I can't say much about what cultures thought clouds were. But I can tell you that the West has known what clouds are at least since Aristotle.

Aristotle's Meteorologica deals with clouds, and he describes them correctly as water being heated up by the sun, becoming a vapour, rising up, and condensing in the atmosphere, where it falls back down.

This is how Aristotle describes it:

The efficient and chief and first cause is the circle in which the sun moves. For the sun as it approaches or recedes, obviously causes dissipation and condensation and so gives rise to generation and destruction. Now the earth remains but the moisture surrounding it is made to evaporate by the sun's rays and the other heat from above, and rises. But when the heat which was raising it leaves it, in part dispersing to the higher region, in part quenched through rising so far into the upper air, then the vapour cools because its heat is gone and because the place is cold, and condenses again and turns from air into water. And after the water has formed it falls down again to the earth.

The exhalation of water is vapour; air condensing into water is cloud. Mist is what is left over when a cloud condenses into water, and is therefore rather a sign of fine weather than of rain; for mist might be called a barren cloud.

So we get a circular process that follows the course of the sun. For according as the sun moves to this side or that, the moisture in this process rises or falls. We must think of it as a river flowing up and down in a circle and made up partly of air, partly of water. When the sun is near, the stream of vapour flows upwards; when it recedes, the stream of water flows down : and the order of sequence, at all events, in this process always remains the same.

(Aristotle, Meteorologica, 346b16-34, web: https://archive.org/stream/workstranslatedi03arisuoft#page/n41/mode/2up )

It would be interesting to see some other perspectives on this:

• How did other civilizations think about meteorology? e.g. China, Africa, America)
• How did the West think about meteorology in the middle ages, before the 12th century?
• What were the folk theories of meteorology in ancient Greece, or in the middle ages? Were these very different from the opinions of the scholars?

Unfortunately, I don't know the answer to these questions from the top of my head. I only know of Aristotle (because I attended a lecture on Aristotle's meteorology not too long ago), and the developments in early modern science (because that's my own area of expertise).

But it would be nice if someone else could shed some further light on this question.