Skip to content
FIS Book
MET

MeteorologyLesson 4 of 4

Back to subjects
MET 004

Clouds

What a cloud is

Visible water, droplets or ice crystals, suspended in the earth’s atmosphere. Different atmospheric conditions produce different types of cloud, classified according to height, shape and behaviour.

Drawn to scaleMagnified 100 timesA cloud dropletCondensation nucleusabout 0.0002 mmRaindrop, about 2 mmCloud droplet, about 0.02 mm
A raindrop is about a hundred cloud droplets across, so it takes about a million of them to make one.
Small and far apartCloud droplets are very small, and mostly relatively far apart.
Kept aloftThey fall very slowly, because of friction with the air, and air currents keep them aloft.
A surface to form onWater vapour needs a non-gaseous surface to change state on. Condensation nuclei are that surface.
Condensation nuclei+Saturated air=Cloud

How air becomes saturated

Cloud forms when air is saturated. There are three ways to get it there, and the third needs the air to be lifted.

Way 1Adding moisture

Daytime heating causes evaporation from water.

Precipitation, too, evaporating as it falls. Virga is a streak of precipitation not reaching the ground.

Way 2Cooling from below

Warm air moving over a cold surface: advection.

Way 3Adiabatic cooling

Air that is lifted expands and cools until it is saturated. Five things lift it, below, and the next section shows why lifting cools it.

What lifts the air
ConvectionFree

Uneven heating of the earth’s surface causes warmer air to start rising. The air eventually becomes saturated, and forms clouds.

Frontal lifting

Warm air forced up because of underlying colder air, as at a warm front.

Orographic upliftForced

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.

Why rising air cools

Adiabatic cooling, step by step: nothing takes heat away from the air, and yet it cools.

1 000 ft2 000 ft3 000 ft4 000 ft5 000 ft6 000 ft20 °CSurface17°14°11°9.5°8°6.5°Expandingcools itDALR3 °C per 1 000 ftwhile unsaturatedSALRAbout 1.5 °C per 1 000 ftonce saturatedLifting condensation levelDew point: cloud base
A parcel of air rising from warm ground, drawn every 1 000 ft, and larger as it expands.
  1. The ground heats unevenlyA patch that warms faster warms the air touching it, by conduction (MET 003).
  2. That air rises on its ownIt is lighter than the air around it: convection.
  3. The pressure around it fallsPressure always decreases with height (MET 002), so the rising air expands.
  4. Expanding cools itThe key stepTo expand, the air has to push the air around it out of the way. The energy for that comes from its own heat: its molecules slow down, and its temperature falls. No heat leaves it, which is what adiabatic means.
  5. It cools at the DALR3 °C per 1 000 ft while it is unsaturated: 20, 17, 14, 11 °C.
  6. It reaches its dew pointHere at 3 000 ft, the lifting condensation level. The water vapour condenses on condensation nuclei, and the cloud base forms.
  7. Above it, it cools more slowlyCondensing releases latent heat, which makes up part of the cooling: about 1.5 °C per 1 000 ft, the SALR. 9.5, 8, 6.5 °C.

It works the same way in reverse: sinking air is compressed as the pressure rises, and warms.

The föhn wind

Moist air forced over a mountain arrives on the far side warmer than it started.

1 000 ft2 000 ft3 000 ft4 000 ft10 °C8.5 °C7 °C5.5 °C8.5 °C11.5 °C14.5 °C13 °CDew point10 °C17.5 °CWindward sideLee sideDALR, 3 °C per 1 000 ftSALR, 1.5 °C per 1 000 ft
Up the windward side it cools at the DALR to its dew point, then at the slower SALR in cloud. Down the lee side there is no cloud left, so it warms at the DALR all the way, and arrives 4.5 °C warmer.

Lapse rates

How fast temperature falls with height. Two are adiabatic, for air that is rising or sinking; the third is the air around it.

DALRDry adiabatic lapse rate
3 °Cper 1 000 ft
  • Unsaturated air.
  • Cooling or warming at this rate.
  • Applies to rising or subsiding air.
SALRSaturated adiabatic lapse rate
1.5 °Cper 1 000 ft, on average
  • Saturated air: condensation occurs.
  • Slower, because of the release of latent heat.
  • Varies strongly with temperature and pressure.
  • Applies to rising or subsiding air.
ELREnvironmental lapse rate
2 °Cper 1 000 ft, on average
  • The actual change of temperature with altitude.
  • Applies to still air.
  • Can even be a temperature increase with height: an inversion.
  • The ISA’s is 1.98 °C (MET 002).

Stability

How an air mass reacts to vertical movement.

StableUnstableNeutralPushed backPushed awayFinds a newresting place
A ball nudged in a bowl, on a hilltop and on the flat.
StableHas a restoring force, which tends to return it to its original position.
UnstableHas a force that moves it farther away from its original position.
NeutralWhen moved, it finds a new resting place.

Stable and unstable air

Lift a parcel 2 000 ft, and compare. Unsaturated air cools at the DALR, saturated air at the SALR, and the air around it follows the ELR. At the top, a parcel colder than the air around it sinks back; a warmer one keeps rising.

