In brief: Meteorologists recognise ten main cloud types, called genera. High clouds include cirrus, cirrocumulus and cirrostratus. Middle-level clouds include altocumulus, altostratus and nimbostratus. Low-based clouds include stratus, stratocumulus, cumulus and cumulonimbus. Their shapes, heights and development can help identify fair-weather conditions, approaching fronts, widespread rain and developing showers. A cloud’s name alone does not provide a guaranteed forecast.
The ten types of clouds at a glance
The World Meteorological Organization (WMO) uses ten cloud genera in its International Cloud Atlas. The familiar division into high, middle and low clouds describes their typical levels, but several types extend through more than one level. Cumulus and cumulonimbus may have low bases and tops that reach high into the troposphere.
| Cloud type | Typical appearance | Usual weather association |
|---|---|---|
| Cirrus (Ci) | Separate wisps, feathers or hair-like fibres | Often fair at ground level; sometimes an early sign of frontal change |
| Cirrocumulus (Cc) | Fine, small white grains or ripples | High-level moisture; precipitation rarely reaches the ground |
| Cirrostratus (Cs) | Transparent, widespread veil, sometimes with a halo | Can precede a warm front and later precipitation |
| Altocumulus (Ac) | Mid-level patches, rolls or rounded cloudlets | Variable; some forms indicate instability aloft |
| Altostratus (As) | Extensive grey or bluish sheet | Often associated with an approaching or present frontal system |
| Nimbostratus (Ns) | Thick, extensive layer obscuring the Sun | Sustained rain or snow is common |
| Stratocumulus (Sc) | Low, broad rolls or rounded masses with gaps | Often dry, sometimes drizzle or light precipitation |
| Stratus (St) | Low, uniform grey sheet | Cloudy weather, mist-like conditions or drizzle |
| Cumulus (Cu) | Individual heaps with flatter bases and rounded tops | Shallow forms commonly fair; taller forms can bring showers |
| Cumulonimbus (Cb) | Deep towers, often with an anvil top | Heavy showers, thunderstorms, hail and strong gusts possible |
Why clouds have different shapes
Clouds become visible when atmospheric water vapour condenses into droplets or forms ice crystals. The shape of the cloud depends on how air is lifted, how much moisture is available, the temperature profile and the movement of air at different heights.
Uniform lifting across a broad area, such as near a warm front, often creates extensive layers of cloud. Strong localised upward currents caused by daytime heating or instability favour heap-shaped cumulus and cumulonimbus clouds. Wind and turbulence can arrange cloud into streaks, ripples or rolls. A cloud’s visible form therefore gives clues about the processes taking place, although different processes can produce similar-looking clouds.
High clouds: cirrus, cirrocumulus and cirrostratus
In temperate regions, the WMO places the usual high-cloud level at approximately 5–13 km above the Earth’s surface. This is a broad classification range, not a fixed base for every cloud.
Cirrus: wisps, feathers and hooks
Cirrus consists mainly of ice crystals and appears as detached, white, fibrous strands. Some strands are nearly straight; others curve into hooks or tails. Cirrus can occur in stable weather, but an expanding field of cirrus may be one of the first visible signs that a weather system is approaching.
Cirrocumulus: fine white ripples
Cirrocumulus is made up of numerous very small cloud elements, commonly appearing as grains or ripples high in the sky. From the ground the elements usually show little or no shading. The pattern is sometimes called a mackerel sky, although that informal term is also applied to some altocumulus patterns. Cirrocumulus should not be treated as a dependable stand-alone rain forecast.
Cirrostratus: a transparent high veil
Cirrostratus forms a thin, extensive veil of ice-cloud through which the Sun or Moon is normally visible. A halo around either light source is a useful identification clue. Cirrostratus can develop ahead of a warm front; if the veil grows denser and is followed by lower, thicker cloud, precipitation may become more likely.
Middle clouds: altocumulus, altostratus and nimbostratus
The WMO describes the usual middle-cloud level in temperate regions as roughly 2–7 km. These clouds are often made of liquid water droplets, ice crystals or a mixture. Nimbostratus commonly extends beyond the middle level.
Altocumulus: patches, rolls and tufts
Altocumulus contains rounded cloudlets, patches or layers with some grey shading. Its elements usually appear larger than those of cirrocumulus. A widespread layer may indicate moisture at middle levels without promising rain. Vertically developed, turreted forms such as altocumulus castellanus can indicate instability at their level, which may be relevant to later convective development.
