Here's a look at what you need to know about tornadoes, which are funnel-shaped clouds that forms under thunderclouds and contain rapidly rotating air.
Most tornadoes form from severe thunderstorms. Hurricanes can also produce tornadoes.
Tornado winds may exceed 300 mph (480 kilometers).
Tornadoes can lift cars, mobile homes, and animals into the air.
Tornadoes are sometimes called "twisters."
The damage path of a tornado is usually less than 1,600 feet wide.
Most tornadoes move at less than 35 mph.
Most tornadoes last only a few minutes.
The most destructive and deadly tornadoes occur from supercells, which are rotating thunderstorms with a well-defined radar circulation called a mesocyclone. Supercells can also produce damaging hail, severe non-tornadic winds, unusually frequent lightning, and flash floods.
A tornado over a body of water is called a "waterspout."
The United States has the highest number of tornado occurrences in the world with an average of 1,000 tornadoes reported each year.
According to the National Weather Service, in 2012 there were 70 tornado-related deaths in the U.S.
Most of the tornadoes in the United States strike in Tornado Alley, which spans the Midwest and Southern states.
Tornadoes usually occur during the spring and early summer, most often in the late afternoon and early evening.
A tornado watch is issued by the National Weather Service when atmospheric conditions promote the forming of tornadoes.
A tornado warning is issued when Doppler radar detects a mesocyclone in a thunderstorm, or when a funnel cloud has been spotted.
A tornado emergency is enhanced wording in a tornado warning indicating a large tornado is moving into a heavily populated area. Significant widespread damage and numerous fatalities are likely. The term was coined by forecasters in May 1999 and is used sparingly.
Enhanced Fujita Scale: The Fujita scale is used to estimate the wind speed of a tornado by the damage the tornado causes.
EF0 is the weakest point on the Enhanced Fujita scale and EF5 is the strongest.
An EF5 tornado can tear a house off its foundation.
Category EF0 Wind in mph: between 65 - 85. Light damage. Some damage to chimneys; branches broken off trees; shallow-rooted trees pushed over; sign boards damaged.
Category EF1 Wind in mph: between 86 - 110. Moderate damage. Peels surface off roofs; mobile homes pushed off foundations or overturned; moving autos blown off roads.
Category EF2 Wind in mph: between 111 - 135. Considerable damage. Roofs torn off frame houses; mobile homes demolished; boxcars overturned; large trees snapped or uprooted; light-object missiles generated; cars lifted off ground.
Category EF3 Wind in mph: between 136 - 165. Severe damage. Roofs and some walls torn off well-constructed houses; trains overturned; most trees in forest uprooted; heavy cars lifted off the ground and thrown.
Category EF4 Wind in mph: between 166- 200. Devastating damage. Well-constructed houses leveled; structures with weak foundations blown away some distance; cars thrown and large missiles generated.
Category EF5 Wind in mph: 200+. Incredible damage. Strong frame houses leveled off foundations and swept away; automobile-sized missiles fly through the air in excess of 100 meters (109 yards); trees debarked; incredible phenomena will occur.
March 18, 1925 - One of the worst tornado disasters in the United States; 695 people in the tri-state area of Missouri-Illinois-Indiana are killed. It is the longest-lived and has the longest path of any recorded U.S. tornado.
1950 - The U.S. begins keeping official records about tornadoes.
April 3-10, 1974 - There are 148 tornadoes in 16 states.
May 12-18, 1995 - There are 173 tornadoes in 18 states.
May 5-10, 2003 - There are 395 tornadoes reported in 19 states.
Feb. 2, 2007 - At least 20 people are killed in Lake and Volusia counties in Florida after at least three tornadoes touch down in the middle of the night.
March 1, 2007 - At least 20 people are killed, one in Missouri, 10 in Alabama, and nine in Georgia from a string of tornadoes. In Alabama, eight of the 10 killed are teenagers from Enterprise High School in Enterprise, Ala.
February 5, 2008 - At least 56 people are killed, 32 in Tennessee, 13 in Arkansas, seven in Kentucky, and four in Alabama from a string of tornadoes.
March 14, 2008 - A tornado reaching EF-2 strength at times hits downtown Atlanta, damaging the World Congress Center, CNN Center, the Georgia Dome, Cotton Mill Lofts, and many other buildings.
May 9-11, 2008 - A series of tornadoes kills 22 in three states including six in Ottawa County, Okla.; 13 in Newton County, Mo.; one in Jasper County, Mo.; one in an area of Purdy in Barry County, Mo., and one in Laurens County, Ga.
April 14-16, 2011 - At least 114 tornadoes touch down in Oklahoma, Kansas, Louisiana, Mississippi, Alabama, Missouri, Illinois, Georgia, South Carolina, North Carolina, Virginia, Maryland, Arkansas and Pennsylvania. Of the 46 fatalities reported, 23 occur in North Carolina.
April 25-28, 2011 - An outbreak of 201 confirmed tornadoes occurs from 8 a.m. ET April 25 to 8 a.m. ET April 28, 2011. There are approximately 321 fatalities in six states during the entire outbreak from April 25 to April 28. The majority of fatalities occur in Alabama, where as many as 243 people perish. Other states reporting fatalities are Mississippi, Tennessee, Georgia, Virginia and Arkansas. In terms of multi-day outbreaks, this outbreak holds the record for the largest number of tornadoes.