy showers, squalls

Code	FEW	SCT	BKN	OVC
Oktas (eighths)	1–2	3–4	5–7	8

The ceiling is the height above aerodrome level of the lowest BKN or OVC layer. Report bases are above aerodrome level, so a base of 1200 ft at an aerodrome 600 ft high is 1800 ft AMSL. Cloud bases are measured by laser ceilometer; tops are estimated from soundings and pilot reports.

Cumulus grows from humilis → mediocris → congestus to cumulonimbus, first calvus (smooth top) and then capillatus (fibrous anvil top).
The anvil forms where the rising air meets the stable tropopause. Mamma under the anvil means strong sinking air and turbulence.
Fair-weather cumulus: bumpy below the base, usually smooth above the tops. Cumulus bases rise as the day warms.
Contrails form when exhaust water vapour condenses and freezes in cold, humid air aloft.

🌧️ Cloud Formation & Precipitation 48 questions

Cloud needs air cooled to its dew point, almost always by lifting. Precipitation needs the tiny cloud droplets to grow a million times bigger.

🎯 Five ways to lift air🎯 Bergeron vs coalescence🎯 Showers vs continuous🎯 Freezing rain

💡 Key idea

Air is lifted in five ways: turbulence (mixing up to the top of the friction layer, giving St or Sc under an inversion), orographic uplift, convection, frontal ascent, and convergence in lows and troughs. Stable air lifted gives layer cloud; unstable air lifted gives heap cloud.

Bergeron (ice crystal) process

Cloud that reaches below about 0°C has supercooled droplets and ice crystals together. The air is saturated over water but supersaturated over ice, so crystals grow at the droplets' expense until they fall. This is the main mechanism in mid-latitudes.

Coalescence

Cloud that stays warmer than 0°C, as in the tropics and summer. It needs a range of droplet sizes, so that big drops fall faster, collide with small ones and grow.

Type	Detail
Drizzle	Droplets of about 0.2–0.5 mm, from stratus. Reduces visibility more than light rain does
Rain	Drops of about 0.5–5.5 mm
Snow	Gives the poorest visibility of all precipitation. Flakes are largest near 0°C
Hail	5 mm or more; formed in Cb by repeated lifting through supercooled water
Graupel / ice pellets	Soft white pellets from wintry showers / clear frozen raindrops under 5 mm
Freezing rain	Rain from a warm layer aloft falls into sub-zero air, supercools and freezes on impact
Virga	Trails of precipitation that evaporate before reaching the ground

Showers come from heap cloud and are short. Continuous precipitation comes from layer cloud (classically nimbostratus) without a break for at least an hour. Intermittent precipitation comes from layer cloud and stops and starts without the sky clearing.

⛈️ Thunderstorms 59 questions

The most violent weather a pilot routinely meets, packed into one cloud: every hazard in this guide at once.

🎯 Three ingredients🎯 Air-mass vs frontal🎯 Three stages🎯 Avoidance distances

💡 Key idea

A thunderstorm needs three things: a deep layer of unstable air reaching well above the freezing level (about 10 000 ft deep or more), plenty of moisture, and a trigger: surface heating, high ground, a front or convergence. Subsidence in an anticyclone suppresses them.

Cumulus (initial) stage: updraughts throughout the cell, and no precipitation reaching the ground yet.
Mature stage: precipitation starts to fall from the base and drags down cold downdraughts beside updraughts of several thousand fpm. The gust front can be felt 15 nm or more ahead. The anvil spreads at the tropopause.
Dissipating stage: mainly downdraughts throughout the cell, and the precipitation dies away.

Type	Character
Air-mass (heat)	Isolated, over land on summer afternoons, often in cols or slack lows
Frontal	Mostly at cold fronts and occlusions; can be embedded in layer cloud and hidden
Squall line	A line ahead of an active cold front; fast-moving
Orographic	Moist unstable air forced up high ground
Supercell	A single, long-lived rotating cell with deep instability and strong shear; large hail and tornadoes. Moves to the right of the 500 hPa wind

⚠️ Hazards and avoidance Turbulence in, around, above and below the cloud; hail up to about 45 000 ft and under the anvil miles from the core; severe icing between about 0°C and −45°C; lightning (most strikes within about 5000 ft of the freezing level); microbursts and windshear; compass errors and static. On radar: avoid echoes by 5 nm below FL200 (10 nm if growing) and by 20 nm above FL300. Visually, keep at least 10 nm laterally and 5000 ft vertically from a tall, growing or anvilled storm.

✈️ Pilot tip Weather radar sees water, not ice. A dry, frozen top or hail shaft can look weaker than it is, so treat a weak echo high in the storm as worse than it looks. Lightning detectors map electrical discharges instead.

🌫️ Visibility & Fog 55 questions

Fog closes more aerodromes than any other weather. Knowing which kind you face tells you when it will form and when it will clear.

🎯 Mist, fog, haze🎯 Radiation vs advection🎯 How fog clears🎯 RVR

💡 Key idea

Fog is visibility below 1000 m in water droplets; mist is 1000 m or more (usually up to 5000 m); haze is the same reduction from dry particles. Poor visibility belongs to stable air, inversions and light winds, which is why highs and cols are the foggy pressure systems.

Radiation fog

Clear night, moist air, light wind of about 2–8 kt (calm gives dew or frost; above 10–15 kt gives low stratus instead). Over land only, in autumn and winter, first in valleys, thickening until just after sunrise. Cleared by the sun heating from below or by stronger wind. In a winter high it can persist all day.

Advection fog

Warm, moist air moving over a colder surface. It can form with moderate wind, at any time of day, and over the sea. Over land in north-west Europe it is a winter feature; over the sea, late spring and early summer. It clears with a change of air mass, or turns to stratus above about 15 kt.

⛰️ Hill fog

Low cloud with its base below the hilltops.

♨️ Steaming fog

Very cold air over much warmer water: Arctic sea smoke near Greenland, Iceland and in fjords.

🌧️ Frontal fog

Rain saturating the air under a front until the base reaches the ground; a belt up to about 200 nm wide.

🥶 Freezing fog / ice fog

Supercooled droplets that rime surfaces / ice crystals in extreme cold.

Runway visual range

RVR is how far a pilot on the centreline can see the runway markings or lights. It is measured by transmissometer or forward-scatter meter at the touchdown zone, mid-point and stop end. It is reported when visibility or RVR falls below 1500 m, or with shallow fog. It is usually greater than the meteorological visibility because bright lights can be seen further away. RVR is observed, never forecast.

✈️ Pilot tip Above a haze or fog layer, the horizontal view can be excellent while the slant view down to the runway is poor, and haze is worst looking into the sun. Near a city, the visibility is better on the upwind side.

🧊 Icing 56 questions

Water can stay liquid well below 0°C, down to about −40°C. When those supercooled drops hit an aircraft, they freeze on it.

🎯 Supercooled water🎯 Clear, rime, mixed, hoar frost🎯 Freezing rain🎯 Engine icing

💡 Key idea

Airframe icing needs supercooled liquid droplets and an airframe at or below 0°C. The bigger the drops and the closer the temperature is to 0°C, the more of each drop runs back before freezing, and the worse the ice. Below about −20°C, most cloud is ice crystals, which do not stick.

Type	How it forms	Character
Clear (glaze)	Large drops; part freezes at once and the rest flows back. Cu, Cb and Ns, 0 to about −20°C; freezing rain	Dense, heavy, sticks hard, distorts the aerofoil; the most dangerous
Rime	Small drops freeze instantly, trapping air. Layer cloud and the colder parts of heap cloud	White, opaque, light, little runback, mainly on leading edges
Mixed	Large and small drops together, usually 0 to −20°C	Rough and heavy
Hoar frost	Vapour deposits directly on a cold-soaked skin, for example after a fast descent into warm, moist air	White crystals; adds drag and obscures windscreens; clears in warmer air

🌧️ Freezing rain Rain falls from a warm layer aloft into sub-zero air below, typically in a shallow layer ahead of a warm front or occlusion in winter. It builds clear ice very fast; even short encounters can be hazardous, and ice protection may not cope. Leave at once, for example by turning back or climbing into the warm layer.

