MET review

Exam review

The exam topics of MET in one place, from the MET lessons and made to print. Every part keeps the wording of its lesson, and a line under it names that lesson for the detail.

Each section shows its syllabus level, the depth the course asks for: 1 know it (state, list, define), 2 explain it (explain, describe, differentiate), 3 apply it (decode, calculate, appreciate). The levels come from the EU training rules, Regulation (EU) 2015/340, and every level is exam material.

Left out as nice to know: the storms by region, the thunderstorm types, lightning in detail, and the layers above the troposphere in detail.

Units and instruments

What an airport measures, the unit it is reported in, and the instrument that measures it. Weather reports in Europe use these units.

Syllabus level 3apply itMETB 1.1.1, 2.1.3, 3.4.2, 4.1.3, 4.3.3

Measured	Unit	Instrument
At the aerodrome: made at least once an hour
Wind	° Degrees, its directionkt Knots, its speed	Anemometer and wind vane: together they measure the wind’s speed and direction, about 10 m above the runway. A windsock gives a visual indication.
Temperature, dew point and humidity	°C Degrees Celsius% Per cent, relative humidity	Temperature and humidity probes
Visibility	km, m Kilometres or metresA local report: metres under 5 km, kilometres from 5 km. A METAR: always metres.	Transmissometer or forward scatter meter. Each visibility sensor gives both the visibility and the RVR.
Runway visual range, RVR	m Metres only	Transmissometer or forward scatter meter, at up to three points along the runway
Present weather	Light, moderate or heavy	Present weather sensor: the type and intensity of precipitation
Air pressure	hPa Hectopascals	Dual sensors: an electronic pressure sensor or an aneroid barometer
Cloud base and vertical visibility	ft Feet	Ceilometer
Above the aerodrome
Upper air, up to 30 km	–	Radiosonde, carried up by a balloon: pressure, temperature, humidity and wind. The wind comes from tracking its position.
Wind aloft	–	Wind profiler: radar or sound waves
Other sources	–	Weather radars, satellites and aircraft observations

True in a METAR, magnetic to an aircraft. A METAR gives the direction the wind blows from in degrees true. When ATS gives the wind to an aircraft for take-off or landing, the direction is in degrees magnetic.

From MET 001, Introduction to meteorology; MET 002, The atmosphere (pressure); MET 005, Atmospheric circulation (wind); MET 007, Visibility; and MET 009, Application of meteorological information (the units in reports).

The meteorological organisation

Who sets the standard, the centres that serve the whole planet, and the offices that work for ATS.

Syllabus level 2explain itMETB 1.2.2, 1.3.1, 1.3.2

Who	What it does
Who sets the standard
ICAO Annex 3 Meteorological Service for International Air Navigation	Where the rules are found. Its objective is the safety, regularity and efficiency of international air navigation.
WMO World Meteorological Organization	Works with ICAO to provide the framework.
The world centres
WAFC World Area Forecast Centre 2: London and Washington	Prepares gridded global forecasts. WAFS, the World Area Forecast System, provides them in a uniform, standardised format, and SADIS distributes them.
VAAC Volcanic Ash Advisory Centre 9 worldwide	Forecasts where an ash cloud will move. Updates at least every 6 hours, while there is volcanic ash to report on.
TCAC Tropical Cyclone Advisory Centre 7 worldwide	Monitors tropical cyclones. Updates at least every 6 hours.
SWXC Space Weather Centre 5 providers	Advisories on space weather that might affect HF radio, communications via satellite, GNSS and radiation exposure at flight levels.
National and local
The national met provider In Denmark, DMI	Each state has its own meteorological provider.
MWO Meteorological watch office For the ACC and FIS	Provides meteorological information for specific FIRs. Issues SIGMET, and AIRMET where the authority decides the traffic below FL 100 needs it. Denmark issues SIGMET only.
Aerodrome meteorological office For TWR, AFIS and APP	The local service: flight-related forecasts for its aerodrome. Among them the TREND, a trend forecast for landing, valid for 2 hours.

A meteorological office shall be associated with each ATS unit. Which kind depends on the unit.

From MET 001, Introduction to meteorology.

The atmosphere and the ISA

A mixture of gases surrounding the earth. The first 80 km above the surface contain 99% of its mass.

Syllabus level 2explain itMETB 2.1.1, 2.1.2, 2.2.1, 2.2.2, 2.5.1

What it is made of

78%Nitrogen

21%Oxygen

1%Other gases

Up to 4%Water vapour, in the lower atmosphere. The amount varies.

The layers Five, each topped by a pause. On the way up, temperature falls, rises, falls and rises again.

Layer	Height	What to know
Troposphere	Surface to 7–18 km	Almost all the water vapour and weather. Heated from below, which promotes vertical mixing. Temperature usually falls with height.
Tropopause	7 to 18 km, about 23 000 to 60 000 ft	The top of the troposphere: not a hard boundary, it works a little like a lid. High over the hot equator, low over the cold poles; it depends on the latitude and the season. Found by measuring the change of temperature with height.
Stratosphere	Up to 51 km	Small vertical movement. Temperature stable, then increasing with height from 20 km, due to the high amount of ozone.
Mesosphere	–	Temperature falls again.
Thermosphere	–	Temperature rises again. The aurora borealis is up here.
Exosphere	–	The outermost layer.

The International Standard Atmosphere Assumed, to simplify the atmosphere. Used to calibrate instruments and to assess aircraft performance, uniformly.

1013.25 hPaPressure at mean sea level

15 °CTemperature at mean sea level

1.225 kg/m³Density

NoneWater vapour

Good to know: the temperature lapse rate is 1.98 °C per 1 000 ft, the pressure lapse rate about 1 hPa per 30 ft. The tropopause is at 36 090 ft (11 km), −56.5 °C.

Pressure, temperature, density and height

Temperature. Usually decreases as altitude increases.

Pressure. Unlike temperature, it always decreases as altitude increases.

Moisture. Warm air can hold more water vapour than cold air.

Density. The mass per unit volume of the atmosphere. It decreases with height, because the pressure does.

What lowers density. Lower pressure, higher temperature, more moisture. Moisture has the smallest effect of the three.

