Tornadoes

The Enhanced Fujita scale became operational on 1 February 2007, replacing the original Fujita scale.

It runs from EF0, with estimated three-second gusts of 65 to 85 mph, to EF5, with gusts over 200 mph.

The scale stops at EF5. Damage cannot be rated higher, however strong the wind.

The wind speeds are estimates worked out from damage to 28 types of structure and tree, not readings taken inside the tornado.

Size does not show strength. Some tornadoes more than a quarter of a mile wide have caused only EF0 to EF1 damage, while some narrow rope tornadoes have caused EF4 or EF5 damage.

A tornado can last from several seconds to more than an hour, but most last less than 10 minutes.

Since 1950 the average tornado has travelled about 3.5 miles.

Forward speed ranges from almost stationary to more than 60 mph, with 10 to 20 mph typical.

Tornadoes occur mostly in spring and summer, usually in the late afternoon and early evening.

Most tornadoes in the Northern Hemisphere rotate counterclockwise, and most in the Southern Hemisphere rotate clockwise.

Recent years point to about 1,200 tornadoes a year in the United States, give or take a few hundred.

The true long-term average is unknown, because spotting and reporting methods have changed so much that older records are believed to be incomplete.

Official tornado records in the United States go back only to 1950.

Tornadoes have been reported in all 50 states, and they are most common in the Central Plains and the southeastern states.

Tornado Alley is a popular nickname for a broad swath of the central United States where tornadoes are relatively frequent, and maps of it differ.

The tornado near El Reno, Oklahoma, on 31 May 2013 had a maximum path width of 2.6 miles, the widest on record.

It narrowly exceeded the F4 tornado at Hallam, Nebraska, on 22 May 2004.

Surveyors rated the El Reno tornado EF3, because they found no damage that supported a higher rating, even though radar had measured much stronger winds.

A University of Oklahoma mobile Doppler radar detected near-surface winds as strong as 295 mph in the El Reno tornado, and a research paper reports ground-relative Doppler velocities of at least 135 metres per second.

The Tri-State tornado of 18 March 1925 killed 695 people across parts of Missouri, Illinois and Indiana, and produced F5 damage.

It is the deadliest tornado in United States history.

Death counts for events in the 1800s and early 1900s are estimates, because comprehensive damage surveys did not yet exist.

The Joplin, Missouri, tornado of 22 May 2011 killed 158 people.