Why don't clouds fall to the ground?

04 October 2016

Question

Why don't clouds fall to the ground?

Answer

We put Isabella's question to Chris Smith... Chris - Now the reason that clouds stay up where they are is because there are strong winds pushing the clouds upwards. What do I mean by that?

Well, clouds are full of water and, in fact, if we weighed a cloud that was one kilometer by one kilometer by one kilometer, because scientists have measured how much water there is in a cloud it would weigh about five hundred tons. But the water isn't in one giant blob, it's in lots of tiny particles and ice crystals which are called hydrometeors, and these have a very big surface area compared to their size. And because the Earth is constantly being heated up by the Sun shining on the Earth's surface, warming the surface, the surface warms the air above the surface, and the warm air rises because it expands and become less dense. So there's a column of warm rising air which is pushing upwards and this hits these tiny water particles which are trying to fall down under their own weight and it keeps them up there.

And that's why clouds have a flat bottom, because that's the point at which the tiny water crystals particles inside, which are trying to fall downwards, meet the warm air coming upwards and the two balance each other out, and the form the cloud base. So that's why clouds stay up there and also why five hundred tons of water in one cloud doesn't come crashing straight down to the Earth. Good thing really, isn't it!

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