What causes clouds to form?

Clouds are possibly the most interesting (and beautiful) of all weather phenomena. While there is a wide variety of cloud shapes and sizes, they are all made of the same thing: condensed water or ice. The air in the cloud has been cooled (almost always because it is rising) and it can no longer hold all of thewater vapor it contains. Some of that (invisible) water vapor condenses to form (visible) cloud droplets or ice crystals.