The three main types of clouds that you will see in the sky are cumulus, stratus and cirrus clouds. Cumulus clouds, which look like giant cotton balls, are formed by warm, moist air that rises and cools off quickly. Stratus clouds are formed by air rising very slowly over large areas, and are the clouds you see in the sky on those dark, gray days that often bring lots of steady rain. Cirrus clouds are the wispy clouds you see high in the sky. They are so high that the low temperatures cause them to be made up completely of ice crystals.