The Fujita and Pearson Tornado Scale was developed in 1971 by University of Chicago professor T. Theodore Fujita (1920–1998) and Allen Pearson (1925–), who was then the director of the National Severe Storms Forecast Center. It ranked tornadoes by their wind speed, path, length, and width. Tornadoes are not assessed based on actual wind speed and damage, but rather the scale determines wind speed based on damage. Sometimes known simply as the Fujita scale, the rankings ranged from F0 (very weak) to F6 (inconceivable).

F0—Light damage: damage to trees, billboards, and chimneys.
F1—Moderate damage: mobile homes pushed off their foundations and cars pushed off roads.
F2—Considerable damage: roofs torn off, mobile homes demolished, and large trees uprooted.
F3—Severe damage: even well-constructed homes torn apart, trees uprooted, and cars lifted off the ground.

F4—Devastating damage: houses leveled, cars thrown, and objects become flying missiles.
F5—Incredible damage: structures lifted off foundations and carried away; cars become missiles. Less than two percent of tornadoes are in this category.
F6—An F6 tornado has never been recorded, but we surmise the damage would be devasting.