Why do F-scale ratings differ between SPC and others tornado records?
The F-scale is based on one’s subjective judgment of damage intensity, and hundreds of people have rated tornadoes for Storm Data over the years. Currently, damage ratings for the “official” database are made by meteorologists at each local forecast office where tornadoes are reported. Sometimes, first-hand damage surveys are done, but because of travel budget constraints, ratings often must be made from spotter, chaser and/or media accounts. In order to assign F-scales to tornadoes from before about 1978 (the year varies from state to state), NSSFC contracted with college students to cross reference the NSSFC file with newspaper articles. When information that conflicted with Storm Data were uncovered, a judgment call was made as to what most likely occurred; and if necessary, the NSSFC information was “corrected” in the final data base. Also, several “new” tornadoes (previously unreported ones) were uncovered and added to the record. There were enough of these changes that the NSSFC
