If clouds are made of water, how do they keep the wind?
Using known values for volume, wind magnitude, and gravitational field, it gives a buoyancy force of 11.8 newtons, or 2.7 pounds.
Now, let’s replace that air block with another block of the same shape and size. But at this hour, assume that it is a cubic meter of water with a thickness of ρwater = 1,000 kg / m3.
Since it has the same amount of floating air, this block has the same amount of buoyancy force. It doesn’t matter what you put in that space, if it has a maximum of 1 m3, it has a buoyancy speed of 11.8 newtons. But for this cube of water, that’s not enough to make it float. The strong gravitational pull of it down could be even greater – it’s 9,800 newtons. The water cube will just fall.
In order for buoyancy to be much greater than gravitational force, you have to fill that space with a component that has a thickness that is much less airborne. There are two common ways for this to work in real life. One is to use a thin rubber tube filled with a low-density gas. (Think of a helium balloon.) Others use an indoor low-mass container to hold hot air, which is less dense than cold air and rise on top of it. (Think of a hot-air balloon.)
So if you want a cloud to float, it has to have a density that is much smaller than air. But how can such a low be low when the cloud has a strong wind and water?
Because the clouds never floated.
Why Does the Size of Water Matter?
Let’s say a cloud consists of air plus a set of very small water droplets. The size of the three is important. You might be surprised to learn that even though they are both made of water and the same shape, the small drops don’t act like the big drops. To know the difference between them, we need to look at the wind resistance.
Let’s start with a quick demonstration. Stretch your arm in front of you to open your hand. Now swing your arm back and forth so that your hand can move quickly in the air. Do you have a feeling? It may be small, but there must be some interaction between your hand and the wind, a force that holds the back that we call wind resistance or air drag. (You’ll definitely notice this when you put your hand in the window of a moving car.)
We can model the air resistance of a moving object with the following equation: