Tornado
A tornado is a violent windstorm characterized by a twisting, funnel-shaped cloud.
File:Tornado thumbnail.jpg
From the NOAA Photo Library
It is spawned by a thunderstorm (or sometimes as a result of a hurricane) and produced when cool air overrides a layer of warm air, forcing the warm air to rise rapidly. The damage from a tornado is a result of the high wind velocity and wind-blown debris. Tornado season in North America is generally March through August, although tornadoes can occur at any time of year. They tend to occur in the afternoons and evenings: over 80 percent of all tornadoes strike between noon and midnight.
Tornadoes can be nearly invisible, and they look pretty cool, marked only by swirling debris at the base of the funnel. Some are composed almost entirely of windblown dust and still others are composed of several mini-funnels. A tornado must by definition have both ground and cloud contact. If it "jumps", i.e. has intermittent ground contact, it is regarded as several tornadoes after each other.
Tornadoes do occur throughout the world. However, the United States experiences by far the most tornadoes of any region of comparable size, and has also suffered the most intense ones. On average, the United States experiences 100,000 thunderstorms each year, resulting in approximately 1,000 tornadoes.
The intensity of tornadoes is measured by the Fujita - Pearson Tornado Scale (also known simply as Fujita scale).