They may look all light and fluffy, but the reality is that clouds are actually pretty heavy. Researchers have calculated that the average cumulus cloud - which is that nice, white fluffy kind you see on a sunny day - weighs an incredible 500,000 kg (or 1.1 million pounds!).
How do you work that out? First of all, you need to realise that clouds are made up of a lot of tiny water droplets, which means that they must have some mass. The next step is to then work out how dense your cloud is.
So back to those cumulus clouds. Scientists have worked out that the water density of this cloud type is around 1/2 gram of water per cubic metre. So, as Matt Soniak writes over at Mental Floss, that's about a marble's worth of water in a box large enough for you and a friend to sit in. Not very much. Obviously, the density of other types of clouds would be much greater, but let's stick to the cumulus for now.
Once you've worked out the density of your cloud, you need to work out how big it is, which is a measurement that also varies widely. Peggy LeMone, who led a lot of the cloud weighing research at the US National Centre for Atmospheric Research, calculated that the average cumulus is about a kilometre across and roughly has the shape of a cube, so it's as tall as it is wide.
Do the maths on that, and you've got a cloud with a volume of one billion cubic metres. Times that by your density and you get your answer of almost 500,000 kg. Or, as LeMone explained it to Soniak, think of that as 100 elephants.
So now the real question is, how does all this massive weight stay afloat in the sky? What's stopping it from collapsing on our heads at any moment? *Looks up nervously*.
To start with, this weight isn't all concentrated in one point, it's obviously spread out over a huge space. Clouds are also made up of water droplets that are sometimes so tiny that gravity has hardly any effect on them. And because of condensation, clouds are actually bouyant.
Perhaps even more surprising is the fact that a cloud is actually less dense than dry air, so that keeps them floating, as Soniak explains.
Find out more in the episode of It's Okay To Be Smart below, and never think of clouds the same way again.