Tornado Facts & History

What is a Tornado?

A tornado appears as a rotating, funnel-shaped cloud that extends from a thunderstorm to the ground with whirling winds that can reach 300 miles per hour. Damage paths can be in excess of one mile wide and 50 miles long. Some tornadoes are clearly visible, while rain or nearby low-hanging clouds obscure others. Occasionally, tornadoes develop so rapidly that little, if any, advance warning is possible.

Quick Tornado Facts

Signs of Danger

  • Dark, often greenish sky
  • Large hail
  • A large, dark, low-lying cloud (particularly if rotating)
  • Loud roar, similar to a freight train

Before a tornado hits, the wind may die down and the air may become very still. A cloud of debris can mark the location of a tornado even if a funnel is not visible. Tornadoes generally occur near the trailing edge of a thunderstorm. It is not uncommon to see clear, sunlit skies behind a tornado.

  • Tornadoes cause an average of 70 fatalities and 1,500 injuries in the U.S. each year.
  • The strongest tornadoes have rotating winds of more than 250 mph.
  • Tornadoes can be more than one mile wide and stay on the ground for over 50 miles.
  • Tornadoes may appear nearly transparent until dust and debris are picked up or a cloud forms within the funnel.
  • The average forward speed is 30 mph but may vary from nearly stationary to 70 mph.
  • Waterspouts are tornadoes which form over warm water. They can move onshore and cause damage to coastal areas.

Where Tornadoes Happen

  • The average tornado moves Southwest to Northeast, but tornadoes have been known to move in any direction.
  • “Tornado Alley” is a nickname given to an area in the southern plains of the central U.S. that consistently experiences a high frequency of tornadoes each year. Tornadoes in this region typically happen in late spring and occasionally the early fall.

When Tornadoes Happen

  • They may strike quickly, with little or no warning.
  • Tornadoes can accompany tropical storms and hurricanes as they move onto land.
  • Peak tornado season in the southern states is March through May; in the northern states, it is late spring through early summer.
  • Tornadoes are most likely to occur between 3 pm and 9 pm, but can occur at any time.

The Enhanced Fujita Tornado Scale

Remember when movies and news reports talked about “F3 tornadoes” or “F5 tornadoes”? That’s the Fujita Scale, devised by Dr. T. Theodore Fujita of the University of Chicago in 1971. For almost forty years, scientists rated tornado windspeed on the F0-F5 scale based on the damage caused by a tornado.

EF Number and Windspeed

  • EF0 -> 65-85 mph
  • EF1 -> 86-110 mph
  • EF2 -> 111-135 mph
  • EF3 -> 136-165 mph
  • EF4 -> 166-200 mph
  • EF5 -> over 200 mph

But the devastating tornadoes in Jarrell, TX in 1997 and Moore/Oklahoma City in 1999 demonstrated to many engineers, emergency managers and meteorologists that there were flaws in the original Fujita system. In 2006, the National Weather Service unveiled the Enhanced Fujita Tornado Scale based on four years of research. In 2007, the EF-scale replaced the original F-scale in all tornado damage surveys in the United States.

Overall, most tornadoes (around 77 percent) in the U.S. are considered weak (EF0 or EF1) and about 95 percent of all U.S. tornadoes are below EF3 intensity. The remaining small percentage of tornadoes are categorized as violent (EF3 and above). Of these violent twisters, only a few (0.1 percent of all tornadoes) achieve EF5 status, with estimated winds over 200 mph and nearly complete destruction. However, given that on average over 1,000 tornadoes hit the U.S. each year, this means that 20 of these can be expected to be violent and one could possibly be incredible (EF5).