The 10 cloud types
Every cloud in the sky belongs to one of ten genera defined by the World Meteorological Organization. They are grouped by the altitude at which they form. Pick one to learn how to recognise it.
High clouds
Above roughly 6,000 m. Made almost entirely of ice crystals, thin and often translucent.
Cirrus
Cirrus are delicate white filaments of ice crystals drawn across the highest part of the troposphere. They are the thinnest clouds in the sky — never dense enough to hide the sun — and their strands are shaped by the wind carrying them.
Cirrocumulus
Cirrocumulus is a high sheet of very small white grains or ripples, arranged in regular rows like the scales of a fish. It is the rarest and shortest-lived of the high clouds, and the only one that shows any granular structure.
Cirrostratus
Cirrostratus is a transparent whitish veil of ice crystals covering all or most of the sky. It is so thin that the sun still casts sharp shadows through it, and it is the cloud responsible for the 22° halo ring.
Middle clouds
Roughly 2,000–7,000 m. A mix of water droplets and ice crystals.
Altocumulus
Altocumulus is a mid-level layer of white or grey rounded masses, rolls or tiles, usually with darker shading on the underside of each element. It is one of the most common clouds in the sky and one of the most varied in appearance.
Altostratus
Altostratus is a uniform greyish or bluish sheet covering the whole sky at mid level. The sun shows through it as a dim, watery disc with no sharp edge, and objects on the ground stop casting shadows.
Nimbostratus
Nimbostratus is a thick, dark grey, shapeless layer that blots out the sun entirely and produces continuous rain or snow. It is the classic all-day rain cloud, and the only genus that is defined by the precipitation it delivers.
Low clouds
Below roughly 2,000 m. Made of water droplets, often grey and featureless.
Stratocumulus
Stratocumulus is a low, lumpy layer of large grey and white rolls or patches, usually with gaps of brighter sky between them. It covers more of the Earth's surface than any other cloud type and rarely produces more than drizzle.
Stratus
Stratus is a uniform, low, light grey layer with no visible structure — essentially a sheet of fog that formed above the ground rather than at it. It is the lowest of all cloud genera and produces at most drizzle.
Clouds with vertical development
Clouds that build upward through several altitude bands, from a low base to great height.
Cumulus
Cumulus are detached, brilliant white heaps with sharp cauliflower-like tops and distinctly flat bases. They are the clouds children draw, the classic sign of a pleasant day — and the seed from which thunderstorms grow.
Cumulonimbus
Cumulonimbus is a vast, dense tower that reaches from a low base to the top of the troposphere, usually crowned with a flattened anvil of ice. It is the only cloud that produces lightning, and the source of hail, downbursts and tornadoes.
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