Scouring the farmland of South Dakota with winds up to 200 miles an hour (322 kilometers an hour), a wedge-shaped tornado touches down near the town of Woonsocket on June 24, 2003. It was one of 67 tornadoes spawned in South Dakota that day by a massive collision of humidity and shearing winds that swept across the northern plains. More than a thousand tornadoes a year are generated in the continental United States.