Strong upper winds in the stratosphere smooth and spread the top of the cloud. This make the cloud look like the top of a mushroom or an anvil. The ice crystals in the anvil cloud give it a fuzzy look. As a thunderstorm grows, water drops or ice crystals inside the cloud hit and mix with each other, getting bigger. The bottom of the cloud grows dark with water about to fall. When the drops become so heavy that the winds inside the cloud cannot keep them up any more, they fall from the cloud as rain or hail. Even as they fall the water drops grow by mixing with the smaller drops, and makes big raindrops. Hail becomes bigger inside the cloud. One thunderstorm cloud usually runs out of energy in about 30 to 50 minutes. But supercell thunderstorms can last for hours until they dissipate by outflow.