Technology

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 buoyant force of 11.8 newtons, or 2.7 pounds.

Now, let’s replace this air mass with another mass of identical shape and size. But this time, let’s say it’s one cubic meter of water with a density ρWater = 1,000 kg/m3.

Since it has the same volume of floating air, this mass will have exactly the same buoyancy force. It doesn’t matter what you put in that space, if it’s 1m in size3, would have a buoyant force of 11.8 N. But for this water cube, it is not enough to allow it to float. The force of gravity pulling it down will be much greater – 9800 Newtons. The water cube will fall.

In order for buoyancy to be greater than the force of gravity, you must fill this space with a substance with a density less than the density of air. There are two common ways to make this work in real life. The first is to use a thin rubber pan filled with a low-density gas. (Think of a helium balloon.) The other is to use a low mass vessel to hold hot air, which 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, its density must be less than the density of air. But how can this density be lower if the cloud contains both air And Water?

That’s because clouds don’t really float.

Why is water volume important?

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

Let’s start with a quick show. Extend your arm in front of you with your hand open. Now move your arm back and forth so that your hand moves quickly in the air. Do you feel anything? It may be slight, but there has to be an interaction between your hand and the air, a back push we call air resistance or air drag. (You will definitely notice this if you raise your hand from the window of a moving car.)

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

Illustration: Rhett Allen



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