Ice reduces lift (by around 30%) and increases drag (by around 40%), raises the stall speed, adds weight, and blocks pitot-static instruments.
In a cumulus the worst icing is in the lower part of the cloud, just above the freezing level. The thinnest parts (tailplane edges, struts, probes) collect ice first.
Carburettor icing needs no sub-zero air: fuel evaporation and venturi expansion can produce it from about −10°C to +25°C or more in moist air.
Turbine intake icing comes from snow or supercooled water, worst at high RPM and low speed. Cold-soaked fuel can frost the wings on the ground.
On the ground the aircraft must be free of frost, ice and snow. Holdover time is how long the de/anti-icing fluid will keep protecting.

🧊🔥 Air Masses & Fronts 57 questions

An air mass takes on the character of the surface it sat over. When it moves, that character changes from below, and where two air masses meet, a front forms.

🎯 Air mass codes🎯 The five European air masses🎯 Warm and cold front sequence🎯 Slopes and speeds

💡 Key idea

Warmed from below, an air mass becomes unstable; cooled from below, it becomes stable. Air moving over a warmer surface gets heap cloud, showers and good visibility. Air moving over a colder surface gets stratus, drizzle, fog and poor visibility. The letters in an air mass code are its moisture (m maritime / c continental), its source (A Arctic, P polar, T tropical), and sometimes whether it is colder (k) or warmer (w) than the surface beneath it.

Air mass over NW Europe	Typical weather
Arctic maritime (mA)	Very unstable after a long sea track: large Cu, heavy snow showers in winter
Polar maritime (mP)	From the north-west: Cu and Cb, showers, bumpy flying, good visibility. Clear nights inland bring radiation fog
Returning polar maritime (mPw)	Has been south and back; arrives from the south-west, warmer and moister below, with more layer cloud
Polar continental (cP)	Winter, from Siberia: very cold and dry. After a North Sea crossing, snow showers on east coasts
Tropical maritime (mT)	South-westerly: low St and Sc, drizzle, poor visibility, advection fog
Tropical continental (cT)	Summer, from North Africa: hot, dry, cloudless, dust haze

A depression moving east. The observer to the east sees, in order, the warm-front clouds lowering and thickening, the warm sector, and then the cold front.

Warm front passes

Ahead of it: pressure falls steadily and continuous rain becomes heavier under Ns. As it passes: the wind veers, temperature and dew point rise, and the pressure fall slows. Then the warm sector: St, drizzle and poor visibility, especially in winter. A warm front moves at about ⅔ of the geostrophic wind component at right angles to it.

Cold front passes

A sharp veer, squalls, heavy showers, a sudden drop in temperature and dew point, and pressure starting to rise. Behind it: Cu and Cb, showers, and very good visibility between them. It moves at about the full perpendicular geostrophic component. Its weather is more intense but narrower than a warm front's, and more intense in winter.

The Polar Front separates polar and tropical air. In the North Atlantic it runs roughly from Florida to SW Britain in winter, and from Newfoundland to NW Britain in summer.
Depressions come in families, the next one forming on the trailing cold front. They move parallel to the warm-sector isobars.
The jet lies parallel to the fronts in the warm air: about 400 nm ahead of the surface warm front and about 200 nm behind the surface cold front.
At 10 000 ft with a 1:150 slope, the warm front surface is 1 500 000 ft (about 250 nm) ahead of its surface position.

🔗 Occlusions 35 questions

The cold front moves faster than the warm front and eventually catches it, lifting the warm sector off the ground.

🎯 Warm or cold occlusion?🎯 Where the rain falls🎯 Embedded Cb🎯 Triple point and trowal

💡 Key idea

The colder of the two cold air masses stays at the surface. If the air ahead is colder, the cold front rides up over it and you get a warm occlusion. This is the typical winter case over Western Europe, when continental air ahead is very cold. If the air behind is colder, it undercuts and you get a cold occlusion, which is more common in summer. A station that is colder ahead of the occlusion than behind it is under a warm occlusion.

