I was driving cross country yesterday, and the sky was a perfect clear summer blue, except for 4 tiny little clouds. I started to wonder: why are the air conditions different enough in those 4 little areas to form clouds, and why do the clouds have edges?
So what was different in those 4 locations, that let clouds form, when the rest of the sky was crystal clear? And why are the boundaries between those areas sharp enough to let me see an persistent edge, rather than blending into the adjoining air?
Not all clouds have sharp edges; some types of stratus and cirrus clouds just fade gradually into the surrounding air.
The most clearly defined clouds are formed by convection, where clearly differentiated bubbles or columns of air rise and cool until they reach the point where water vapour forms into droplets. These rising bubbles or columns are quite different from the cooler, drier air in which they are rising, so they appear quite distinct and sharp-edged. If you could examine them at close quarters - in a hang-glider or paraglider*, perhaps- you would see that at very close quarters they are quite fuzzy.
*Don’t go near a rapidly rising updraft inside a tall cumulus or cumulonimbus in a paraglider, though, as you might go higher than you want to and even lose consciousness. See Cloud Suck
It depends on if the cloud mixes with the surrounding air, and how it mixes. If the air surrounding the cloud has low humidity and the cloud mixes with it, the water and ice particles evaporate and disappear, making a sharp edge. If it’s high humidity, the mixing will cause the air to reach saturation and effectively become cloudy itself, at least right near the cloud. That makes for a less defined edge. Some clouds don’t mix very much at times and those appear to have a more distinct border because, well, they are somewhat self contained and really do have a distinct border.
The edges are not even all that distinct when viewed close-up. Cumulus clouds have a fractal like structure. That puffy shape you see from the ground is replicated again and again at smaller and smaller scales. The big puffs have a surface made of smaller puffs, which are in turn made of still smaller puffs, and so-on.
The bottoms do transition pretty quickly, as this is where the cooling due to adiabic expansion causes the air temperature to reach the dew point. Below this the water evaporates into invisible vapor, above this, it condenses into visible droplets.
Part of it is also probably due to the fact that the human visual system is adapted to finding edges (since most of the objects we deal with are solid, and actually do have well defined edges, and it is good to know where they are). It will occasionally find them when they are not really there.
That said, I have walked up a mountainside into the base of a cloud covering the peak, and the transition was indeed quite sharp.
When I’m in a plane that goes through a cloud it is usually fairly gradual. But when I"m on a mountainside and a cloud envelops me it is usually quite quick, with only about a 20 foot edge. Perhaps if it were gradual I would think of it as fog rather than a cloud
