Why do clouds have edges?

Factual Questions
1

Why do clouds have edges? OK, not really sharp clear edges all the time, but still you can look at a cloud and see a pretty clear demarcation of cloud-to-“not cloud” around most of the cloud shape.

Yet clouds are heaps of water vapor, collected together by air pressure and perhaps some cohesion of water droplets. Shouldn’t there be a gradual smeared visual effect instead of a sharp edge as the water vapor density drops from high in the center to lower toward the edge? My guess is that there is a spot in the center of the cloud that has say 99% water vapor, then a spot further toward the edge that has 80%, then 70% and so on as you get closer to the edge, and then a “non-cloud” part of the sky that has whatever the local humidity allow. But if that were the case, then the cloud should smear out gradually, and not have the fluffy demarcation lines that show white (on a nice day) against blue. Seems to me it should; but it doesn’t. So I’m wrong – but I’d like to know why.

Are the edges:

(a) Physical – some force causing a sharp drop in water vapor at the edge of the cloud?
(b) Optical – something about light diffraction that causes water vapor of, say, X% to look white but (X-1)% to look clear?
© Psychological – my visual system creating edges where no real edges are to be found?

Thanks for your help, and best regards -

  • Rich Gore
2

Cloud is not water vapour, you can’t see water vapour, it’s a gas. Cloud is made up of tiny water droplets which have condensed as the humidity reaches 100%. So what you are seeing is just the shape the group of water droplets makes.

3

Edges would be expected if, for any reason, water droplets preferentially formed next to other water droplets, or do not evaporate as quickly when there are other droplets nearby.
The physical shading of one droplet by another likely slows evaporation, but I suspect that the larger effect comes from the nonlinear relation between droplet size and evaporation rate: the curvature effect.

4

There is a gradual fade at the edge of a cloud, but it’s just over a short enough scale, and the cloud is far enough away, that it looks sharp to you.

5

Some clouds do have whispy, fadingedges.

6

I don’t know if this really answers the question, but if you look at Jupiter or Saturn, you don’t see any fuzzy edges despite the fact that they are nothing but giant balls of gas. The sun, many space nebula, and even galaxies also look like they have sharp edges.

I think it’s just how things work that they look more sharply defined than they really are except when you’re inside or very close.

7

I’ve often wondered this, too. And the edges of clouds can be pretty sharply defined. Quite a few times when I’ve been skiing I’ve seen those fluffy white clouds hugging the slopes, and skied through them. I’d say you go from clear air to dense fog in the space of a metre or two.

8

As clouds are a bunch of dust with water sticking to the particles, I would think that they have their own centre of gravity which would help them to be discrete objects rather than just wafting off into the ether.

9

Exactly right. Try focusing a telescope on the “edge” of a cloud and it will be hard to find.

10

I’ve heard that clouds are often as heavy as a 747. While that is impressive, it’s negligible as far as gravity goes.