Like cotton wool, clouds are al­ways used to de­scribe the lighter things in life. But while they may glide grace­fully around a blue sky, clouds are the heavy­weight giants of our at­mos­phere. When you con­sider the amount of wa­ter that comes from a mas­sive down­pour, imag­ine how heavy the cloud must have been to hold it.

The wa­ter den­sity of an av­er­age fluffy cu­mu­lus cloud is about 0.5 grams per cu­bic me­tre. If you pro­pose a cloud that is one kilo­me­tre long, tall and wide, that gives you a to­tal of 1 bil­lion cu­bic me­tres in vol­ume. That works out at around 500 tons of wa­ter — the same as around two and half blue whales float­ing above our heads! This method also sug­gests that larger and denser cu­mu­lonim­bus clouds could weigh around 1 mil­lion tons! It’s a huge weight, but the sur­round­ing at­mos­phere is denser than the cloud, so it floats. Tem­per­a­ture also plays a part in keep­ing these clouds in the air, as warmer air is less dense than cool.

As we know, when a cloud gets too full of wa­ter, droplets form and we get rain, and the weight of the cloud re­duces as a re­sult. So next time it’s a cloudy day and pour­ing it down, there could be lit­er­ally tons of wa­ter fall­ing over your head.

There are ten main groups of cloud that cross the sky

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