How do clouds disappear?

Why are clouds sometimes clearly demarcated like "sheep clouds", sometimes more diffuse?

In truth, clouds never have such a sharp edge as one might sometimes think from below. Everyone has had this experience in the mountains, for example, or when you penetrate the cloud cover in an airplane. It always starts with a few clouds of fog, which then become increasingly thick. But it's true: in some clouds, the transition takes place over such a short distance that from a greater distance it looks as if it has a sharp edge. At least these types of clouds - the famous sheep or "cauliflower clouds" in the sky form clear, demarcated clusters that can be seen e.g. B. can count. In other cases, on the other hand, a closed cloud cover or a few diffuse veil clouds form.

That depends above all on the respective air movements. I said earlier that clouds are formed when air rises, cools down and the water vapor condenses. Basically, the following applies in the atmosphere: if air rises somewhere, air has to fall again somewhere else. And when the air sinks, the opposite happens: Then clouds tend to dissolve. However, this pattern of rising and falling air can vary in size. To outline two extreme cases: If the air rises evenly over a wide area, then there tends to be a closed cloud cover. While somewhere completely different - maybe a thousand kilometers further, air masses sink again to compensate and there is blue sky.

But it can also be that these air movements are much smaller. For example, that we have a column of air perhaps 500 meters in diameter that rises, and next to it another column in which the air descends. A cloud forms in the column in which the air rises, but not in the other next to it. Such clouds are round and bushy, especially at the top, because the air rises fastest in the middle of the cloud, and this whole process of condensation reaches greater heights more quickly. And if the air rises quickly, it also cools down the faster, the water condenses faster, the droplets become larger, and the larger the droplets are, the more visible they are, so the contrast between cloud and sky appears correspondingly sharper.

So the bottom line: If clouds have an apparently sharp edge, then it is where the local air rises quickly - and the other clouds, which are more diffuse?

They are less caused by rising air than either by rather irregular air turbulence or simply by the fact that the air in the atmosphere simply cools down overall - for example because the sun goes down. In these cases, a rather diffuse situation arises, where, by chance, sometimes here, sometimes there, the temperature falls below the critical temperature for cloud formation, so that a few plumes form in one place, but not in the other, but without it as with the heap clouds, there would be a clearly defined updraft zone where a cloud forms.