Clouds

A cloud is a visible collection of millions of tiny water droplets (or ice crystals) floating in the air. Clouds form when water vapour in the atmosphere cools enough to condense onto microscopic dust particles. They come in many shapes and sizes, from delicate wispy ice clouds high in the sky to dark towering thunderclouds full of rain. Learning to read clouds is one of the oldest forms of weather forecasting: each cloud type tells you something about what the atmosphere is doing and what weather might be coming.

  • Main cloud types10Plus many variations
  • How clouds formCondensationWater vapour turning back into liquid droplets
  • Typical droplet sizeApproximately 0.02 mmAbout the width of a human hair
  • Highest cloudsCirrus, around 6 to 12 km upMade of ice crystals
  • Lowest cloudsStratus, near ground levelSometimes called fog
  • Tallest cloud typeCumulonimbus, up to 16 km tallThe thunderstorm cloud

The 10 main cloud types

The basic cloud classification was set up by English chemist Luke Howard in 1802. He used Latin names that are still used today. Modern meteorologists recognise 10 main types, based on shape and altitude.

High clouds (6 to 12 km up)

  • Cirrus: thin, wispy "horse-tail" clouds. Made of ice crystals. Often a sign that weather is about to change.
  • Cirrocumulus: small rippled patches of cloud, sometimes called "mackerel sky".
  • Cirrostratus: thin, sheet-like clouds that cover much of the sky. Often produce a "halo" around the Sun or Moon.

Middle clouds (2 to 6 km up)

  • Altocumulus: white or grey patches of cloud, often in rolls or waves.
  • Altostratus: grey or blue-grey sheets that often cover the whole sky and produce light continuous rain.

Low clouds (under 2 km up)

  • Stratus: smooth flat grey blankets. Often produce drizzle. When they touch the ground we call them fog.
  • Stratocumulus: lumpy grey rolls. Common in British weather.
  • Nimbostratus: dark grey rain clouds. Often produce steady all-day rain or snow.

Vertical clouds (range across altitudes)

  • Cumulus: classic puffy "cotton wool" clouds. Sign of a fine day if isolated, but can grow into something bigger.
  • Cumulonimbus: enormous towering storm clouds, up to 16 km tall. Produce thunderstorms, heavy rain, hail and sometimes tornadoes.

How clouds form

All clouds form by the same basic process: warm wet air rises, cools as it expands, and the water vapour condenses into tiny droplets. The droplets are small enough that they stay suspended in the air by the constant motion of the rising air around them.

Different cloud types form in different ways.

  • Cumulus and cumulonimbus form when the Sun heats the ground unevenly, causing patches of warm air to rise rapidly.
  • Stratus forms when whole layers of air are gently cooled, often when warm air rises slowly over cold air at a weather front.
  • Orographic clouds form when wind blows wet air over a mountain, forcing it upwards and cooling it.
  • Cirrus forms very high up, where the air is so cold that even small amounts of water vapour condense directly into ice crystals.
Fact Clouds may look light and fluffy, but they are surprisingly heavy. A typical small cumulus cloud weighs around 500 tonnes: about the same as 100 elephants. Large cumulonimbus thunderstorm clouds can weigh millions of tonnes. The reason they stay up is that the weight is spread across millions of tiny droplets, each one supported by the constant upward motion of air rising through the cloud.

What clouds tell us about the weather

  • Cirrus appearing in a clear sky: often means a weather front is approaching within 1 to 2 days.
  • Cirrostratus producing a Sun halo: rain or snow within 12 to 24 hours.
  • Lots of fluffy cumulus: usually fine weather, but watch for them growing taller.
  • Cumulus growing tall and dark: a thunderstorm is likely.
  • Cumulonimbus on the horizon: severe weather possible, including heavy rain, hail or tornadoes.
  • Stratus all day: probably grey damp drizzle (typical British weather).
Did you know? The colour of a cloud tells you a lot about it. White clouds reflect most sunlight; they are usually shallow or contain mostly fine droplets. Grey or black clouds contain enough water that little light gets through. Sunset and sunrise clouds appear yellow, orange, pink or red because the light has travelled through more atmosphere, which scatters away the blue light and leaves only the warmer colours.
Deeper dive: cloud seeding and other unusual clouds

Not all clouds form naturally. Several types are caused, in whole or in part, by humans.

  • Contrails: the long white lines left by aircraft. They form when hot wet jet exhaust meets cold high-altitude air; the water vapour in the exhaust condenses or freezes into a thin cloud. Contrails can persist for hours and slowly spread out into cirrus-like clouds.
  • Pyrocumulus: cumulus clouds produced by the heat of major fires (forest fires, volcanic eruptions). The fire heats the air rapidly, pushing it upwards strongly enough to form clouds at altitude. Very large pyrocumulus can even produce their own lightning and rain.
  • Industrial fog: cooling towers and chimneys release water vapour and other gases that can form small clouds nearby.
  • Cloud seeding: a technique where small particles (often silver iodide) are sprayed into clouds to provide extra condensation nuclei and encourage rain. Used in China, the UAE and parts of the United States, mainly in dry areas.

Some unusual natural cloud types are also worth knowing.

  • Lenticular clouds: smooth lens-shaped clouds that form above mountains. Sometimes mistaken for UFOs.
  • Mammatus: pouch-shaped clouds that hang from the underside of cumulonimbus storm clouds. Often a sign of severe weather.
  • Noctilucent clouds: glowing electric blue clouds at the edge of space (around 80 km up), visible only at summer twilight. The highest clouds in the atmosphere.
  • Asperitas: dramatic wave-like cloud formations, only officially named in 2017. Look like a stormy sea from underneath.

For more, see rain, thunderstorms and condensation.