Clouds are the sky’s own forecast, and learning the ten main types is the fastest way to start reading it. This page covers the full cloud atlas from cumulus to cirrocumulus, with the weather each type tends to bring. Storm-specific clouds like wall clouds and shelf clouds live on the Storm Structure page.
Clouds are grouped by altitude and shape:
Cumulus clouds are the puffy, cotton-ball clouds with flat bases and dome-shaped tops, usually a sign of fair weather. They form when rising thermals condense at the lifted condensation level. Vigorous cumulus growth on a humid afternoon — towering cumulus — is often the precursor to cumulonimbus and thunderstorms.
Stratus clouds are featureless gray sheets that cover the entire sky like a low ceiling, often producing drizzle or light snow. They form when a moist layer is lifted gently or when fog lifts off the surface. The 'overcast and dreary' look.
Stratocumulus clouds are low, lumpy gray-and-white patches or rolls covering most of the sky, usually with breaks of clear air between them. Common behind cold fronts. They rarely produce significant precipitation but can give a moody, broken sky.
Nimbostratus is a dark, thick, featureless cloud layer that produces continuous moderate to heavy rain or snow. They're the classic 'all-day rain' cloud, associated with warm fronts and broad areas of lift in mid-latitude cyclones.
Altostratus is a uniform gray or blue-gray mid-level cloud sheet (6,500-20,000 ft) that often covers the whole sky. The sun appears as a dim disc behind it — 'sun behind ground glass.' Altostratus often thickens into nimbostratus ahead of a warm front, signaling approaching rain.
Altocumulus clouds are mid-level (6,500-20,000 ft) white or gray patches arranged in bands, rolls, or 'cloudlets.' Altocumulus on a humid morning often precedes thunderstorms later that day. Castellanus variants (turreted tops) indicate mid-level instability.
Cirrus clouds are thin, wispy, high-altitude (>20,000 ft) clouds made of ice crystals. The classic 'mares' tails' streaks. Cirrus is usually fair-weather but can be the first sign of an approaching warm front or upper-level disturbance, especially when it thickens to cirrostratus.
Cirrostratus is a thin, milky veil of high-level ice clouds that often covers the entire sky, frequently producing a halo around the sun or moon from light refracting through the ice crystals. Cirrostratus thickening and lowering is a reliable signal that a warm front and precipitation are 12-24 hours away.
Cirrocumulus clouds are small, white, high-altitude patches arranged in ripples or rows — the classic 'mackerel sky.' Made of ice crystals at >20,000 ft. They're relatively rare and short-lived, often indicating instability aloft.