	Warm occlusion	Cold occlusion
Chart line continues	Along the warm front	Along the cold front
Most precipitation	Ahead of the surface occlusion	Behind the surface occlusion
Special risk	Rain ice in very cold air below	Convective weather as in a cold front

The meeting point of the warm, cold and occluded fronts is the triple point (point of occlusion), a favourite place for a new secondary low, especially where mountains hold the fronts back. The lifted warm air forms a trowal (trough of warm air aloft). An occluding low slows down and curves to the left in the Northern Hemisphere; it deepens for about four days and can take ten days or more to fill. On a chart, an occlusion is drawn with triangles and semicircles on the same side of the line.

⚠️ Main hazard Embedded cumulonimbus: the cold front's convective cloud is pushed into the warm front's layer cloud, so the storms are hidden in stratiform cloud, with icing and turbulence. A slow-moving occlusion near the depression centre gives the most persistent weather.

🌀 Other Depressions 48 questions

Not every low is a polar front low. Mountains, heating, cold pools and warm tropical seas all make lows of their own.

🎯 Lee lows🎯 Thermal and polar lows🎯 Cold air pools🎯 Tropical revolving storms

⛰️ Orographic (lee) low

Air flows round the ends of a range, leaving a shortage of air in its lee. The Gulf of Genoa is the classic case as a cold front reaches the Alps. Dry air gives Föhn; moist, unstable air gives Cb and squalls.

☀️ Thermal low

Heating expands the column, so air flows out aloft and surface pressure falls. Weakens with height, possibly to a high aloft. In summer over land it brings storms; in winter it forms over warm inland seas (Mediterranean, Black, Caspian).

❄️ Polar low

Arctic air over a much warmer sea in winter: Cb and heavy snow showers. Not a frontal low.

🥶 Cold air pool

Cold air aloft, seen on thickness or contour charts but not on a surface chart. Over summer land it brings showers and thunderstorms.

➕ Secondary low

A smaller low within a bigger circulation. It may grow into the next main low, or hold up a cold front and make its timing hard to forecast.

🌪️ Tornado

Up to a few hundred metres across, with violent winds and a sudden pressure drop. A hook echo on radar. US peak in the late afternoon; European ones are smaller.

Tropical revolving storms

Requirement / fact	Value
Sea temperature	At least about 26°C, through a deep layer
Latitude	About 5–25°; none within ~5° of the Equator (too little Coriolis), and almost none in the South Atlantic (sea too cool)
Other needs	Little vertical windshear; energy from latent heat
Classification	Tropical depression < 34 kt · tropical storm (named) 34–63 kt · hurricane / typhoon / cyclone 64 kt or more
Structure	Calm, nearly clear eye of sinking air; strongest winds and rain in the eye wall
Track (north)	West in the trades, then recurving north and north-east; decays over land or cold sea
Seasons	N Atlantic June–November (peak Aug–Oct); S Indian Ocean / Australia December–April

✈️ Pilot tip Near a tropical revolving storm in the Northern Hemisphere, turn until you have port drift: the wind is then from the right and you are heading away from the centre. In the Southern Hemisphere, turn until you have starboard drift.

🗺️ Global Climatology 56 questions

The big picture: belts of pressure and wind that move north and south with the sun, and the monsoons that reverse with the seasons.

🎯 Pressure and wind belts🎯 The ITCZ🎯 Monsoons🎯 January vs July

💡 Key idea

From the Equator outwards in each hemisphere: the equatorial trough (ITCZ and the doldrums) → north-east / south-east trade winds → subtropical highs at about 30–35° (the "Horse Latitudes") → the westerlies at about 40–60° → the temperate lows → the polar easterlies → the polar high. The whole pattern follows the sun, much more over land than over the oceans.

