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© Copyright 1997, Jim Loy
We hear that a cloud weighs many hundreds of tons. So, what holds a cloud up in the sky? A cloud is made up of tiny drops of liquid water. These are heavier than air, and slowly filter downward. The smaller the drops, the slower they fall due to air resistance. Turbulence and upward moving air currents can keep them up for quite a while. But, they do fall.
The clouds of steam that we see coming out of the tea kettle are made up of drops of water. Water vapor is an invisible gas, as you can tell by looking at the gap between the tea kettle's spout and the clouds of steam. Water vapor is all around us, and it is lighter than air. It rises with warm air and upward moving air currents. When this moist warm air enters a layer of cold air, the water cools and condenses into tiny drops, becomes part of a cloud, and starts falling. It falls into the warm air, and may evaporate again, and move up into the cold air over and over again. So, parts of a cloud are continually disappearing and reforming. And that is what keeps a cloud up in the sky.
Why are some clouds dark? The sun can shine right through a thin cloud. But, thicker ones are dark on bottom, where the sun's light doesn't penetrate. Storm clouds are sometimes miles thick, and are darkest of all.
Rain is when tiny drops become bigger, mainly by colliding and merging with other drops. Hail and sleet happens when rain drops are blown upward into freezing air. Hail stones may be blown upwards many times, each time gaining a layer of ice. Snow happens when moisture solidifies around a dust particle, in calm air, and the ice crystals build up rather slowly.
Note: In science, "water vapor" is gaseous water. "Steam" is gaseous water that results from boiling. "Vapor" is essentially the same as "gas" except that "gas" is used to describe a substance that is a gas at standard temperature and pressure, while "vapor" describes a substance that is not normally a gas under standard conditions. There is a major conflict here, with the common definition of "vapor." To the non-scientific world, "vapor" seems to mean a visible cloud (condensation or smoke or dust or whatever) coming from something, as your visible breath in cold weather, or condensation coming from your hot coffee. These vapors are all made up of tiny liquid drops (in the case of some clouds) or tiny solid particles (in the case of smoke, dust, or some clouds), suspended in the air.