A cloud is only a fog, higher up in the air.
Do fogs ever ascend in the air?
Yes; they often ascend, and thus produce clouds.
What causes the vapor of the air to be condensed into clouds?
Cold causes it to be condensed into clouds.
Does the air grow colder as we ascend?
Yes; the higher we ascend, the colder we find the atmosphere.
What becomes of the vapor in the air, as it ascends from the earth?
The vapor is condensed into clouds in the upper regions of the air.
Why do we mostly see clouds at the top of a mountain?