Why do clouds float?

They weigh hundreds of tonnes, but even dark clouds remain aloft. Karl Kruszelnicki explains why they seemingly defy gravity.

Michel Tripepi / EyeEm / getty images

Clouds must have weight, because water has weight. A cloud is many, many tiny “clumps” of water, either liquid or frozen.

The liquid droplets are about 0.002 millimetres across (smaller than the thickness of a human hair, which is about 0.050 – 0.070 millimetres). Some of these tiny droplets are so small that it would take a billion of them to make a single raindrop.

Different clouds carry different amounts of water. After all, cloud shapes and sizes can range from thin, wispy cirrus, right up to monstrous cumulonimbus thunderclouds.

A typical cumulus cloud carries about 0.50 grams of water (the weight of a big garden pea) in each cubic metre. But the whole cloud might be one kilometre by one kilometre by one kilometre. So it could carry up to about 500 tonnes of water.

What holds this water up?

There are three main reasons. First, the heat of the Sun warms the ground, which then creates rising currents of air. Second, an advancing storm or weather front running close to the ground can push the air that it runs into upwards. Third, air carrying water vapour can run into a mountain and rise, adding buoyancy to the cloud above.

Air (when it is moving very fast) can easily hold up a 600-tonne jet plane. So a huge volume of slowly moving air can definitely hold up a 500-tonne cloud.

Cloud trivia

— The name “cloud” comes from the 13th-century Old English word “clud” or “clod”, meaning a mass of rock, or a hill. A very dark cumulus cloud can look like a hill.
— Other planets have clouds. On Venus they are made of sulfur dioxide, while the clouds on Mars carry water as ice. The clouds on Jupiter and Saturn are (from the top down) made of ammonia, ammonium hydrosulfide and near the bottom, water. The clouds on Uranus, Neptune and Titan (a moon of Saturn) are made of methane.
— The first successful weather satellite was TIROS 1, launched on 1 April 1960. It survived only 78 days, but proved that weather satellites could be useful.

Credit: Edited extract from Short, Back and Science, Macmillan 2015

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Karl Kruszelnicki is an author and science commentator.
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