I know, or at least think I know, what clouds are. A bunch of very small water droplets. But I don’t understand why they seem to have a definite “floor” or why they don’t simply disperse like smoke. How can you have a few discrete clouds just sitting in an otherwise cloudless sky?
What sets their altitudes?
Air pressure, above or below a certain point they might condense or be evenly spread across the sky.
It’s temperature. There is just about as much water (in the form of vapor) below the cloud, but it’s warm enough there that it doesn’t condense. Above a certain point, it’s cool enough to condense and droplets are formed. As to why clouds have tops, I suppose the air is just too dry beyond a certain point. Humidity drops off very quickly with altitude.
For a pretty good overview, try:
http://ww2010.atmos.uiuc.edu/(Gh)/guides/mtr/cld/home.rxml
Follow the pages beginning with Development and then cross-check it with Cloud Types. The info is s[read pout over quite a few pages, but it loads very quickly and each page has specific information. Actual cloud boundary development is described beginning about four pages into Development.
(I think this is the replacement site for the old COVICS site, although they don’t have a direct mention of that fact.)