376. Why, if they are invisible when they rise, do they became visible when they have ascended?
Because the vapours become cooled in passing through the air, and form a denser body.
377. Why, when they are condensed, do they not follow the course of gravitation, and descend?
Because the vapours form into minute vesicles, which we may call vapour bubbles, and these, being warmed by the sun, are specifically lighter than the air.
Because, also, the lower parts of clouds do partially descend, but again becoming more rarefied by meeting with a warmer atmosphere, they again ascend, and are thus poised upon the air.
Because, also, there is always a degree of atmospheric motion upward, caused by the convection of heat from the earth's surface. And, although there must also be downward movements of the air to supply the place of that which has ascended, still the heat of the ascending air, combined with its upward movement, expands and floats the vapour of the clouds.
378. At what height do clouds usually fly?
They fly at every degree of altitude; but clouds of specific character are said to fly at given altitudes, or to occupy certain ranges of altitude. We will give their probable altitudes when speaking of the specific clouds.