Cumulonimbus clouds are very large and vertically developed into "towers." These towers can exist individually or in a line, known as "squall lines". They are fuelled by convective updrafts, which can be in excess of 50mph. Cumulonimbus can reach 39000 feet (12000 metres) or higher. The top of cumulonimbus spread out as they reache the tropopause, forming a characteristic anvil shape. The base of the cloud consists of water droplets, while the top of the cloud is made up of ice crystals due to the atmospheric temperature. Cumulonimbus are rain clouds.