Tornado - Wikipedia

A tornado is a violent windstorm characterized by a twisting, funnel-shaped cloud.

It is spawned by a supercell 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. Many tornadoes are the tail end of a mesocyclone and they have a characteristic "hook echo" signature on a radar screen. The damage from a tornado is a result of the high wind velocity and wind-blown debris. Tornado winds range from a slow 40 mph at the low end to a possible 300 mph in the strongest storms. 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, marked only by swirling debris at the base of the funnel. Others are composed of several mini-funnels. A tornado must by definition have both ground and cloud contact.

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 over 1,000 tornadoes and approximately 50 death per year. The deadliest US tornado on record is the 18 March 1925 Tri-State tornado that went across southeastern Missouri, southern Illinois, and southern Indiana, killing 695 people.

The intensity of tornadoes is given by the Fujita - Pearson Tornado Scale (also known simply as Fujita scale). The intensity can be derived directly with high resolution Doppler radar wind speed data, or empirically derived from structural damage compared to engineering data. Also, note that intensity does not refer in any way to the size, or width, of a tornado.