Hopefully, this won't disappoint Joni Mitchell fans too much, but clouds are not actually bows of angel hair and ice cream castles in the air. A cloud is a visible mass of water droplets, or ice crystals, or a mixture of both that is suspended above the Earth's surface. Clouds are formed when moist, warm air rises. As it ascends higher and reaches a space that's cooler, the moist warm air cools down, too, and the water vapor condenses back into tiny water droplets and/or ice crystals, depending upon how cold they get. Those droplets and crystals stay massed together because of the principle of cohesion, which we've previously discussed. The result is a cloud [source: Britannica ]. Some clouds are thicker than others because they happen to have a higher density of water droplets.
Clouds are a key part of our planet's hydrologic cycle, in which water continually moves between the surface and the atmosphere, and changes in state from liquid to vapor to liquid, and sometimes to solid as well. If it weren't for that cycle, there probably wouldn't be any life on our planet [source: NASA].
In 1803, a meteorologist named Luke Howard came up with four main cloud classifications, whose names were based on Latin words. Cumulus, which is the Latin word for "pile," describes those heaped, lumpy clouds that we often see in the sky. Cirrus, which means "hair," is the term for high-level clouds that look wispy, like locks of hair. Flat-looking, featureless clouds that form sheets are called stratus, which is the Latin word for "layer." Finally, there are nimbus clouds (the name actually is Latin for "precipitating cloud") are low, grey rain clouds [source: NASA]. And sometimes they combine – like the very tall grey lumpy clouds you see before thunderstorms – called a cumulonimbus!