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Convective storms are the result of warm, moist air rising from the earth, and depending on atmospheric conditions, may develop into tornadoes, hail, thunderstorms with lightning, or straight-line winds. Convective storms are the most common and damaging natural catastrophes in the United States, according to the Triple-I’s May 2020 white paper, Severe convective storms.
According to catastrophe modeling company RMS, insured losses in the United States from these storms average about $17 billion each year, nearly equal to the losses incurred by hurricanes. While scientists cannot say that these storms are increasing, given the relatively short periods of reliable, post-Doppler radar data, it is clear that the losses are increasing, as a result of population growth and economic development. In addition, the geographic span of these storms appear to be changing from what previously was called the “tornado alley” to the southern United States with tornado formation tending to develop further east.
A tornado is a violently rotating column of air that extends from a thunderstorm and comes into contact with the ground, according to the National Oceanic and Atmospheric Administration (NOAA). In an average year about 1,000 tornadoes are reported nationwide, according to NOAA. Tornado intensity is measured by the enhanced Fujita (EF) scale. The scale rates tornadoes on a scale of 0 through 5, based on the amount and type of wind damage. It incorporates 28 different damage indicators, based on damage to a wide variety of structures ranging from trees to shopping malls.
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(1) Original scale: wind speeds represent fastest estimated speeds over one quarter of a mile.
(2) Enhanced scale: wind speeds represent maximum 3-second gusts.
Source: U.S. Department of Commerce, National Oceanic and Atmospheric Administration.
2021 Tornadoes: In 2021, there were there were 1,376 tornadoes in the United States, compared with 1,075 in 2020, according to preliminary data from the National Oceanic and Atmospheric Administration (NOAA). Tornadoes killed over 100 people in 2021, including about 90 in the December 10 series of tornadoes. In 2020, 76 people perished in tornadoes.
The following chart shows the top 10 catastrophes involving tornadoes. It counts severe convective storms that may include tornadoes and other perils such as straight-line winds (derechos) and hail. The costliest U.S. catastrophe involving tornadoes occurred in August 2020 during a multi-day outbreak that included the August 10 Midwest Deracho causing $9.6 billion in insured losses in 2021 dollars.
($ millions)
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(1) Defined by Aon as severe convective storms including insured thunderstorm events and may include tornado, hail, damaging straight-line winds (derechos) and flash flood impacts from events. Includes events that occurred through 2020. Subject to change as loss estimates are further developed. As of February 1, 2022.
(2) Adjusted for inflation by Aon using the U.S. Consumer Price Index.
Source: Aon.
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(1) Tornadoes that cross state lines are counted in every state in which they touch down.
Source: U.S. Department of Commerce, Storm Prediction Center, National Weather Service.
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(1) Ranked by total number of tornadoes.
(2) States with the same number of tornadoes receive the same ranking.
(3) State had no tornadoes in 2021.
(4) The U.S. total will not match data used in other charts because it counts tornadoes that cross state lines.
Source: U.S. Department of Commerce, Storm Prediction Center, National Weather Service.
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(1) Excludes Puerto Rico. A tornado that crosses state lines counts as one event.
Source: U.S. Department of Commerce, Storm Prediction Center, National Weather Service.
Source: U.S. Department of Commerce, National Oceanic and Atmospheric Administration, National Weather Service.