A tornado is a spinning, violent air column connected to the base of a cumulonimbus cloud. It forms during storms when cold and warm air combine, making the spinning air visible as it approaches the ground. Tornadoes most commonly occur in Tornado Alley in the Great Plains, where flat land allows winds to spiral without obstruction. Several conditions are needed for tornado formation, including a combination of cold, dry air and warm, moist air that creates thunderstorms, as well as wind shear causing wind rotation to change with altitude.