If clouds are made of water, how do they stay in the air?

Using known values ​​for volume, air density and gravitational field, this gives a buoyancy force of 11.8 Newtons, or 2.7 pounds.

Now, let’s replace this air unit with another unit that is identical in shape and size. But this time let’s assume that this is 1 cubic meter of water with density ρ_water = 1000 kg / m³.

Since it has the same volume as the floating air, this unit will have exactly the same buoyancy force. It does not matter what you put in this space, if it has a volume of 1 m³, it will have a buoyancy force of 11.8 Newtons. But for this cube of water, this is not enough to let it swim. The gravitational force that pulls it down will be much greater – that’s 9,800 Newtons. The water cube will just fall.

In order for buoyancy to be greater than gravitational force, you must fill this space with a substance with a density lower than air. There are two common methods to make this work in real life. One is to use a thin rubber container filled with low-density gas. (Consider a helium balloon.) The other is to use a low-mass container to hold hot air that is less dense than cold air and will rise above it. (Think of a hot air balloon.)

So if you want a cloud to float, it must have a density lower than that of air. But how can this density be lower if the cloud also contains air and water?

This is because the clouds do not actually swim.

Why does the size of the water matter?

Let’s say a cloud is made up of air plus a bunch of very small drops of water. The size of the drops is important. You may be surprised to learn that even if both are made of water and have the same shape, small drops do not behave like large drops. To understand the difference between them, we need to look at air resistance.

Let’s start with a quick demonstration. Extend your hand in front of you with an open hand. Now rotate your arm back and forth so that your hand moves quickly in the air. Do you feel anything? It may be slight, but there must be an interaction between your hand and the air, a force of repulsion, which we call air resistance or air resistance. (You’ll definitely notice it if you reach through the window of a moving car.)

We can model the air resistance on a moving object with the following equation: