(CNN)Here's some background information about tornadoes, which are funnel-shaped clouds that form under thunderclouds and contain rapidly rotating air.
Most tornadoes form from severe thunderstorms. Hurricanes can also produce tornadoes.
The Fujita scale is used to estimate the wind speed of a tornado by the damage the tornado causes.
March 18, 1925 - Tri-state area of Missouri, Illinois and Indiana - 695 fatalities.
(in 2015 dollars)
May 22, 2011 - Joplin, Missouri - $2.8 billion (actual cost) - $2.92 billion (adjusted for inflation)