Clouds are formed when warm moist air rises into the sky. When it reaches a certain height, the warm air cools down and the water vapour in it condenses into small drops of water, forming clouds. There are several different types of clouds. The ones which are highest are usually made up of ice, and are the thinnest. These are called cirrostratus and cirrocumulus clouds. Lower clouds are called altostratus and altocumulus clouds. Lower still are the stratocumulus clouds and the nimbostratus clouds the lowest of all are the stratus clouds, which are seen as fog over high, mountains regions. Thunder clouds are called cumulus and cumulonimbus clouds, and these are the fat, cauliflower shaped ones.