Eli5 Why the clouds are around that height and not lower in the atmosphere?
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Water vapor (gaseous H2O) exists all throughout our atmosphere in most places. The amount of water vapor x volume of air can hold is determined largely by the pressure that air is under. As you go higher in the atmosphere the pressure drops. Thus the air can hold less water vapor and water molecules start to condense into clouds.
Any higher and the air has already lost enough water vapor. Any lower and the air pressure is enough to keep the water vapor in the air.
A cloud is sort of a patch of air totally saturated with water vapor. If the air could hold more water, you wouldn’t have this mass of visible droplets we call a cloud. We don’t usually have clouds at ground level because it’s warm enough that the air can hold a lot of water (when it can’t hold it all, we get fog.)
As you go higher, the air gets thinner and colder. This means that there’s an altitude where the air is 99% saturated with water vapor, and just above it, an altitude where the air can’t hold any more water, aka its “dew point.” That sharp line you see at the base of clouds is where the air goes from “just warm and dense enough” to “just cold and thin enough.”
Clouds can be at basically any altitude where there’s air. “That height” as you’ve asked, doesn’t exist.
Clouds at ground level are called “fog”
Non-fog clouds can be anywhere to include over 50,000 feet. There are thunderstorms, for example, that are completely clouded from 100ft all the way up to 50,000 feet. You can also have layered clouds, where (for example) you’ve got a cloud layer from 1,000 to 5,000, then clear air from 5k to 10k, then more clouds from 10k to 25k, then clear air above that.
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