The National Weather Service of the US adopted the Fujita scale for rating tornado damage in 1975. Since then, all tornadoes in the US have received a rating. In addition, in the late 1970s, an effort was made to rate all tornadoes in the official NWS archive from 1950-1974 and back to 1916 as part of the “DAPPLE” project, based on damage descriptions that were written at the time of the tornado (e.g., newspaper accounts).

Tornadoes in numerous other countries have also received Fujita scale ratings, some as part of official national weather service efforts and some by individuals outside of weather service duties. The distribution of tornadoes by Fujita scale in most parts of the US and in most other countries is nearly log-linear and shows a consistent slope. In the US, there are two limiting distributions—one associated with Florida, the Front Range counties of Colorado, and the west coast, and the other associated with most of the eastern 2/3 of the US. The second distribution is consistent with processes that are much more likely to produce violent tornadoes than the first, providing evidence that two distinct mechanisms, perhaps one associated with supercells and the other not, leading to tornadogenesis. Most of the countries of the world show distributions much closer to the first distribution than the second.

Studies of proximity soundings associated with US tornadoes from 1957-1999 indicate that environments associated with F2 and stronger tornadoes have been relatively consistent from 1973 on, but that many previous F2 tornadoes occurred in environments that would not typically be associated with tornadoes currently. I hypothesize that the tornadoes in the 1957-1972 period were rated more strongly than at present, perhaps because of the retrospective nature of the ratings and because of different groups of people being responsible for verifying tornado reports (state climatologists prior to 1972 and the NWS since then.)