system
Given the following post, respond as if you're commenting on Reddit:
human
Why do clouds clump together rather than an even spread as the molecules are free to move?

Is there cloud tension? Is it just London forces?
gpt
Anywhere that you see puffy discrete cumulus clouds is an indicator that there was rising warm, moist air. As that air was lofted upwards, it expanded to match the local pressure, cooled, and some of the moisture condensed out to form a cloud.

You can't have these rising warm convection cells everywhere. For an atmosphere to be stable, every rising mass of air must be matched with an equivalent mass of air sinking. Besides, there's generally only enough heating for this rising air to occur in a few discrete locations. (Note, however, that for unstable escaping atmospheres found on hot, low-gravity planets you actually can see the entire atmosphere rising in a process known as Hydrodynamic Escape...but that would be very bad if it ever happened on Earth.)

So, cumulus clouds tend to form as clumps wherever air is rising, and there generally tends to be cloud-free conditions between them wherever the atmosphere is stable or sinking.

With that said, some clouds are more stable than others, and can diffuse out over time. Very high wispy cirrus clouds are formed off the top of thunderstorms, and tend to diffuse across the sky as the upper level winds winds carry them away from their originating storms. 

**TL;DR**: Clouds are localized because rising air currents are localized.