Droplets too small to fall
How Does Something That Heavy Float?
A cloud weighs tonnes, but it is made of droplets so tiny that air resistance almost stops them falling.
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A cloud really is astonishingly heavy.
A modest fluffy cloud holds hundreds of tonnes of water, which is the weight of a fully loaded jumbo jet sitting in the sky.
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But it is not one lump of water.
All that water is divided into an enormous number of separate droplets, each far too small to see, spread through a huge volume of air.
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Tiny things fall extraordinarily slowly.
Air resistance depends on surface area while weight depends on volume, so the smaller a droplet is, the more the air holds it back.
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Gentle rising air is enough to hold them up.
Cloud droplets sink at roughly a centimetre a second, and the slow updraught that formed the cloud easily matches that.
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Rain starts when droplets merge.
Droplets collide and combine, and once one grows large enough its weight finally beats the air resistance and it falls out as rain.
A cloud droplet is about 0.02 mm across and falls at 1 cm per second; a raindrop is 100 times wider and falls at 6 metres per second.
The short version
Clouds float because their water is divided into droplets so tiny that air resistance nearly cancels their fall, and gentle updraughts do the rest.
Try it yourself
Watch steam above a kettle. It hangs and drifts rather than falling, for exactly the reason a cloud does.
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