There are dozens of types of clouds, but they can all be classified into three main categories: cirriform, stratiform, and cumuliform. Cirriform clouds are feathery and wispy; they are made of ice crystals and occur at high elevations. Stratiform clouds are sheet-like and spread out across the sky. Cumuliform clouds are the ubiquitous cloud—puffy and individual, they can be harmless or they can be the source of torrential storms and tornadoes.