Feature	January (northern winter)	July (northern summer)
Asia	Intense Siberian high; very cold because it is far from the sea, with long nights and snow cover	Huge thermal low near Pakistan (the Baluchistan low)
Oceans	Deep Icelandic and Aleutian lows; Azores high further south	Lows weaker; Azores and Pacific highs stronger and further north
India	NE monsoon: dry, offshore	SW monsoon: SE trades cross the Equator and veer towards the Asian low, giving heavy rain. Upper winds become easterly (tropical easterly jet)
ITCZ	Furthest south; reaches about Darwin	Furthest north, deep into Asia and West Africa

A monsoon is a seasonal wind reversal caused by a winter high or a summer low over a continent.
Trade-wind weather is fine on the poleward and eastern sides of the oceans and more showery to the west and near the Equator.
Climate zones: warm, wet equatorial; arid subtropical; cool, wet temperate; cold, dry polar. North-west Europe is Köppen class C.
In the Southern Hemisphere, polar front lows centre near 50°S all year. Near the South Pole, the surface winds are strong easterlies.

🏔️ Local Winds & Weather 34 questions

Named winds are a shorthand. Each name stands for a pressure pattern, a direction and a hazard.

Wind	Where / type	Character
Föhn / Chinook	Alps / east of the Rockies	Warm, dry downslope wind; the Chinook can clear snow within hours
Mistral	Rhône valley to the Gulf of Lions	Cold northerly, funnelled; winter and spring, with high pressure to the north and a Genoa low
Bora	Adriatic coast	Cold, katabatic north-easterly that sets in suddenly with violent gusts
Gregale	S Italy and Malta	Strong north-easterly, usually in winter
Sirocco · Ghibli · Khamsin	North Africa	Hot, dusty southerlies ahead of lows moving east; the Khamsin is mostly a spring wind. The Sirocco crossing to southern Europe becomes moist, with low cloud and fog
Etesians (Meltemi)	Aegean	Steady, dry summer northerlies with clear skies
Levanter	Strait of Gibraltar	Easterly; a banner cloud off the Rock and turbulence near the airfield
Harmattan	West Africa (e.g. Kano), Nov–Apr	Dry, dusty north-easterly from the Sahara; improves towards the Gulf of Guinea
Haboob	Sudan	Duststorm driven by thunderstorm outflow
Sumatras	Straits of Malacca, SW monsoon	Night-time thunderstorm squall lines
Southerly Buster	Sydney	A squally southerly change with a sharp temperature drop behind a summer cold front

✈️ Pilot tip Fjords and mountain valleys funnel strong winds into sudden low-level turbulence and windshear. The Bora and the Mistral are both at their worst in winter, when cold continental air pours towards a Mediterranean low.

🌍 Area Climatology 43 questions

What to expect, season by season, over the regions this material concentrates on: Europe and the North Atlantic, the Mediterranean, North-west Africa, Arabia and the Gulf.

🌊 North Atlantic

Grand Banks advection fog in late spring and early summer, as warm air crosses the cold current. Winter depressions track towards NW Europe.

🏔️ Europe

Cold fronts are held up by the Alps, with frontal and orographic cloud on the north side. Persistent winter radiation fog in the Po valley.

☀️ Mediterranean

Summer: little cloud under the Azores high. Winter: lee lows near Genoa, in the northern Adriatic, south of the Atlas and near Cyprus.

🏜️ NW Africa

South of about 20°N, the ITCZ brings the wet season and a SW monsoon in spring. In winter, polar-front cold fronts bring rain to northern Morocco.

🛢️ Arabia & Gulf

Winter: Mediterranean lows and cold fronts with showers, storms and dust, and a NW Shamal. Summer inland: almost rainless and sometimes above 50°C.

✈️ Route Climatology 40 questions

Seasonal weather along some classic long-haul routes: across South and South-east Asia, the Western Pacific, Australia and East Africa.