2 000 ft1 000 ftSurface Stable air ELR 1 °C per 1 000 ft 20°19°18° 20.5 14.5 Sinks Unsat 20.5 17.5 Sinks Sat Unstable air ELR 4 °C per 1 000 ft 20°16°12° 22 16 Rises Unsat 22 19 Rises Sat Conditionalinstability ELR 2 °C per 1 000 ft 20°18°16° 20 14 Sinks Unsat 20 17 Rises Sat
All in °C. Stable: the air around cools more slowly than either parcel. Unstable: faster than either. Conditional: in between.
Stable airAny displacement of either unsaturated or saturated air will cause the air to return to its original position.
Unstable airAny displacement of either unsaturated or saturated air will cause the air to continue to rise.
Conditional instabilityStable for unsaturated air, unstable for saturated air. The ELR lies between the SALR and the DALR.
Which cloud forms
Stable airStratiformCloud in layers. The air does not rise on its own, so the cloud spreads out rather than up.
Unstable airCumuliformHeaped cloud, with vertical development. Lifted air keeps rising on its own, so the cloud builds upwards.
Try it: drag the ELR
Try
The ELR2 °C per 1 000 ft
The actual change of temperature with height. Below zero, it warms with height.
Conditionally unstableStable for unsaturated air, unstable for saturated air. The ELR lies between the SALR and the DALR.
  • Unsaturated parcel, cooling at the DALR: 14 °C at 2 000 ft, in air of 16 °C. Colder: it sinks back.
  • Saturated parcel, cooling at the SALR: 17 °C at 2 000 ft, in air of 16 °C. Warmer: it keeps rising.
2 000 ft1 000 ftSurface10°15°20°25°14°17°Air 16°
DALR, unsaturatedSALR, saturatedELR, the air around
Where the ELR sits decides it
StableConditionally unstableConditionalUnstable
ELR 2 °C SALR 1.5DALR 3

An inversion, below zero, is off the left end, and more stable still.

Cloud forms

Four forms, by shape and behaviour.

CumuliformDetached clouds with convective development.
StratiformLayered clouds, usually broad and widespread.
CirriformHigh, wispy clouds composed of ice crystals.
NimboformClouds that produce precipitation.

Cloud types by height

Where each type forms, and what to expect in it.

CICirrusCSCirrostratusCCCirrocumulusASAltostratusACAltocumulusSTStratusSCStratocumulusCUCumulusNSNimbostratusCBCumulonimbusHigh16 500 to45 000 ftMiddle6 500 to23 000 ftLowSurface to6 500 ftCirrusWispyStratusLayeredCumulusHeapedVertical developmentBase: ground to 10 000 ft
Read down a column for the shape in the name, across a row for the height. Not to scale: middle and high clouds overlap between 16 500 and 23 000 ft.
High clouds16 500 to 45 000 ft
CICirrusCCCirrocumulusCSCirrostratus
PrecipitationNoneIcingNoneTurbulenceNone or light
  • Entirely ice crystals.
  • A thin layer.
  • CI and CS are common with an advancing warm front.
  • The heights are for temperate regions.
Middle clouds6 500 to 23 000 ft
ACAltocumulusASAltostratus
IcingLight to moderateTurbulenceLight to moderate
  • Ice crystals and/or water droplets, which can be supercooled.
  • Usually no great vertical extent, but thicker than high clouds.
Low cloudsSurface to 6 500 ft
SCStratocumulusSTStratus
  • Water droplets, ice crystals and/or snowflakes.
Clouds with vertical developmentBase: ground to 10 000 ft
CBCumulonimbusNSNimbostratusCUCumulus
  • Mainly water droplets, but ice crystals may form.
  • CB is the only cloud to produce thunder, lightning or hail.

The letters are ICAO’s, from its abbreviations, Doc 8400, and CB is how a METAR or TAF writes it. Meteorology texts write the same letters as Ci, Cc, Cb and so on.

Other clouds
Lens-shapedLenticularis

Lens-shaped clouds.

Thin and icyNoctilucent clouds

Thin, icy clouds, at 76 to 85 km. Also in MET 002, among the layers.

Cloud base and ceiling

Two heights that are easily mixed up, and two codes for no cloud.

20 000 ftBKNFEWCloud baseCeiling
Every cloud has a base. The ceiling is the base of the lowest layer covering more than half the sky: 5 oktas or more, BKN or OVC.
Cloud baseThe lowest height of the visible portion of a cloud. Also in MET 001.
Cloud ceilingThe 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. The sky is 8 oktas, so half is 4, and 5 is already more than half.

NCDNo cloud detected
NSCNil significant cloud

Reading material

Meteorology

MET 004 Clouds PowerPoint / PDF
EPN Group Training Document Meteorology, chapter 4 Reading list, course material
Ahrens, C. Donald, Meteorology Today: An Introduction to Weather, Climate, and the Environment, 8th edition, 2007 Reference, paid
WMO-No. 731, Guide to Meteorological Observing and Information Distribution Systems for Aviation Weather Services, 2014 edition Reference, free
European Union Aviation Safety Agency, Easy Access Rules for Air Traffic Management/Air Navigation Services (Regulation (EU) 2017/373), March 2025 Reference, free