eli5 : Why do cloud move as a whole thing and not dissolve in the wind?



I live in a windy place. Right now, I am looking at clouds moving really fast. But cloud are just water vapor no? So how can they keep their shape and move as a whole piece instead of just being blown away by the wind?

Edit : thank you for all those detailed explanation ! I guess I’ll spend way more time looking at clouds now !!

In: Physics

They do not move as a whole. Each individual tiny droplet of water in the cloud is moving independently of all the other tiny little droplets of water.

But they are all moving independently based on essentially the same forces being applied to them. If the wind is blowing in One direction and doesn’t have a lot of different cheering forces, then it is going to push the whole cloud in one direction, and there won’t be a bunch of forces present to rip the cloud apart.

Think of a cloud as a three dimensional graph in the sky of a volume where the relative humidity has reached 100% and moisture has condensed into tiny droplets of liquid water. You might see a cloud moving across the sky as a unified thing when in fact new droplets are condensing into liquid at the leading edge while droplets are evaporating back into vapor at the trailing edge.

Besides the good descriptions of why they can move as a group (the vapor feels the same general forces) there are a few other factors.

First, the wind itself is holding the cloud up. Water as a vapor in a cloud is a *liquid* and will fall. It’s only the updraft created by rising warm air, and reinforced through heat released when the water gas turns to liquid, that keeps the entire thing up. And as it isn’t a solid, this means the water droplets are constantly circulating. It is a turbulent environment for the tiny water droplets, not a smooth, floating experience you’d imagine by looking at the entire cloud.

And the edges of the cloud are constantly dispersing, or falling back into the cloud, if you watch a cloud closely, it is often growing and shifting in the direction it’s moving, not just drifting.

You feel wind as something blowing past you. But the column of air is all moving together, and the cloud is simply visible suspended particles of water within that column.

Look closely to clouds, especially if they are well-defined and have that cauliflower look, before they start to pull apart like cotton candy. While they are still sharp, if you focus on one spot you can actually see them expanding like slow mushroom clouds. It’s very cool.

The behaviour you’re describing is a property of gasses. They want to spread out as much as possible.

But clouds are a mix of solid particles and liquid water.

They still get ripped up though, but just on a gigantic scale.

Just one quick thing to add: Clouds are really really big. It’s hard to gauge how big they are because they tend to be rather far away.

They look substantial because you’re looking through a lot of water droplets. The light needs to pass through enough volume of cloud to be scattered.

But they don’t have a sharp edge, so the parts that look like “cloud” are very large and so despite the fact that there’s all that wind, the small scale changes aren’t very noticeable because there needs to be a significant change of a large volume to change the scattering of light.

Clouds are liquid water – not water vapour. If they were vapour there would be no cloud to see.