Region	Key points
Bay of Bengal & Malaysia	SW monsoon (summer): storms on windward coasts. NE monsoon (January): heavy orographic Cu and Cb on the east coast of West Malaysia. The ITCZ crosses Singapore northbound around March and southbound around Nov–Dec. Cyclones mainly early and late in the summer
Western Pacific	Typhoons form at about 10–15°N, drift west at 10–15 kt and often recurve north. Crachin (Jan–Apr): fog, stratus and drizzle on the South China coast. Summer sea fog east of northern Japan over the cold Oyashio current
Australia & NZ	The ITCZ reaches just south of Darwin in January. Dust haze in the SE trades in the southern winter. Subtropical jet near 25°S in winter. Southerly Busters at Sydney
East & Southern Africa	Khartoum's wet month is July, when the ITCZ is just north. The Guti brings stratus to the high veld. Johannesburg in summer: early low cloud, then afternoon storms. North of the ITCZ in July the wind is northerly, an extension of the Etesians

🛰️ Satellite Observations 27 questions

Two kinds of orbit and three kinds of picture. Knowing which one you are looking at tells you what the brightness means.

Geostationary

Over the Equator at about 36 000 km, one orbit in 24 h. A continuous view of about a third of the globe (Meteosat at 0°: about 70°W–70°E), good for time-lapse. Distorted towards the poles.

Polar orbiting

About 850 km up, about 100 minutes per orbit, scanning a strip. Sharper detail and polar coverage, but each place is seen only about twice a day.

Image	Shows	Watch for
Visible	Reflected sunlight; daytime only	Sea dark; thick cloud bright
Infrared	Temperature of the top surface, day and night; cold = white	Fog and low stratus hard to see (warm tops); thin cirrus shows well
Water vapour	Moisture in the middle and upper troposphere	Upper flow and jets, even in clear air

Bright on both visible and IR means thick, high-topped cloud such as Cb. Bright on visible but grey on IR means low cloud or fog. Patterns to recognise: frontal bands and comma clouds round depressions, open cells behind cold fronts (cold air over warm sea), lee-wave lines downwind of mountains, and spiral bands round the eye of a tropical revolving storm. A satellite cannot show CAT directly, only hints such as jet-stream cirrus.

📟 METARs 66 questions

A METAR is an observation, not a forecast: what the weather was at one aerodrome at one moment, in a fixed order of groups.

🎯 Group order🎯 Visibility and RVR🎯 Weather codes🎯 CAVOK, NSC, TREND

💡 Key idea

Read it left to right; the order never changes: type · station · day and time (UTC) · wind · visibility · RVR · present weather · cloud · temperature/dew point · QNH · supplementary (recent weather, windshear, runway state) · TREND. METARs are issued every 30 minutes (hourly at some aerodromes), and a SPECI is issued in between when conditions change significantly.

METAR EGXX 141050Z 24018G32KT 200V280 4000 1200NE R27/0650D +SHRA BKN012 SCT030CB 08/06 Q0998 RERA WS R27 TEMPO 2000 TSRA

Group	Meaning
141050Z	14th day, 1050 UTC
24018G32KT 200V280	240° true, 10-minute mean 18 kt, gusting 32 kt; direction varying 200°–280°. Calm is 00000KT; VRB04KT is light and variable
4000 1200NE	Prevailing visibility 4000 m (reached or exceeded over at least half the area), minimum 1200 m to the north-east. 9999 = 10 km or more; 0000 = less than 50 m
R27/0650D	RVR runway 27, 650 m, decreasing (U up, N no change). Reported when visibility or RVR is below 1500 m. P2000 = above the maximum the system can assess
+SHRA	Heavy showers of rain (− light, no sign moderate, + heavy; VC in the vicinity)
BKN012 SCT030CB	5–7 oktas at 1200 ft, 3–4 oktas of Cb at 3000 ft, above aerodrome level. Only CB and TCU are named. VV001 = sky obscured, vertical visibility 100 ft
08/06 Q0998	Temperature 8°C, dew point 6°C (M = minus); QNH 998 hPa, rounded down
RERA · WS R27	Recent moderate or heavy rain; windshear reported on runway 27
TEMPO … / NOSIG	TREND: a forecast for the 2 hours after the observation; NOSIG = no significant change expected