Reduced density. Less lift, a less efficient rotor or propeller, less performance from a piston engine, and less drag: the one effect that helps.

Hypoxia. Higher up, the air is thinner, so each breath holds less oxygen. The lack of oxygen causes hypoxia.

From MET 002, The atmosphere.

Pressure settings

An altimeter measures air pressure, and the pressure falls as you climb. What it shows depends on the setting in its subscale window.

Syllabus level 2explain itMETB 2.5.2

Setting	Zero at	Reads	The pilot says
QNH	Mean sea level	Altitude	“Altitude”, or no word at all
QFE	The aerodrome’s elevation	Height above the aerodrome	“Height”
1013.25 hPa	The standard pressure	Flight level	“Flight level”

QNH. The local pressure reduced to mean sea level according to the ISA: the setting that makes the altimeter read the aerodrome’s elevation on the ground. Given routinely in Denmark.

QFE. The pressure at the aerodrome’s elevation. Given on request only in Denmark.

1013.25 hPa. The standard setting, the ISA’s pressure at mean sea level. A flight level is in hundreds of feet above the 1013.25 hPa level: FL 50 is 5 000 ft.

Elevation. How high the aerodrome sits above mean sea level.

About 1 hPa for every 30 ft It turns a difference in pressure into a difference in height. In your head, round 1013.25 to 1013, as the course does in its sums.

QFE = QNH − elevation ÷ 30An aerodrome 300 ft above the sea, QNH 1003: QFE 993.

Altitude = FL × 100 − (1013.25 − QNH) × 30FL 50, QNH 1003: 10 hPa × 30 = 300 ft, so about 4 700 ft. Lower QNH, lower aircraft.

A QNH below 1013.25 puts the 1013.25 hPa level below the sea. Flight levels are measured from it, so the flight level reads higher than the altitude.

Climbing, the altimeter goes from QNH to 1013.25 hPa passing the transition altitude; descending, it goes back to QNH passing the transition level. Between the two is the transition layer. The transition in full is in ATM 005.

From MET 002, The atmosphere; MET 001, Introduction to meteorology (QNH and QFE in Denmark); and ATM 005, Altimetry.

Heat and water

Heat and temperature are not the same phenomena. How heat moves, and what water does as it changes state.

Syllabus level 2explain itMETB 2.3.1, 2.3.3, 2.4.1

Temperature. An expression of the heat condition of a body. Determined by the kinetic energy, the average speed, of the body’s molecules.

Heat. The transfer of thermal energy. Heat flow is the transfer of energy because of a temperature difference alone.

Heat capacity. The amount of energy required to change the temperature of a substance by 1 degree. Different materials can have different heat capacities.

Temperature inversion. The temperature rises with altitude: warm air over cold air. For example, cold fronts. It indicates reduced performance.

How heat moves In the atmosphere the three work together.

Way	What it is
Conduction	Heat transfer between two objects that are in contact. The better the conductor, the more rapidly the heat transfers.
Convection	Heat transport in the atmosphere by upward currents of air. Warm air is lighter than cold air, and starts to rise on its own. The dominant heat transfer mechanism in gases and liquids.
Radiation	Heat transfer by the emission of electromagnetic waves. The earth receives all its heat from the sun as radiation, and loses heat to space as radiation too.

Water in three states Solid (ice), liquid (water) and gas (water vapour).

Change	From and to
Towards the gas: latent heat is absorbed, and the surroundings cool
Melting	Ice to water
Evaporation	Water to water vapour
Sublimation	Ice straight to water vapour
Towards the solid: latent heat is released, and the surroundings warm
Freezing	Water to ice
Condensation	Water vapour to water
Deposition	Water vapour straight to ice

Latent heat. Latent means hidden: the heat changes the state of the water, not its temperature.

Humidity. The amount of water vapour in the air.

Relative humidity. The degree of saturation of the air, measured in per cent.

Saturation. The air is incapable of holding more water vapour. It is reached when the air is cooled, or when water vapour is added by evaporation.

Dew point. The temperature a parcel of air must be cooled to for its water vapour to condense into water: the saturation temperature. Important for calculating the likelihood of fog, carburettor icing and so on.

Supercooling. Lowering the temperature of a liquid below its freezing point without it becoming solid. A liquid needs a nucleus to crystallise. Supercooled droplets freeze when they hit an object.

The smaller the difference between temperature and dew point, the nearer the air is to saturation, and to mist or fog forming.

From MET 003, Heat, temperature and water.

Lapse rates, stability and cloud formation

How fast rising air cools, whether it keeps rising, and how that makes cloud.

Syllabus level 2explain itMETB 2.3.2, 4.1.1

Adiabatic heating and cooling. A change in the temperature of air because of a change in its pressure. No heat is added or taken away. Rising air expands and cools; sinking air is compressed and warms. Clouds, rain and snow are all created because of it.

Lapse rate	Rate	What it is for
DALR Dry adiabatic lapse rate	3 °C per 1 000 ft	Unsaturated air, rising or subsiding.
SALR Saturated adiabatic lapse rate	1.5 °C per 1 000 ft, on average	Saturated air, rising or subsiding. Slower, because condensation releases latent heat. Varies strongly with temperature and pressure.
ELR Environmental lapse rate	2 °C per 1 000 ft, on average	The actual change of temperature with altitude, in still air. It can even be an increase with height: an inversion. The ISA’s is 1.98 °C.

Not the ISA lapse rate. The ISA’s 1.98 °C per 1 000 ft is how temperature falls with height in the standard atmosphere, not how rising air cools.

Stability How an air mass reacts to vertical movement. Neutral: when moved, it finds a new resting place.

The air	When	Lifted air	Cloud
Stable	The ELR is less than the SALR	Has a restoring force: unsaturated or saturated, it returns to its original position.	Stratiform: in layers. The cloud spreads out rather than up.
Conditionally unstable	The ELR lies between the SALR and the DALR	Stable for unsaturated air, unstable for saturated air.	–
Unstable	The ELR is more than the DALR	Moves farther away: unsaturated or saturated, it continues to rise.	Cumuliform: heaped, with vertical development. The cloud builds upwards.

