Tetsuya Theodore "Ted" Fujita (1920-1998) is famous for developing the Fujita Tornado Intensity Scale, a scale used to measure the strength of a tornado based on the damage it produces.
Fujita was born in Japan and studied the damage caused by the atomic bomb in Hiroshima. He developed his scale in 1971 while working as a meteorologist with the University of Chicago. The Fujita Scale (also known as the F-Scale) typically consists of six ratings from F0 to F5, with damage rated as light to incredible. Sometimes, an F6 category, the "inconceivable tornado" is included in the scale.
Since the Fujita Scale is based on damage and not really wind speed or pressure, it is not perfect. The primary problem is that a tornado can only be measured in the Fujita Scale after it has occurred. Secondly, the tornado can not be measured if there is no damage when the tornado occurs in an area without any features to be damaged. Nonetheless, the Fujita Scale has proven to be a reliable measurement of the strength of a tornado.
Tornado damage needs to be examined by experts in order to assign a Fujita Scale rating to the tornado. Sometimes tornado damage appears worse than it actually is and sometimes, the media may overemphasize certain aspects of the damage tornadoes can cause. For example, straw can be driven into telephone poles at speeds as low as 50 mph.
The Fujita Tornado Intensity Scale
F0 - Gale
With winds of less than 73 miles per hour (116 kph), F0 tornadoes are called "gale tornadoes" and cause some damage to chimneys, damage sign boards, and break branches off of trees and topple shallow-rooted trees.
F1 - Moderate
With winds from 73 to 112 mph (117-180 kph), F1 tornadoes are called "moderate tornadoes." They peel surfaces off of roofs, push mobile homes off of their foundations or even overturn them, and push cars off of the road. F0 and F1 tornadoes are considered weak; 74% of all measured tornadoes from 1950 to 1994 are weak.
F2 - Significant
With winds from 113-157 mph (181-253 kph), F2 tornadoes are called "significant tornadoes" and cause considerable damage. They can tear the roofs off of light frame houses, demolish mobile homes, overturn railroad boxcars, uproot or snap large trees, lift cars off the ground, and turn light objects into missiles.
F3 - Severe
With winds from 158-206 mph (254-332 kph), F3 tornadoes are called "severe tornadoes." They can tear the roofs and walls off of well-constructed houses, uproot the trees in a forest, overturn entire trains, and can throw cars. F2 and F3 tornadoes are considered strong and account for 25% of all tornadoes measured from 1950 to 1994.
F4 - Devastating
With winds from 207-260 mph (333-416 kph), F4 tornadoes are called "devastating tornadoes." They level well-constructed houses, blow structures with weak foundations some distances, and turn large objects into missiles.
F5 - Incredible
With winds from 261-318 mph (417-509 kph), F5 tornadoes are called "incredible tornadoes." They lift and blow strong houses, debark trees, cause car-sized objects to fly through the air, and cause incredible damage and phenomena to occur. F4 and F5 tornadoes are called violent and account for a mere 1% of all tornadoes measured from 1950 to 1994. Very few F5 tornadoes occur.
F6 - Inconceivable
With winds above 318 mph (509 kph), F6 tornadoes are considered "inconceivable tornadoes." No F6 has ever been recorded and the wind speeds are very unlikely. It would be difficult to measure such a tornado as there would be no objects left to study. Some continue to measure tornadoes up to F12 and Mach 1 (the speed of sound) at 761.5 mph (1218.4 kph) but again, this a hypothetical modification of the Fujita Scale.