The lowest rating on the original Fujita scale was F0.
Introduced in 1971 by meteorologist Ted Fujita, the scale ranked tornadoes from F0 through F5 in increasing order of damage. F0 represented the weakest category and was associated with estimated winds of roughly 40–72 mph. Typical effects included minor roof or chimney damage, broken branches, damaged signs, and some broken windows.
F0 is often confused with EF0, but the labels belong to different systems. EF0 is the corresponding lowest category on the newer Enhanced Fujita scale, which replaced the original Fujita scale in the United States on February 1, 2007. The original scale also did not use T ratings; “T” categories are associated with other tornado-intensity systems or informal terminology.
Both scales are fundamentally damage-based rating systems. Their wind-speed ranges are estimates inferred from damage surveys, and they are not equivalent to a direct measurement of the tornado’s maximum wind.