How do clouds float in the air if they contain a lot of water?

Despite having a fluffy facade of unbearable lightness, clouds are actually pretty heavy things in the sky.

Take even a small, fluffy cloud measuring a mere cubic kilometer. The water in that cloud would typically weigh about a million kilograms, about the same as 500 mid-size automobiles. So how does all that weight stay up there?

Well, it turns out that the air in that same cubic-kilometer cloud is also heavy—about a thousand times heavier than the water is. The water vapor is less dense than the surrounding air, so the surrounding air exerts an upward force on the cloud, causing it to float and keeping it up in the air.

But wait, there’s more. Clouds are usually formed in warm air that is moving upward, adding even more buoyancy. So, as long as the water is in the form of tiny droplets, it floats just fine in the warm up-currents.

It’s only when the air cools that the mist condenses into larger drops, and the downward pull of gravity trumps the upward push of rising air.

Then rain falls, and the moisture that is the lifeblood of the cloud slowly gets sapped away.