There are few things prettier than fluffy white clouds floating in a blue sky. We tend to think of such clouds as having no more substance than fog. However, clouds are actually pretty hefty. The average cumulus cloud—made out of water droplets—weighs 550 metric tonnes (500 tons).
Such a hefty behemoth can float because the atmosphere around it is pretty heavy. It’s easy to forget that we and the cumulus cloud exist close to the bottom of the atmosphere. Air molecules have weight, and its air pressure is roughly 1 kilogram (2.2 lb) per square centimeter, or 17.2 metric tonnes (15.5 tons).
That’s quite a lot of force against a typical 168 centimeter (5’6″), 63.5 kilogram (140 lb) human. We aren’t crushed by this weight though. Because it presses equally over both the inside and outside of our bodies.Also, air is a fluid just like water. This allows the Archimedes Principle to come into play.
The upward buoyant force on the cloud is equal to the weight of the air it displaces. This close to Earth’s surface, that multi-tonne cloud floats in the air for the same reason that a cruise ship floats on water.