How cloud forms Condensation nuclei and saturated air make cloud.

Adding moisture. Evaporation from water in daytime heating, or precipitation evaporating as it falls.

Cooling from below. Warm air moving over a cold surface: advection.

Adiabatic cooling. Air that is lifted expands and cools until it is saturated.

Lifting condensation level. Where rising air reaches its dew point: the water vapour condenses on condensation nuclei, and the cloud base forms.

What lifts the air	How
Convection	Uneven heating of the earth’s surface causes warmer air to start rising, until it is saturated and forms clouds.
Frontal lifting	Warm air forced up because of underlying colder air, as at a warm front.
Orographic uplift	Air is forced upwards by terrain.
Convergence	Air flowing together near the ground has nowhere to go but up. Unlike frontal lifting, it needs no warm and cold air.
Turbulence	Wind over rough ground is stirred up and down, and the air carried up cools.

From MET 003, Heat, temperature and water, and MET 004, Clouds.

Clouds, cover and ceiling

Visible water, droplets or ice crystals, suspended in the earth’s atmosphere, classified according to height, shape and behaviour. Then how much of the sky is covered, and how high the cloud starts.

Syllabus level 2explain itMETB 4.1.2, 4.1.3, 4.1.4, 4.1.5

Level	Height	Types	What to expect
High	16 500 to 45 000 ft	CI Cirrus CC Cirrocumulus CS Cirrostratus	Entirely ice crystals. No precipitation, no icing, none or light turbulence. CI and CS are common with an advancing warm front.
Middle	6 500 to 23 000 ft	AC Altocumulus AS Altostratus NS Nimbostratus	Ice crystals and/or water droplets, which can be supercooled. Light to moderate icing and turbulence. NS is almost always in the middle level, but usually extends into both the low and high levels.
Low	Surface to 6 500 ft	SC Stratocumulus ST Stratus	Water droplets, ice crystals and/or snowflakes. CU and CB are low clouds too, with considerable vertical extent: tops reaching the middle or high levels.
Vertical development	Base: ground to 10 000 ft	CU Cumulus CB Cumulonimbus	Mainly water droplets, but ice crystals may form. Bases normally in the low level, tops may extend into the middle and high levels. CB is the only cloud to produce thunder, lightning or hail.

The heights are for temperate regions; middle and high clouds overlap between 16 500 and 23 000 ft. Four forms: cumuliform, detached clouds with convective development; stratiform, layered clouds, usually broad and widespread; cirriform, high, wispy clouds of ice crystals; nimboform, clouds that produce precipitation.

Cover, in oktas The sky in eighths.

FEWFew: 1 to 2 oktas

SCTScattered: 3 to 4 oktas

BKNBroken: 5 to 7 oktas

OVCOvercast: 8 oktas

VVVertical visibility, in feet, when the sky is obscured

NSCNil significant cloud

NCDNo cloud detected

CAVOKVisibility, cloud and present weather better than prescribed values or conditions

CAVOK. Visibility 10 km or more, no cloud of operational significance, no significant weather. A cloud of operational significance is one with its base below 5 000 ft, or below the highest minimum sector altitude if that is higher, or any cumulonimbus or towering cumulus. The course spells the letters out as “ceiling and visibility OK”.

Cloud base. The lowest height of the visible portion of a cloud. A ceilometer measures it.

Ceiling. The height above the ground or water of the base of the lowest layer of cloud below 20 000 ft (6 000 m) covering more than half of the sky.

More than half the sky is 5 oktas or more, not more than 5 oktas: a ceiling is a BKN or OVC layer. CAVOK needs no significant weather, not the absence of a SIGMET.

From MET 004, Clouds, and MET 001, Introduction to meteorology.

Global circulation and the forces on wind

Wind is air in motion, caused by differences in pressure: the air moves to even them out. It is given as its speed and the direction it blows from.

Syllabus level 2explain itMETB 3.1.1, 3.4.1, 3.4.3

Force	What it does
Pressure gradient force	The air flows from high to low pressure. The pressure gradient is how much the pressure changes over a given distance: the closer the isobars, the stronger the wind.
Coriolis effect	Anything moving freely over the turning Earth seems to bend. In the northern hemisphere, moving air is deflected to the right.
Friction	Below about 3 000 ft the ground drags on the air and slows it. A slower wind means a weaker Coriolis effect, so the pressure gradient force wins: the wind turns in across the isobars towards the low.

The geostrophic wind is a theoretical wind: the balance between the Coriolis effect and the pressure gradient force. It blows parallel to the isobars, anticlockwise round a low in the northern hemisphere. Above about 3 000 ft the real wind is close to it: normally within 10% in speed and 10° in direction.

Near the ground. Slower than the geostrophic wind and turned in towards the low, so air flows into a low and out of a high.

Buys Ballot’s law. In the northern hemisphere, with the wind at your back, the low is on your left.

Why wind matters. It affects an aircraft’s motion over the ground. Landing and take-off are made into the wind. It makes the track differ from the heading, and the groundspeed from the airspeed. Strong head- and tailwinds, such as a jet stream, can mean a new route or level.

Four scales. Global: trade winds. Synoptic: cyclones, low- and high-pressure systems, fronts. Mesoscale: land and sea breeze, föhn wind, squall lines. Microscale: individual thunderstorms, local turbulence.

The global circulation The large-scale movement of air, driven by the different amounts of solar radiation absorbed at different latitudes.

Northern hemisphere	Between	Along the ground
Polar cell	The pole and 60°N	The air flows from the pole to 60°N.
Ferrel cell	60°N and 30°N	From 30°N to 60°N, bent to come from the south-west: the westerlies.
Hadley cell	30°N and the equator	From 30°N to the equator, bent to arrive from the north-east: the trade winds.

Highs and lows. Air rises at the equator and at 60°N, where the pressure at the surface is low, and sinks at 30°N and at the pole, where it is high.

The polar front. The boundary near 60°N between the cold air of the polar cell and the warmer air of the Ferrel cell.

ITCZ. The intertropical convergence zone: a belt of low pressure round the Earth, where the trade winds of the northern and southern hemispheres come together. It moves north and south with the seasons.