Altostratus: a broad, grey sheet
Altostratus typically covers a substantial area of sky. The Sun may appear as a dim disc, as though viewed through frosted glass; sharper shadows are usually absent. Thickening altostratus can precede nimbostratus in a frontal cloud sequence. Unlike an individual passing shower, this development often reflects uplift over a broad area.
Nimbostratus: widespread precipitation cloud
Nimbostratus is an extensive, often dark cloud layer associated with continuous rain or snow. Its base may appear diffuse because precipitation is falling beneath it, and the Sun is generally obscured. It can cover a large area and produce hours of wet weather. Thunder, anvil-shaped towers and sharply localised heavy downpours are more characteristic of cumulonimbus than nimbostratus.
Low clouds: stratus, stratocumulus, cumulus and cumulonimbus
The usual low-cloud level extends from the ground to about 2 km in the WMO’s temperate-region classification. The bases of vertically developed cumulus and cumulonimbus often belong to this level even when their upper parts are much higher.
Stratus: an almost featureless grey layer
Stratus resembles fog that is lifted above the surface. It can form a continuous low sheet, sometimes producing drizzle. Its base may be close to the ground, particularly in damp, stable conditions. A stratus-covered sky does not necessarily mean substantial rainfall is approaching.
Stratocumulus: low rolls or lumpy layers
Stratocumulus appears as broad, rounded masses or rolls, often joined into extensive fields with occasional openings. It is commonly dry but can produce light precipitation. Its relatively large individual elements and low base help distinguish it from altocumulus.
Cumulus: separate heaps with flat bases
Cumulus typically has a comparatively flat base and rounded, cauliflower-like tops. Shallow cumulus humilis often develops in fair daytime weather. Cumulus mediocris is more vertically developed. Cumulus congestus grows into substantial towers and can produce showers. Cloud growth should be watched over time rather than judged from a single glimpse.
Cumulonimbus: deep storm cloud
Cumulonimbus is a very large cloud with powerful vertical development. The top may become fibrous and spread into an anvil as the rising air reaches a stable layer near the tropopause. Cumulonimbus can produce intense rain, snow, hail, lightning, thunder and strong downdraughts. It may be embedded within broader cloud cover and therefore difficult to recognise from below.
How to distinguish commonly confused clouds
Cirrus versus cirrostratus: cirrus consists of separate wisps or narrow bands; cirrostratus generally makes an extensive veil. A prominent halo is more characteristic of cirrostratus.
Cirrocumulus versus altocumulus: both can form a rippled sky. Cirrocumulus elements normally appear much finer and lack obvious shading, while altocumulus elements look larger and often have grey portions. Apparent size is affected by perspective and is not infallible.
Altostratus versus nimbostratus: altostratus may let a dim Sun remain visible, whereas nimbostratus usually obscures the Sun and produces widespread precipitation. Thickening altostratus can develop into nimbostratus.
Stratus versus stratocumulus: stratus has little visible structure and appears as a uniform sheet. Stratocumulus displays distinct lumps, rolls or rounded elements.
Cumulus versus cumulonimbus: an ordinary cumulus cloud has rounded, sharply outlined tops. Mature cumulonimbus reaches great heights and frequently develops a fibrous or anvil-shaped upper portion; thunder or lightning definitively indicates a thunderstorm cloud in the vicinity.
Which clouds indicate rain?
The strongest common clues are nimbostratus bringing widespread precipitation and cumulonimbus bringing showers or thunderstorms. Tall cumulus congestus may precede local showers. Altostratus thickening beneath an expanding cirrostratus layer can suggest an approaching front. Thin cirrus, shallow cumulus or a field of stratocumulus cannot reliably determine whether rain will occur at your particular location.
The sequence of cloud changes is often more revealing than a single cloud name. A transition from high, thin cloud to widespread thick layers suggests a different weather process from isolated clouds growing upward into towers. Wind, pressure, radar and current forecasts help distinguish the possibilities.
How to observe cloud types accurately
Begin with the overall shape. Is the cloud a layer, a sheet of rounded elements, isolated heaps or thin fibres? Then estimate whether it appears high, middle or low. Look for diagnostic features: a halo, a flat base, sharply defined cauliflower tops, a fibrous anvil or rain visibly falling beneath a cloud.
Observe again after 30–60 minutes. Note changes in coverage, height and vertical growth. Avoid estimating cloud height from colour alone; a dark cloud may simply be optically thick or poorly illuminated. Cloud colours change considerably with sunlight and viewing angle.
Cloud classification is descriptive, and some observations are genuinely ambiguous. The WMO’s International Cloud Atlas contains formal definitions, identification comparisons and photographic examples.
Related guides
How to Predict the Weather: A Complete Beginner’s Guide