system
Given the following post, respond as if you're commenting on Reddit:
human
Why are clouds often discrete?

Especially those stereotypical bunny clouds on a windy day. Are all clouds big enough that each one represents a pocket of air where the conditions for condensation are right? If so why isn't the upper air more uniform?

Are individual clouds sometimes directly from individual lakes, or other smaller isolated geological features? (not mountains or coasts)

I've never really been satisfied with my understanding of cloud formation, but I get the theory (i think). There was a similar question on yahoo answers, but the answer wasn't very helpful for me.
gpt
Clouds appear to have a very well defined boundaries because of a property of optics. If you went to a cloud an measured the density of water vapor you would find that at the boundary between the cloud and not cloud changed a little but not a lot. This is because the opacity of air is proportional to exp(density of water vapor), so that a small change in the density of water vapor will lead to a dramatic change in the opacity. If you could go up close to a cloud you would see a small transition region, but from far away it looks like a really sharp transition from transparent to opacity. This is what causes clouds to have their distinct shapes and features.