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
Related reading: How clouds complicate global warming
Originally published by Cosmos as Why do clouds float?
Karl Kruszelnicki
Karl Kruszelnicki is an author and science commentator.
Read science facts, not fiction...
There’s never been a more important time to explain the facts, cherish evidence-based knowledge and to showcase the latest scientific, technological and engineering breakthroughs. Cosmos is published by The Royal Institution of Australia, a charity dedicated to connecting people with the world of science. Financial contributions, however big or small, help us provide access to trusted science information at a time when the world needs it most. Please support us by making a donation or purchasing a subscription today.
