Cloud Types Explained: The Core Four and the Ten Basic Clouds by Height
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How we wrote this: A plain-English summary of NOAA's JetStream online weather school pages on clouds, which draw on the World Meteorological Organization's International Cloud Atlas, checked October 29, 2026.
Learning a handful of names makes the sky much easier to read. NOAA's JetStream weather school starts with four core forms, then builds them into ten basic cloud types sorted by height.
The core four
| Form | Name means | What it looks like |
|---|---|---|
| Cirro-form | "curl of hair" | High, whitish, wispy, made of ice crystals. Often the first clouds ahead of a storm system |
| Cumulo-form | heap | Detached "cotton ball" clouds with flat bases and sharp outlines, showing rising air |
| Strato-form | layer | Broad, blanket-like sheets with diffuse edges, from non-convective rising air |
| Nimbo-form | rain | The rain or snow-producing clouds |
JetStream explains that 19th-century naturalist Luke Howard noticed many clouds combine features of two forms. Those combinations became the basis of the ten basic types below, defined in the WMO International Cloud Atlas.
High-level clouds
Typically thin and white, but brightly colored when the sun is low.
- Cirrus (Ci): delicate white filaments in patches or bands, with a hair-like or silky look. Always ice crystals.
- Cirrocumulus (Cc): thin, white patches arranged in small grains or ripples, without shading.
- Cirrostratus (Cs): a transparent, whitish veil that can cover the whole sky.
Mid-level clouds
Mostly water droplets, though ice when cold enough. JetStream notes that "alto" means high, yet these are mid-level: the name separates them from their low-level cousins.
- Altocumulus (Ac): white or gray patches, sheets or layers of rounded masses or rolls.
- Altostratus (As): gray or bluish sheets that let the sun show through as if through ground glass. Streaks of rain (virga) may hang below.
- Nimbostratus (Ns): thickened altostratus. A dark gray layer of falling rain or snow, thick enough to blot out the sun.
Low-level clouds
Composed of water droplets.
- Cumulus (Cu): dense, detached clouds rising in mounds or towers, often like a cauliflower.
- Stratocumulus (Sc): gray or whitish patches or layers, usually with a dark honeycomb pattern or rolls.
- Stratus (St): a uniform gray layer that may produce drizzle or snow grains. The sun's outline is clear through it.
- Cumulonimbus (Cb): the thunderstorm cloud. A heavy tower that reaches into the high levels, usually with a flattened anvil top. JetStream notes it can also produce hail and tornadoes.
Why it matters
Cirrus thickening into cirrostratus and then altostratus often means a storm system is approaching. A cumulus tower growing into cumulonimbus means lightning is possible: see our beach weather and lightning safety guide. Clouds also decide whether you'll see a sky event like the northern lights or a meteor shower.