High, puffy little clouds do not change until a lot of moisture is added. This moisture helps the updraft. Moisture can add heat, which means the cloud will warm up inside and go up even faster. The moisture makes the cumulus cloud "mushroom" upwards and turn into a tall cumulus cloud. The winds inside this cloud are very strong. Thunder and lightning begins when the top of the cloud is 25,000 feet (7,280 m) high. The inside of the cloud is, by this time, cold enough so that the water drops are turned into ice crystals. A cumulus cloud stops growing when it hits the warm stratosphere. Some thunderstorms grow twice as high as Mount Everest!