Clouds are made up of very tiny drops of water or tiny crystals of ice that float in the air. Clouds form when warm air, containing moisture evaporated from the earth's surface, rises into the sky and begins to cool. As the air cools, some of the vapor it contains condenses into tiny water droplets. If the cloud forms very high in the sky, where the temperature is freezing, the condensing water forms tiny ice crystals instead of water droplets. The droplets or bits of ice cluster together and make rain clouds, snow clouds, or fair weather clouds.