Jet streams. The polar and the subtropical jet: bands of very strong wind, usually from west to east, where the tropopause breaks, at about 60°N and 30°N.

Jet streams and ATS. Flying in and out of one is bumpy. A lot of headwind means a longer flight time, more fuel and delays, and can mean a level change or a re-routing. A lot of tailwind is favourable.

From MET 005, Atmospheric circulation.

Pressure systems, air masses and fronts

How pressure is drawn on a weather map, the air masses over Europe, and the boundaries between them. All for the northern hemisphere.

Syllabus level 1know itMETB 3.2.1, 3.2.2, 3.2.3

On the map	What it is
Isobars	Lines connecting points of equal pressure, generated from mean sea level pressure reports.
Low Cyclone	Surrounded by higher pressure. Near the ground the air flows in (convergence), and it rises in the middle. Anticlockwise: a cyclonic flow.
High Anticyclone	Surrounded by lower pressure. The air sinks in the middle and flows out near the ground (divergence). Lighter winds, clockwise. The sinking air dries out the air mass: usually clear skies and fine weather.
Trough	An elongated area of low pressure, often with fronts.
Ridge	An elongated area of high pressure: the opposite of a trough.
Col	An area of slack pressure between two highs and two lows, arranged crosswise.

Air masses A volume of air over a large region, with uniform characteristics, defined by its temperature and its water vapour content.

AArctic: very cold and dry

mPMaritime polar: cold and humid

cPContinental polar: cold and dry

mTMaritime tropical: warm and humid

cTContinental tropical: warm and dry

EEquatorial: hot and humid

The small letter is the source region, maritime or continental. The capital is its latitude: Arctic, Polar, Tropical or Equatorial.

Cold air over a warm surface. Heated from below, so it becomes unstable, with convection: convective cloud, a fairly high base, showers, good visibility except in precipitation.

Warm air over a cold surface. Cooled from below: stable and smooth. Layered cloud, a generally low base, continuous precipitation, reduced visibility with a risk of fog, mist or haze.

Fronts A front is the boundary between two air masses with different characteristics. The denser cold air undercuts the warm air.

	Cold front	Warm front
Movement	Fast: it passes quickly	Slow
As it passes	The temperature drops, and the wind changes direction	After it passes, the cloud gradually breaks up
Frontal surface	Steep: the cold air pushes under the warmer air	A very gentle slope: it reaches the tropopause 300 to 500 NM from the frontline
Type of cloud	Convective: rapid lifting of the warm air gives forced convection	Layered
Cloud base	Fairly high	Generally low, and gradually lowering
Precipitation	Showers, over a relatively small area	Continuous, with a risk of fog
Visibility	Good, except in precipitation	Reduced, with a risk of fog, mist or haze

Occluded front. Forms when the cold front overtakes the warm front, usually round a low. The warm air is lifted above the two colder air masses. It can bring many kinds of weather.

Stationary front. A boundary between two air masses where neither is strong enough to replace the other. It can stay over the same area for a long time.

From MET 002, The atmosphere, and MET 005, Atmospheric circulation.

Local winds

Wind on the mesoscale: weather systems smaller than the synoptic scale but larger than the microscale, from about 5 km to several hundred kilometres across.

Syllabus level 2explain itMETB 3.3.1, 3.3.2

Wind	Blows	Why
At a coast: both come from the difference in temperature between the land and the sea
Sea breeze	From the sea to the shore, usually in the afternoon	By early afternoon the land is warmer than the sea, so warm air rises over it. Cooler, denser air from the sea flows in to take its place.
Land breeze	From the land to the sea, at night	At night the land cools faster, and becomes colder than the sea. The air sinks over the land and rises over the sea. Generally weaker than the sea breeze.
In a valley
Anabatic	Up the slopes, by day	By day the slopes are warmed. They warm the air on them, and it flows up the valley sides.
Katabatic	Down the slopes, by night	At night the slopes lose heat by radiation. They cool the air on them; it gets denser and flows down the valley sides.
Over a mountain: not a valley wind
Föhn	Down the lee side, warmer than it started	Moist air forced up the windward side cools at the DALR to its dew point, then at the slower SALR in cloud, where rain falls. Down the lee side there is no cloud left, so it warms at the DALR all the way.

The föhn worked through: moist air at 13 °C, its dew point 10 °C, forced over a mountain 4 000 ft high. Cloud forms at 1 000 ft, it is 5.5 °C at the summit, and 17.5 °C at the foot of the lee side: 4.5 °C warmer than it started.

Mountain waves. High ground disturbs the horizontal air flow. On its lee side, downwind, the flow rises and falls in waves. Lenticular clouds can form in the waves if the air holds enough moisture, and rotors, rolling eddies, turn under them near the ground.

From MET 005, Atmospheric circulation, and MET 004, Clouds (the föhn wind).

Precipitation

Cloud particles too heavy to stay suspended in the air fall to the ground as precipitation. It usually brings a lowering cloud base and reduced visibility.

Syllabus level 2explain itMETB 4.2.1, 4.2.2

How it forms	What happens	What it creates
The coalescence effect	Small droplets collide and make bigger droplets.	Drizzle and light rain
The Bergeron process	Deposition: water vapour freezes directly onto the ice crystal, with no liquid stage, so the crystal grows larger.	Rain, snow, hail, ice pellets and more

The cloud decides	Stratiform clouds	Convective clouds
The cloud	Layered clouds	Clouds with vertical development
Duration	Continuous	Intermittent
What falls	Drizzle, rain, snow	Showers of rain or snow, and hail
Intensity	Steady, normally light to moderate	Varying, moderate to heavy
Particles	Smaller	Larger

What reaches the ground	The air it falls through
Rain	The snow melts in the warm air and reaches the ground as rain.
Freezing rain	The rain falls into a shallow layer below freezing at the ground. It becomes supercooled, and freezes on impact.
Sleet	The snow part-melts in a shallow warm layer, then refreezes in the deep cold layer below.
Snow	Below freezing all the way down, so it reaches the ground as snow.

Virga. Precipitation evaporating before it reaches the ground. As it evaporates it adds moisture to the air.

