Extending from the tropics to the polar regions, the continental plains of North America and Asia are the favored locations of tornadoes. A tornado is a violently revolving funnel cloud usually a few hundred meters in diameter. Intense upward flow of air at its center leads to very low pressure there. The funnel itself is visible because it contains small water droplets formed by cooling and condensation of air expanding into the low pressure region. Heat given up by the condensation evidently provides the energy to drive the rapidly rotating air of the outer funnel at speeds as high as 500 miles per hour.
Most tornadoes make a roaring noise and some are silent.
They often occur in pairs. A man in Iowa watched a tornado pass by his farm, then turned around to find his chicken house located only a few feet from where he stood, silently stolen away by a second tornado.