When we see clouds what we see are molecules of water that have gone from being vapor to being liquid, they have condensed. When we stop seeing a cloud that seems to fade, it is because the thermodynamic conditions in the atmosphere have changed and the water goes back to the gaseous state (water vapor).
The drop of liquid water is subject to a force that is its weight, greater than the equivalent volume of air, but the air around it exerts another force, friction, which is one of the causes why it does not fall . So that the force of that friction (upwards) is greater than that exerted by the weight downwards, the drop has to be very small. This is the main explanation for why clouds float: they are formed by drops of water so small that their weight is a force lower than the force of friction exerted by the air molecules that surround them.
Those drops of water collide with each other and add, increase their size and at the end, when that weight is higher than the force of friction exerted by the air around, precipitate, that is, fall. That's why we call precipitation to rain or snow.
In parallel, other things happen. Below, clouds have layers of air in motion. When a fluid is in motion it generates a turbulence that favors that things stay suspended in it, that "float". Like when we are in a pool, if we stand and we are motionless, we sink but if we move our arms and feet we float and this is because we generate a turbulence in the water below us that sustains us without sink That same effect causes the turbulence of the air that surrounds the clouds. So that to "float" both processes overlap, on the one hand the friction with the air pulls them up and on the other hand the air in movement that is under them sustains them.
There is another curious effect that explains a characteristic of the rain in which surely you have noticed at some time. While the drops remain in the cloud and we see this floating in the sky, it is because there is a balance of the forces that act on it: there is the weight pulling down and that force of friction upwards. But the force of friction is greater when the drop is still than when the drop begins to move because the friction exerted between two bodies is greater at rest than in motion (in Physics, it is explained that the static friction is greater than friction dynamic). It's like when you want to move a closet full of clothes in your bedroom and the first push costs you a very big effort but, once it is in motion, it costs you less to drag it. That is why when it is quiet what you have to overcome are the static pressures that are greater and when it is in motion the force that must be overcome is that of the dynamic pressures that are smaller. Because the water drop in the cloud is the same, it has to acquire a large size to overcome the static friction, which is greater, but once it starts to fall, the friction is lower and this helps the precipitation process. So at the beginning of the rain the drops are bigger than when it has been raining for a while, they are smaller. I had a teacher who explained this with a very graphic example, if you drive and begin to drop drips on the windshield that get smaller as you go, it means that you are moving in the opposite direction to the rain front, but If what falls on the car are small drops, it means that you are moving from the end towards the beginning of the cloud front.
María José Polo She is a PhD in Engineering Agronomy. Professor of Hydraulic Engineering at the University of Córdoba. Interuniversity Institute for Research of the Earth System in Andalusia.
Question asked via email by Esther del Carmen Millán Rosado
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