The codes Drizzle and rain part at 0.5 mm, hail and small hail at 5 mm.

DZDrizzle: drops under 0.5 mm, only from stratiform cloud

RARain: drops over 0.5 mm

SNSnow: six-sided ice crystals

SGSnow grains: under 1 mm, the solid equivalent of drizzle

PLIce pellets: they show a layer of freezing rain above the surface

GRHail: 5 mm or more, only from cumulonimbus

GSSmall hail or snow pellets: under 5 mm

-RALight: FBL in plain language and local reports

RAModerate: MOD

+RAHeavy: HVY

VCVicinity: within 16 km of the aerodrome, but not on it

A METAR writes the sign, never FBL, MOD or HVY. Sleet has no METAR code: refrozen snow is reported as ice pellets, PL, and rain and snow falling together as RASN.

From MET 006, Precipitation.

Visibility, fog and mist

Four kinds of visibility, the two kinds of particle that reduce it, the seven kinds of fog, and the runway visual range.

Syllabus level 2explain itMETB 4.3.1, 4.3.2, 4.3.3, 4.3.4

Flight visibility. The visibility forward from the cockpit of an aircraft in flight.

Ground visibility. The visibility at an aerodrome, as reported by an accredited observer or by automatic systems.

Prevailing visibility. The greatest horizontal distance seen over at least half of the horizon circle. It is taken as the representative visibility at a particular location.

Vertical visibility (VV). How far up an observer or a sensor can see into an obscuring layer, such as fog or heavy snow. When the sky is obscured, it replaces the cloud groups, in feet.

Why it matters. VFR flights need it to see and avoid other aircraft and obstacles; IFR flights to taxi, and to see the runway in the final stage of the flight. Low visibility can change the procedures, the separation methods and the minima, and cause delays and diversions.

Hydrometeors. Liquid or solid water particles: mist and fog, and precipitation. They can greatly reduce visibility.

Lithometeors. Solid material that is not ice, such as volcanic ash.

Fog, mist and the lithometeors Fog and mist are both water droplets in the air: the visibility tells them apart.

FGFog: visibility less than 1 000 m

BRMist: visibility 1 000 to 5 000 m

FUSmoke: small particles produced by combustion

DUDust: small particles of earth or other matter, raised by the wind

SASand: particles of sand, raised by the wind

HZHaze: extremely small, dry particles, invisible to the naked eye

VAVolcanic ash: fine rock powder that can stay in the air for a long time

Haze is dry. That is what separates it from mist, which is water droplets.

Fog	Conditions	How it forms
Radiation fog	Night, little or no cloud, little wind. The night must be long enough.	The land cools down by radiating heat. The moist air near the ground loses heat to the cold ground by conduction and condenses.
Advection fog	Warm, moist air over a cold surface	The warm air loses heat and condenses. Sea fog is one example.
Hill fog or upslope fog	Moist air is forced up by rising ground (orographic lifting)	The air is cooled by expansion and becomes saturated.
Precipitation fog or frontal fog	Rain ahead of a warm front or an occlusion	Some of the falling rain evaporates, and the colder air below the main cloud bank becomes saturated.
Valley fog	–	Cold, dense air settles into the low part of a valley, condenses and forms fog.
Sea smoke	Extremely cold air passes over a warm, moist surface	The two mix, and the air becomes saturated.
Freezing fog	Fog of supercooled water	It forms a layer of ice or rime on impact.

Runway visual range Always in metres. Reported when the visibility or the RVR is less than 1 500 m.

Where and how. Measured at up to three points along the runway: the touchdown zone, the mid-point and the stop end. Each point has a transmissometer or a forward scatter meter.

R26L/0900RVR runway 26L, 900 metres

R01R/P2000RVR runway 01R, more than 2 000 metres

From MET 007, Visibility.

Wind shear, turbulence and icing

The hazards of the wind and of supercooled water: what each is, where it is found, and what it does to an aircraft.

Syllabus level 2explain itMETB 1.2.3, 4.4.1, 4.4.2

Wind shear. A change in wind speed or direction, or both, over a short distance: it is microscale. When severe, it can displace an aircraft from its flight path. Downbursts out of a CB are particularly hazardous.

Turbulence is found	Where and when
Weather fronts	Squall lines and cold fronts.
Thunderstorms	In and near the CB, and in micro- and macrobursts.
Mountains	When the wind blows over them, vertical shear on the lee side: mountain waves.
Inversion layers	Where the temperature rises with height.
Clear air turbulence (CAT)	Turbulence in clear air, so nothing shows where it is.
Wake turbulence	A type of CAT. It comes from aircraft, not from the weather.

Icing Rime ice against clear ice, and how icing is forecast, reported, removed and prevented.

Icing. Supercooled droplets freezing on impact with an aircraft. It occurs when liquid water is at 0 °C or less.

Five effects. Disturbed airflow, more drag, added weight, engine damage in jets and reduced visibility.

	Rime ice	Clear ice
Temperature	Colder than about −15 °C	0 to about −15 °C
Droplets	Small	Large
The ice	Ice with trapped air	Solid and smooth
Forms on	Leading edges and engine intakes	–

Cumuliform cloud. Icing is generally in CB and TCU (towering cumulus). They have large droplets, so ice builds fast.

Stratiform cloud. Usually smaller droplets, but icing in it can still be bad, for example in NS.

Forecast. On the significant weather (SIGWX) chart (also written SWC), and in a SIGMET when it is severe.

Report. In an air-report, AIREP (ARP), or a special air-report, AIREP SPECIAL (ARS).

De-icing. Removal of existing ice, frost and snow.

Anti-icing. A chemical to prevent or delay the formation of ice.

Severity One severity scale for turbulence and icing.

Severity	Turbulence symbol	Icing symbol	In icing, a change of heading or altitude
Light	–	One bar	Not necessary
Moderate	One peaked line	Two bars	Desirable
Severe	Two peaked lines	Three bars	Immediate change necessary
Extreme	–	–	–

The symbols, drawn in MET 008, are also on the significant weather chart. ICAO charts show moderate and severe icing only.

