Cumulus

What it looks like

Separate puffs with a flat, slightly grey base and a bright cauliflower top. They grow on bubbles of warm air rising from the ground, so they appear mid-morning on sunny days, drift with the wind and fade after sunset. All the bases sit at the same height because rising air reaches its dew point at the same level everywhere.

What it means

Small, flat cumulus are fair-weather clouds. If they keep growing upwards through the afternoon, showers become likely; watch whether the tops are still crisp and rounded or starting to look fibrous. Over the sea they form at night, over land by day.

Not to be confused with

Stratocumulus, which is what cumulus become when they spread out under a stable layer into a lumpy sheet, and altocumulus, which is higher and made of much smaller elements.