As big, dark thunderstorm clouds roll in, a siren sounds. It is time for people to seek shelter from this dangerous storm. Tornadoes are violent windstorms that can develop inside thunderstorms when warm, moist air meets cold, dry air. A tornado forms when the warm air rises above the cold and twists into a funnel-shaped cloud. This is why tornadoes are sometimes called twisters.
Funnel clouds can drop out of thunderstorms with little or no warning.
These snake-like clouds extend down and sometimes touch the ground. When they do touch down, they can cause damage. Tornado funnels act like big vacuum cleaners. Tornadoes pick up dirt, trees, buildings and everything else that their violent winds destroy. Tornado wind speeds can reach 300 miles per hour.