🌫️ Obscurations

BR mist · FG fog · FZFG freezing fog · MIFG shallow · BCFG patches · PRFG partial · HZ haze · FU smoke · VA volcanic ash · DU / SA dust / sand

🌧️ Precipitation

DZ drizzle · RA rain · SN snow · RASN rain and snow · GR hail · GS small hail / snow pellets · FZDZ / FZRA freezing drizzle / rain

⛈️ Other

TS thunderstorm (thunder heard within 10 minutes) · SQ squall · FC funnel cloud · SS / DS sand / dust storm · BLSN blowing snow (2 m or more) · DRSN drifting snow (below 2 m)

🤖 Automatic

AUTO = no human input · //// = not determined · NCD = no cloud detected · FEW008/// = cloud type not determined

☀️ CAVOK and NSC CAVOK replaces visibility, weather and cloud when all of these hold: visibility 10 km or more; no cloud below 5000 ft or the highest MSA (whichever is greater); no CB or TCU; no significant weather. A layer at 8000 ft does not prevent CAVOK. NSC means no significant cloud (none below 5000 ft or MSA, no CB/TCU) when CAVOK cannot be used, for example because the visibility is too low. A runway state group with R88 means all runways; SNOCLO means the aerodrome is closed by contamination.

🗓️ TAFs 43 questions

A TAF is the forecast for one aerodrome over 9 to 30 hours, and the document you plan your destination and alternates against.

🎯 Validity groups🎯 FM, BECMG, TEMPO, PROB🎯 What a TAF leaves out🎯 Reading a time slice

TAF EGXX 050500Z 0506/0612 22012KT 9999 SCT030
     BECMG 0512/0514 25020G35KT 5000 RA BKN012
     TEMPO 0514/0518 2000 +RA
     PROB30 TEMPO 0600/0606 0300 FG

Change group	Meaning
FM051430	From 1430 UTC on the 5th, a rapid change to a complete new set of conditions. It always gives a full forecast
BECMG 0512/0514	A lasting change at some time in the period. Only the elements given change; the others stay as before. During the period, conditions could be either the old or the new
TEMPO 0514/0518	Temporary fluctuations, each less than an hour and together less than half the period
PROB30 / PROB40	30% or 40% probability (the only two used). With no PROB group, the probability is 50% or more
TX18/1415Z TN06/1505Z	Maximum 18°C at 1500 UTC on the 14th, minimum 6°C at 0500 UTC on the 15th
NSW · NSC · AMD · CNL	No significant weather (the weather ends) · no significant cloud · amended · cancelled

Worked example — reading the TAF above

At 1300 UTC on the 5th the BECMG change is in progress, so the wind could be anywhere between 220/12 and 250/20G35. Plan for the worse. At 1600 UTC: 5000 m in rain with BKN012, and temporary spells of 2000 m in heavy rain. Early on the 6th there is a 30% chance of temporary fog with 300 m.

0506/0612 means valid from 0600 UTC on the 5th to 1200 UTC on the 6th. 9-hour TAFs are issued every 3 hours; longer ones every 6 hours.
A TAF gives prevailing visibility only (no minimum by direction), names only CB, and never forecasts RVR or QNH.
Cloud heights are above aerodrome level, in hundreds of feet.

🗺️ World Area Forecast System 40 questions

Two centres, London and Washington, produce the global upper wind, temperature and significant weather forecasts that every flight plan uses.

🎯 Fixed-time charts🎯 SIGWX symbols🎯 Wind barbs🎯 Averaging route winds

💡 Key idea

WAFS charts are fixed-time charts: each is valid only at its stated time. They are issued every 6 hours, about a day ahead. For a flight from 1000 to 1400, use the 1200 chart and interpolate at the ends. High-level SIGWX covers FL250–FL630; medium-level covers FL100–FL250 (or up to FL450 in some regions).

