Although people take it for granted, it doesn't mean that clouds remain up there in the sky just because we're accustomed to. There is a number of reasons for which this happens, and these include water particles' size, air and wind pushing or weight.

Surely, this is a question that reminds us of the IgNobel prizes motto, which are awarded for “accomplishments that first make people laugh, and then make them think”. At first sight, this matter seems to be a dumb one, since, let's face it, how many of you thought of it, even as kids? But on a second, more serious thought, you have to admit that not many people know the answer to what keeps clouds from falling onto our heads. The answer to this is a little bit more complex than one would imagine.

First of all, it involves the fact that clouds are made out of water drops and icy crystals of different sizes. Sure, they are denser and heavier than the air, but most of them are too light and have too large of a surface (by comparison with their weight, not in general terms) to be able to overcome the force of the air that pushes them back. If it helps you imagine it, it's like keeping a kite high in the air, but on a much smaller scale. And, just as kites, they do fall, slowly, helped by gravity, and here's where the wind comes into the equation.

This type of wind is not similar to the one we know from our daily experiences (unless you're a weather scientist), which blows parallel to the Earth's surface. Instead, this kind of wind, called an updrift, is blowing upwards, rising like currents of air and vapors. This further prevents water droplets from reaching lower altitudes. In time, the condensation process makes droplets gather into larger drops, which can break the air and updrift resistance and eventually fall to the ground in the form of rain. But this is a slow process, and not all drops fall at once, which still makes a cloud outlast its rains.