The aircraft type. A pilot who reports turbulence also gives the aircraft type. Turbulence feels worse in a small aircraft, so the type lets others judge how severe it will be for them.

From MET 008, Meteorological hazards.

Thunderstorms

What a thunderstorm needs, its life cycle, and what a CB brings.

Syllabus level 2explain itMETB 4.4.1, 3.3.1

Only a CB makes a thunderstorm (TS). A CB can also hide inside layer cloud: an embedded CB.

Three ingredients	Why it is needed
A lifting force	One of four: a frontal surface (a frontal thunderstorm), differential surface heating (an air mass thunderstorm), convergence, or sloping terrain.
Instability	Unstable air makes the air continue to rise.
Moisture	Condensation releases latent heat. Without enough water vapour, the cloud does not grow.

The life cycle	The air
1 Cumulus	Rises into the cloud from below.
2 Mature	Goes both up and down inside it. The cloud spreads into an anvil at the top.
3 Dissipating	Sinks out of the dying cloud and spreads out at the ground.

What a CB brings	In brief
Downbursts	Air rushes down out of a CB and spreads out at the ground. A microburst is less than 4 km in diameter, a macroburst more than 4 km.
Turbulence and icing	Under Wind shear, turbulence and icing, above.
Gusts, heavy rain and hail	With the thunder and lightning often come gusts and heavy rain, and sometimes hail.
Lightning	A discharge of static electricity. It can damage aircraft.
Funnel cloud (FC)	A violent, rotating column of air extending from a CB.
Tornado	A funnel cloud in contact with the ground.
Waterspout	A funnel cloud in contact with water.

From MET 008, Meteorological hazards.

The local routine report

The local routine report and the special report: the weather at the aerodrome, for the traffic there.

Syllabus level 3apply itMETB 5.1.1

Only used locally. Used only at the aerodrome where the observations are made. Transmitted verbally by radio, or on the ATIS.

Abbreviated plain language. Written in abbreviated plain language, not in a code.

SPECIAL. If the weather changes significantly between two routine reports, a special report is sent.

How often. ICAO: observations every hour, or every half hour by regional agreement. The EU: a METAR every half hour, at aerodromes with scheduled international commercial air transport. Many countries send them at 20 and 50 minutes past the hour. Kastrup and Billund report every half hour.

The report, item by item The items in order: one report from Kastrup, EKCH, written both ways.

Item	MET REPORT	METAR
1Type of report	`MET REPORT`	`METAR`
2Location	`EKCH`	`EKCH`
3Time	`070650Z`	`070650Z`
4Automated or missing reportOnly when it applies: AUTO for a report from an automatic station, NIL for a missing report. This Kastrup report is neither, so the item is left out	–	–
5WindDirection in degrees, speed in knots	`WIND 220/8KT`	`22008KT`
6VisibilityIn metres under 5 km, in kilometres from 5 km. A METAR: always in metres	`VIS 1200M`	`1200`
7RVROnly when the visibility or the RVR is less than 1 500 m. A METAR gives the touchdown zone’s value	`RVR TDZ 1400M MID 1400M END 1300M`	`R22L/1400N`
8Present weather	`BR`	`BR`
9CloudThe amount, then the height of the base: in feet, or in a METAR in hundreds of feet	`CLD BKN 500FT`	`BKN005`
10Temperature and dew pointIn degrees Celsius	`T12 DP11`	`12/11`
11QNHIn a local report, also the QFE, when it is given	`QNH 1012HPA`	`Q1012`
12Supplementary information	`RERA`	`RERA`
TrendAdded at the end, after the report: see The end of a report, below	`TREND NOSIG`	`NOSIG`

More local report groups

Wind

WIND 240/5KT VRB BTN 220/ AND 350/240°, 5 kt, varying between 220° and 350°The direction varies by 60° or more but less than 180°, at 3 kt or more: the mean and the two extremes
WIND VRB BTN 350/ AND 050/2KTVarying between 350° and 050°, 2 ktThe same, under 3 kt: the two extremes, without a mean direction
WIND VRB2KTVariable, 2 ktThe direction varies by 180° or more: VRB alone
WIND CALMCalmLess than 1 kt
WIND 310/15KT MAX35 MNM9310°, 15 kt, at most 35 kt, at least 9 ktA wind that is not constant

Visibility and RVR

VIS 5KM5 kmFrom 5 km: in kilometres. Under 5 km: in metres
RVR TDZ 400M MID 600M END 300MTouchdown zone 400 m, mid-point 600 m, stop end 300 mUsually all three parts of the runway
RVR 800M800 m at the touchdown zoneSometimes only the touchdown zone

Cloud

CLD BKN TCU 4000FTBroken towering cumulus, base 4 000 ftTCU and CB are the only cloud types named
CLD FEW CB 2000FTFew cumulonimbus, base 2 000 ft

Temperature and dew point

TMS02 DPMS02Temperature −2 °C, dew point −2 °CMS: minus

Reading it aloud How the numbers are said, and which words may be left out.

Numbers. Single digits, but whole hundreds and thousands for altitude, cloud height, visibility and RVR.

May be left out. If no confusion results: “surface” with the wind direction and speed; “visibility”, “cloud” and “height”; “hectopascal”.

How it starts. The report and the place, then the time. The date is left out.

Report	Said
`MET REPORT EKCH`	“MET REPORT KASTRUP”
`070650Z`	“TIME ZERO SIX FIVE ZERO” The date, the 7th, is left out
`WIND 220/8KT`	“WIND TWO TWO ZERO DEGREES EIGHT KNOTS” “Surface” is left out
`VIS 1200M`	“ONE THOUSAND TWO HUNDRED METRES” “Visibility” is left out
`RVR TDZ 1400M MID 1400M END 1300M`	“RVR TOUCHDOWN ONE THOUSAND FOUR HUNDRED METRES, MIDPOINT ONE THOUSAND FOUR HUNDRED METRES, STOP END ONE THOUSAND THREE HUNDRED METRES”
`BR`	“MIST”
`CLD BKN 500FT`	“BROKEN FIVE HUNDRED FEET” “Cloud” and “height” are left out
`T12 DP11`	“TEMPERATURE ONE TWO, DEW POINT ONE ONE”
`QNH 1012HPA`	“QNH ONE ZERO ONE TWO” “Hectopascal” is left out
`RERA`	“RECENT RAIN”

The trend at the end, TREND NOSIG, is read as it stands.