SIGWX symbol	Meaning
Scalloped line	An area of significant weather (CB, icing, turbulence). Cb implies thunderstorms, moderate or severe turbulence, icing and hail
Heavy dashed line, numbered	An area of clear air turbulence, explained in the legend
ISOL · OCNL · FRQ · EMBD	Isolated · well separated · little or no separation · hidden in other cloud
XXX as a base or top	Extends beyond the vertical limits of the chart
Thick line with arrowheads	Jet stream: barbs for speed, core flight level, and the depth of the 80 kt layer (e.g. FL250/370). A double bar marks a change of 20 kt or 3000 ft
Number in a box	Tropopause height (flight level)
Fronts, volcano, cyclone symbols	Surface fronts with movement; an eruption (check SIGMETs and NOTAMs); a tropical revolving storm with its name

Wind barbs on upper charts. The shaft points the way the wind is blowing towards; the feathers sit at the end it comes from. Temperatures are negative unless marked +.

✈️ Pilot tip To find a route wind, sample the chart at regular intervals and average direction and speed. Polar stereographic charts need care because north changes direction across the chart as the meridians converge.

⚠️ Warning Messages 38 questions

Warnings come in layers: at the aerodrome, on the approach, en route for everyone, en route for low-level flights, and special ones for ash and cyclones.

Message	Covers	Notes
Aerodrome warning	Hazards to parked aircraft and ground operations: gales, thunderstorms, hail, snow, frost, freezing precipitation, fog	Some aerodromes also warn of a marked inversion (a rise of 10°C or more in the first 1000 ft)
Windshear warning	Approach and departure paths, up to about 1600 ft above the aerodrome	Based on observations, forecasts and pilot reports
SIGMET	Specified hazardous weather en route in an FIR: TS (OBSC, EMBD, FRQ, SQL), severe turbulence and severe icing not linked to convective cloud, severe mountain waves, heavy dust or sandstorm, volcanic ash, tropical cyclones	Issued by the meteorological watch office. Valid up to 4 h (6 h for VA and TC, with an outlook). Cancelled by a new SIGMET with CNL. INTSF / WKN / NC = intensifying / weakening / no change; STNR = stationary
AIRMET	Lesser hazards for low-level flights, such as moderate icing or turbulence	—
GAMET	An area forecast for low-level flights in abbreviated plain language	—
VA / TC advisories	Volcanic Ash Advisory Centres; Tropical Cyclone Advisory Centres	Ash positions at +6, +12, +18 h. Cyclone position, movement, pressure, winds and a forecast "cone" that widens with uncertainty

🌋 Volcanic ash Ash melts in the combustion section and fuses onto the turbine, so engines can flame out. It also abrades windscreens and leading edges and blocks pitot tubes. It barely shows on weather radar and is hard to see at night. Colour code: GREEN normal · YELLOW elevated unrest · ORANGE heightened unrest or eruption with little ash · RED eruption with significant ash, or imminent.

📻 Met Information in Flight 30 questions

Once airborne, the weather picture is kept current by broadcasts, data link and the reports pilots themselves send back.

VOLMET

Weather for a group of aerodromes in plain language: METARs with TREND, sometimes TAFs and SIGMETs. Regional services on VHF; long-range ones on HF at fixed times each hour (Shannon matters for the North Atlantic). Updated about every half hour. Useful for destination and alternates, though you may have to wait for the aerodrome you need.

ATIS

A continuous broadcast for one aerodrome: runway, wind (magnetic), visibility, weather, cloud, temperature, dew point, QNH, transition level and warnings. Each update gets a new letter, which you quote on first contact. Sometimes on a VOR voice channel, or separate for arrivals and departures. Even with the ATIS acknowledged, ATC still passes the QNH.

A special air-report is required for severe turbulence or icing, severe mountain waves, thunderstorms, volcanic ash and similar hazards. Give position, level, time, intensity and type. Automated reports by data link give routine wind and temperature data.
Data link gives D-ATIS and METAR/TAF on request, independent of broadcast schedules.
Checking METARs from aerodromes upwind of the destination shows what is on its way.

✈️ Pilot tip A destination METAR well below minima with NOSIG means no improvement is expected within two hours, so plan the diversion now rather than holding in hope.

This guide explains general meteorology for study purposes. Always use current official reports and forecasts for flight planning.