Clouds form where air is lifted and cools to its dew point, and their shapes record how the air was moving. Heaped clouds grow in rising bubbles of unstable air; flat sheets form where stable air is lifted gently over a wide area. A cloud's name tells you both its height and its shape.
Key idea
Cloud names are built from a few Latin roots: cirro- (high, wispy, like a curl of hair), alto- (middle height), strato- (a flat layer), cumulo- (a heap) and nimbo- or -nimbus (bringing rain or snow). High clouds are about 5 to 13 km up, middle clouds about 2 to 7 km, and low clouds below about 2 km; cumulonimbus towers through all three.
Why it works
Shape follows motion. A bubble of warm, unstable air rises on its own, so its cloud bulges upward in heaps (cumulus); if the bubble keeps rising past the freezing level and to the top of the troposphere, it becomes a cumulonimbus, a thunderstorm. Stable air lifted slowly over a large area, such as ahead of a warm front, cools evenly and makes a flat, wide sheet (stratus, altostratus). Height follows temperature: high clouds form where it is far below freezing, so they are made of ice crystals, which is why they look thin and wispy and can make halos around the Sun or Moon.
| Level | Clouds | What to look for |
|---|---|---|
| High (about 5–13 km) | cirrus, cirrostratus, cirrocumulus | ice crystals: wisps, a thin veil with a halo, tiny ripples |
| Middle (about 2–7 km) | altostratus, altocumulus | a gray sheet with a dim "watery" Sun; rows of gray puffs |
| Low (below about 2 km) | stratus, stratocumulus, nimbostratus | a gray blanket; lumpy gray rolls; a thick, dark layer with steady rain or snow |
| Rising through all levels | cumulus, cumulonimbus | flat bases and heaped tops; a huge tower with an anvil top, lightning and heavy showers |
Worked example: Name the cloud
A thin, milky veil covers the sky, and a bright ring surrounds the Sun. What cloud is it?
- A ring (halo) is made by light bending through ice crystals, so the cloud is high.
- A veil that covers the sky is a layer: strato-.
- High plus layered: cirrostratus.
Answer: Cirrostratus
Worked example: Rain or showers?
One afternoon brings brief, heavy downpours with thunder; another day brings light, steady rain from morning to night. Which cloud made each?
- Brief, heavy showers with thunder come from a tall heap that rose fast in unstable air: cumulonimbus.
- Long, steady, gentle rain comes from a thick, wide layer: nimbostratus.
Answer: Cumulonimbus, then nimbostratus
Common mistake
Thinking dark clouds are made of something different from white ones, such as smoke or dirt. Every cloud is made of tiny water droplets or ice crystals. A thick or dense cloud scatters and absorbs most of the sunlight before it gets through, so its base looks dark from below; the same cloud seen from an airplane above is bright white. Check: the darkest clouds are usually the thickest ones, like nimbostratus and cumulonimbus.