From MET 009, Application of meteorological information (the local routine report, the report item by item, and reading a report aloud).

METAR and SPECI

The coded report: the same weather as the local report, for use beyond the aerodrome.

Syllabus level 3apply itMETB 5.1.1

The name. METAR is ICAO’s aerodrome routine meteorological report. The course spells the letters out as “meteorological aviation routine weather report”.

Content. The same items as the MET REPORT, but coded.

SPECI. If the weather changes significantly between two reports, a SPECI is sent.

	MET REPORT and SPECIAL	METAR and SPECI
Where and what for	At the aerodrome only, for take-off and landing	The aerodrome and its vicinity, within 16 km, to plan a flight
Written in	Abbreviated plain language	A coded format
Sent	By radio and on the ATIS	On the AFTN

The METAR groups, in the order of the report

Wind

00000KTCalm
10017G30KT100°, 17 kt, gusts 30 ktG: gusts
170P99KT170°, 100 kt or moreP99: more than 99 kt
35075GP99KT350°, 75 kt, gusts 100 kt or more
34016G29KT 310V030340°, 16 kt, gusts 29 kt, direction varying between 310° and 030°

Visibility

999910 km or more
3800 1700SEPrevailing 3 800 m, lowest 1 700 m to the south-east
9999NDV10 km or more, no directional variations availableNDV: added by an automatic station

RVR

R19/0350V0600RVR runway 19: between 350 and 600 m
R04L/0500 R04R/0650RVR runway 04L: 500 m, runway 04R: 650 mTwo parallel runways, a group each

RVR tendency

R01L/0750NRVR runway 01L: 750 m, no changeN: no change
R19R/P1500URVR runway 19R: more than 1 500 m, going upP: more than; U: upward
R26/M0050DRVR runway 26: less than 50 m, going downM: less than; D: downward

Weather: descriptor

MIShallowLess than 2 m above the ground
BCPatchesFog in patches, here and there over the aerodrome
PRPartialFog over a substantial part of the aerodrome, the rest clear
DRLow driftingDust, sand or snow raised by the wind to less than 2 m
BLBlowingRaised by the wind to 2 m or more
SHShowers
TSThunderstorm
FZFreezing

Weather: other

UPUnknown precipitationAutomatic stations only: the type could not be identified
PODust or sand whirls
SQSquall
FCFunnel cloud
DSDust storm
SSSandstorm

Cloud: amount and base, in hundreds of feet

FEW025Few, base 2 500 ft
SCT040Scattered, base 4 000 ft
BKN012Broken, base 1 200 ft
OVC008Overcast, base 800 ft

Cloud: type

BKN040TCUBroken towering cumulus, base 4 000 ftTCU or CB follows the group: the only cloud types named
FEW020CBFew cumulonimbus, base 2 000 ft

Temperature, dew point and QNH

M10−10 °CM: minus
07/M05Temperature 7 °C, dew point −5 °CThe temperature, then the dew point
Q0987QNH 987 hPa

Missing data

240//KTWind 240°, speed missingA slash for each missing figure
////Visibility missing
///05Temperature missing, dew point 5 °C

The precipitation, intensity and obscuration codes are under Precipitation and under Visibility, fog and mist, above; the cloud amounts, NSC, NCD and CAVOK under Clouds, cover and ceiling.

The end of a report After its items, a report can end with supplementary information and a TREND.

TREND. Trend forecast: for the next 2 hours from the time of the report. Not a report of its own: part of the report it ends, a MET REPORT or a METAR.

Where it goes. At the end of the report: after the word TREND in a MET REPORT; without it in a METAR, where the change group (NOSIG, BECMG or TEMPO) follows the report straight away.

Supplementary information and the TREND’s groups

Recent weather

RERARecent rainRE: since the last report, but not now. With reports every half hour, that is the last 30 minutes. One group, in a local report and a METAR
RETSRARecent thunderstorm with rain

Wind shear

WS R27Wind shear, runway 27In a METAR. A group of its own, not a remark
WS ALL RWYWind shear on all runwaysIn a METAR

The sea

W09/S5Sea surface temperature 9 °C, state of the sea 5W: sea surface temperature; S: state of the sea. From offshore stations only

Change groups

NOSIGNo significant change
BECMGBecoming: changes and stays
TEMPOTemporary: changes, then goes back

Times

AT1130At 11:30AT: at. After BECMG only
TL1200Until 12:00TL: till. After BECMG or TEMPO
FM1100From 11:00FM: from. After BECMG or TEMPO

For example, TREND BECMG AT1130 CLD OVC 1000FT: at 11:30 the cloud becomes overcast at 1 000 ft, and stays so.

From MET 009, Application of meteorological information (METAR and SPECI, and the end of a report), with the weather codes from MET 007, Visibility.

TAF

The forecast for an aerodrome, and the change groups it uses.

Syllabus level 3apply itMETB 5.1.1

TAF. Terminal aerodrome forecast: the expected meteorological conditions at an aerodrome, written in code, as a METAR is. Valid for 9, 24 or 30 hours.

TAF AMD. An amended TAF. It is issued if the expected weather changes significantly.

The TAF’s own change groups

Change groups

FMFrom: new weather in the forecast, from that time
PROBProbability of the weather named

A TAF uses BECMG and TEMPO as a TREND does, but each is followed by its period in days and hours: BECMG 0710/0712. FM, then the day, hour and minutes, starts new weather from that time and replaces everything before it. PROB gives the probability of other weather. A TAF has no NOSIG, AT or TL. In a TREND, FM only times a BECMG or TEMPO change, and PROB is never used.

