Tornadoes are associated with large (supercell) thunderstorms that often grow to over 40,000 feet. Heat during the day sinks to the surface of the Earth, and a column of warm humid air will begin to rise very quickly, creating instability in the atmosphere.
In the above animation, winds that are at two different altitudes in the column blow at two different speeds and directions, creating what is known as wind shear.
That causes a horizontally rotating column of air in the atmosphere. As a thunderstorm matures, the column of air gets caught in an updraft, stretching it vertically. The speed of the spinning motion continues to increase, and the column tightens up, creating a funnel cloud. As the funnel cloud rotates and makes contact with the ground, a tornado is formed. A tornado can be confirmed by the debris ball at the surface.
Tornadoes are rated on a scale EF0 to EF5, the later producing winds in excess of 200 mph. EF5 tornado widths at the surface can measure over a mile wide and can leave destruction in the millions of dollars. The National Weather Service has confirmed that the tornado that hit Moore, Okla., was a top-of-the-scale EF-5. An F5 tornado roared through Moore, Oklahoma on May 3rd, 1999, leaving 36 people dead and 1.1 billion dollars in damage.