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Home›Weather and climate

Clouds

A small cumulus cloud 1 km across holds about 500 tonnes of water and still floats. Learn how clouds form, the ten cloud types in the WMO International Cloud Atlas and the heights they live at, and what each one tells you about the weather. The 3D block puts cirrus, cumulus, stratus and cumulonimbus at their real heights.

Progress

About this course

A small cumulus cloud 1 km across holds about 500 tonnes of water and still floats, because that water is spread at about half a gram in each cubic metre. The names we use for clouds are young. Luke Howard, a London industrial chemist who had kept weather journals since he was ten, published his Essay on the Modification of Clouds in 1803. At the time many people thought clouds changed too fast to be sorted into types. Howard gave the main forms Latin names in the way botanists name plants, and the names stuck. Goethe read the essay in translation in 1815, wrote to Howard and composed a poem in his honour.

The six units start with how clouds form and the story of Howard's names. Low clouds, with bases below 2 km, come next: cumulus, stratus and stratocumulus. Middle and high clouds follow, and at 9 km, where cirrus float, the standard atmosphere is about −43 °C, so these clouds are made of ice. Cumulonimbus can stretch from 1 km above the ground to the tropopause, more than 10 km up, in a single tower. The last unit teaches reading the sky, with observers reporting cover in eighths called oktas, where 0 is clear and 8 is overcast. The 3D block places cirrus, cumulus, stratus and cumulonimbus at their real heights, with a ruler at the back. Visit each cloud from the menu, see the cumulonimbus rise from a dark base near 1 km to a flat anvil at about 11 km, and change the temperature from 25 °C to 5 °C to see several things change together.

Before you start

Clouds form when rising air cools and water vapour condenses, which The water cycle covers in its condensation unit. That course is helpful but optional. A window or an open patch of sky lets you practise naming clouds as you read the last unit.

When you finish

Afterwards you can explain what makes a cloud visible and why its edges keep changing, and name the ten cloud types in the International Cloud Atlas by height. You can tell stratus from altostratus, spot virga under a cloud, explain why cirrus is made of ice, and report how much of the sky is covered in oktas.

  1. 1How clouds formA cumulus cloud holds about half a gram of water in each cubic metre, so a cloud 1 km on each side carries about 500 tonnes.
  2. 2Naming the cloudsThe cloud names used by weather services today were published by a London chemist in 1803.
  3. 3Low clouds: cumulus, stratus, stratocumulusLow clouds have their bases below 2 km, close enough to hide the top of a tall building.
  4. 4Middle and high cloudsAt 9 km, where cirrus float, the standard atmosphere is about −43 °C, so these clouds are made of ice.
  5. 5Cumulonimbus: the thunderstorm cloudA cumulonimbus can stretch from 1 km above the ground to the tropopause, more than 10 km up, in a single tower.
  6. 6Reading the skyWeather observers report how much of the sky is covered in eighths, called oktas: 0 is a clear sky and 8 is overcast.

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