What Is a Tornado?

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, NOAA says about 1,000 tornadoes are reported nationwide.

I.I.I. says tornadoes have caused $97.8 billion in insured losses in the U.S. between 1990 and 2009, making these weather events second only to hurricanes ($152.4 billion) over this same time period as the costliest natural disasters.

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.

Source: NOAA

2008-2010

A tornado funnel cloud touches down in Orchard, Iowa on June 10, 2008 (AP Photo/Lori Mehmen)

From 2008-2010, severe thunderstorms and tornadoes have caused about $30 billion in insured damages.

The number of tornadoes and the deaths they caused were up in 2010, compared with 2009, but below the levels seen in 2008, I.I.I. says. NOAA reports that there were 1,282 tornadoes in 2010, up from 1,146 in 2009, and fatalities reached 45, after dropping to 21 in 2009.

Tornadoes and thunderstorms caused $13.2 billion in economic losses and $9.5 billion in insured losses in 2010, according to Munich Re. This was the third consecutive year thunderstorms caused over $9 billion in insured losses. Thunderstorms and tornadoes accounted for the largest share of insured losses from natural disasters in 2010, topping winter storms ($2.6 billion), floods ($1.1 billion), earthquakes ($128 million) and tropical cyclones ($120 million), according to Munich Re.

Two Red Cross workers walk away from a barn in Mount Pleasant, Wis. that was lifted off its foundation by a tornado on Oct. 26, 2010. (AP Photo/Carrie Antlfinger)

2011

Blue tarps cover the roofs homes still livable along the path of an April 2011 tornado in Tuscaloosa, Ala. (AP Photo/Dave Martin)

The year 2011 saw the two costliest tornado events as far as insured losses, as well as the deadliest tornado since modern record-keeping began in 1950.

According to the National Oceanic and Atmospheric Administration (NOAA) there were 1,691 tornadoes in 2011, including 758 tornadoes in April, the highest monthly toll on record.

Storms that spawned the tornadoes in Tuscaloosa, Alabama in April accounted for $7.5 billion in insured losses, good for the 10th costliest natural-catastrophe event, according to ISO. The storm system that caused the May 2011 tornado in Joplin, Missouri caused $7 billion in insured damages, and was only recently bounced from the top-10 list by Superstorm Sandy. The Joplin tornado caused 158 deaths, making it the deadliest tornado since modern record-keeping began, according to NOAA.

Overall, insured losses from tornadoes/thunderstorms were over $25 billion in 2011, more than double the previous record, I.I.I. says.

A scene from Joplin, Mo. after a deadly tornado struck in May 2011 (Credit: Bryant Rousseau, PC360)

2012-2013

A photo taken May 24, 2012 shows a tornado seconds after lifting off the ground near Cornlea, Neb. (AP Photo/Nick Brichacek)

Severe U.S. thunderstorms, including tornado events, cost $14.9 billion in insured losses and $27.7 billion in economic losses in 2012, according to Munich Re.

This year, I.I.I. says the “devastating tornadoes which hit Oklahoma this May bring to the mind a powerful string of tornadoes that hit Oklahoma, Kansas, Texas, Georgia and 13 other states in 1999.”

Those tornadoes, according to ISO, resulted in $1.5 billion in insured losses ($2.0 billion in 2012 dollars) in all the affected states, with nearly $1.0 billion ($1.4 billion in 2012 dollars) in Oklahoma alone. Moore, Okla. was one area heavily impacted by the 1999 tornadoes.

Scenes from Moore, Okla. in on May 4, 1999 after a tornado flattened many houses. The powerful tornado in suburban Oklahoma City Monday, May 20, 2013, loosely followed the same path as the 1999 twister. (AP Photo/J. Pat Carter, File)

Editor's Note: Thanks to I.I.I. for compiling the above information.

Corrected to reflect that the storm systems that spawned the Tuscaloosa and Joplin tornadoes caused the $7.5 billion and $7 billion in insured damages, rather than the tornadoes by themselves.