A TAF for Kastrup, line by line	Read
`TAF EKCH 070500Z 0706/0806 24012KT 9999 SCT030`	Kastrup, issued on the 7th at 05:00 UTC, valid from 06:00 on the 7th to 06:00 on the 8th: wind 240° at 12 kt, visibility 10 km or more, scattered cloud at 3 000 ft.
`BECMG 0710/0712 27022KT`	Between 10:00 and 12:00 the wind becomes 270° at 22 kt, and stays so.
`TEMPO 0714/0718 4000 SHRA BKN014`	Between 14:00 and 18:00, for a while, the visibility drops to 4 000 m in showers of rain, with broken cloud at 1 400 ft; then it goes back.
`FM072200 31008KT 9999 FEW020`	From 22:00 there is new weather: wind 310° at 8 kt, visibility 10 km or more, a few clouds at 2 000 ft.

Each change group is printed on a line of its own.

From MET 009, Application of meteorological information.

SIGMET and other en-route messages

Significant meteorological information, and the other messages on the weather en route.

Syllabus level 3apply itMETB 5.1.1

En-route weather. A SIGMET describes the occurrence, or the expected occurrence, of en-route weather: OBS observed, FCST forecast.

Cancelled. A SIGMET is cancelled as soon as the weather has ended.

Message	From	What it is	Limits
`SIGMET`	The meteorological watch office	Significant meteorological information. En-route weather, observed or forecast, for an FIR.	Valid no longer than 4 hours; 6 hours for volcanic ash or a tropical cyclone.
`AIRMET`	The meteorological watch office (MET 001)	Airmen’s meteorological information. En-route weather weaker than a SIGMET needs. It is information, not a warning.	Flights below FL 100. Denmark issues none (MET 001).
`ARS`	An aircraft in flight	Special air-report. A meteorological report, made when the aircraft meets certain weather, such as severe icing (MET 008).	–
`GAMET`	–	Area forecast for low-level flights.	Denmark issues none.

A SIGMET, group by group	Meaning
`EKDK`	FIR indicator. The København FIR
`SIGMET`	Message type. Significant meteorological information
`I01`	Sequence number. Icing, number 1
`VALID 150900/151300`	Validity. The 15th, 09:00 to 13:00 UTC
`EKMI-`	Location indicator of the MWO. The Danish Meteorological Institute, in København
`EKDK KOBENHAVN FIR`	FIR. The København FIR
`SEV ICE`	Phenomenon. Severe icing
`OBS AT 0840Z`	Observed or forecast, and the time. Observed at 08:40 UTC, and expected to continue
`WI N5700 E00830 - N5700 E01000 - N5500 E01000 -`	Position: the area. Within a box over Jutland, 57°N to 55°N, 8°30′E to 10°E
`N5500 E00830 - N5700 E00830`	Position: the area, closed. The last corner, 55°N 8°30′E, then the first again
`FL060/140`	Level. FL 60 to FL 140
`MOV E 10KT`	Movement. Moving east at 10 kt
`NC=`	Change in intensity. No change

From MET 009, Application of meteorological information, with MET 001, Introduction to meteorology.

Weather charts

How to read the upper wind and temperature chart and the significant weather chart. The drawn charts and their symbols are on the lesson.

Syllabus level 3apply itEPN objective: decode and apply information from the most commonly used weather charts

The upper wind and temperature chart The forecast wind and temperature at one flight level, as an arrow and a figure at each point of a grid.

The arrow. Runs out from the grid point towards where the wind comes from. The barbs are at its outer end.

The speed. Add up the barbs: a half barb is 5 kt, a full barb 10 kt and a pennant 50 kt. An arrow with no barbs is calm.

The temperature. Read as minus, unless it is marked PS, for plus: 04 is minus 4 °C, PS03 is plus 3 °C.

CalmNo barbs

5 ktA half barb

10 ktA full barb

50 ktA pennant

65 kt50 + 10 + 5

045/10A north-east wind at 10 knots, from 045°

04Minus 4 °C

PS03Plus 3 °C

The significant weather (SIGWX) chart Issued by the world area forecast centres: the weather that matters most to air transport.

On the chart	How it reads
Jet stream	A bold arrow along its core, in the direction of the wind. Its barbs give the speed, added up as on the upper wind chart, and its flight level is printed beside it: for example 110 kt at FL 340.
Clear air turbulence (CAT)	Dashed lines enclose areas of similar turbulence. A number in a box points to the key, which gives the intensity, then the top over the base: 370/290 is from FL 290 to FL 370. XXX as the top means above the chart.
Tropopause heights	Flight levels in boxes. A box pointing up, marked H, is a high of the tropopause; a box pointing down, marked L, is a low.
CB areas	Scalloped lines ring an area of cloud weather, with a label such as ISOL EMBD CB 380/XXX: isolated CB, embedded in layer cloud, top FL 380, base below the chart. The course writes EMB for embedded.
Fronts	The four fronts at the surface: cold, warm, occluded and quasi-stationary.
Volcanoes	An active volcano is shown with its name and position. The latest SIGMET gives more.

The course teaches this form. Since January 2025 the world area forecast centres’ charts reach FL 600, show no embedded CB and no CB base, draw the tropopause as contours and mark turbulence areas; surface fronts are drawn on charts that start at the surface.

On the chart, a CB implies thunderstorm, hail, and moderate or severe turbulence and icing.

The symbols, drawn in MET 009: moderate and severe icing, moderate and severe turbulence, thunderstorms, freezing precipitation; drizzle, rain, snow, shower and hail; widespread mist, fog and blowing snow; and the four fronts at the surface.

From MET 009, Application of meteorological information.

Also on the syllabus

The SNOWTAM and three services that broadcast information to aircraft, one line each, and why meteorology matters to aviation.

Syllabus level 1know itMETB 1.2.1, ATMB 1.3.5

Name	What it is
`SNOWTAM`	Special series NOTAM on snow, ice, slush, frost or standing water. Read and decoded in LAW 006.
`ATIS`	Automatic terminal information service. One way a local report reaches aircraft. A transition level can come by ATIS, too (ATM 002).
`VOLMET`	Meteorological information for aircraft in flight.
`OFIS`	Operational flight information service. Sent out as broadcasts.

Why MET matters. The objective of ICAO Annex 3, Meteorological Service for International Air Navigation: the safety, regularity and efficiency of international air navigation.

From MET 009, Application of meteorological information, and MET 001, Introduction to meteorology (Annex 3).