Tornadoes have occurred in all 50 states and throughout much of the world, from Europe and Africa to Asia and Australia. Approximately 1,200 tornadoes strike the U.S. each year, accounting for numerous deaths and billions of dollars of damage. The current costliest tornado on record is the EF5 tornado that struck Joplin, Missouri, on May 22, 2011, causing 161 deaths and nearly $3 billion in damage.
Rotating supercell thunderstorms, or mesocyclones, frequently spawn deadly tornadoes. The rotation of the thunderstorm causes an updraft of warm air (Step 1) that creates a horizontal vortex in the thunderstorm. A cool air downdraft (Step 2) pushes against the warm updraft creating a vertical spin. As more air and moisture get sucked into the vortices, the funnel gains definition and strength (Step 3).
Tornado Alley is a media term for the central plains that is known for a high frequency of tornadoes. The collision of cold, dry air from Canada with warm, moist air from the Gulf of Mexico creates perfect ingredients for supercell thunderstorms.
The Fujita Scale
Tetsuya “Ted” Fujita, a meteorologist at the University of Chicago, devised his tornado intensity scale in 1971. The F-scale, as it became known, is an estimate of wind speed based on damage done to homes and other buildings. The scale classifies tornadoes into six damage categories from F0, the weakest tornado, to F5, the most destructive.
The Fujita Scale correlates with the Beaufort Wind Speed Scale (in use by ships since 1805) and Mach numbers (ratio of the speed of an object to the speed of sound). Although the Fujita Scale ranges up to F12, the strongest possible tornadoes are in the F5 range.
In 2007, the Enhanced Fujita (EF) Scale replaced the original Fujita scale. The EF-Scale is based on eight levels of damage to 28 different types of structures and vegetation known as “Damage Indicators.”