The classification of tornado intensity has long relied on post-storm damage surveys to estimate the wind speeds involved. For decades, the primary tool for this assessment was the Fujita Scale, or F-Scale, developed in 1971 by Dr. Tetsuya Theodore Fujita. This scale provided meteorologists with a standardized method for ranking tornadoes from F0 to F5, based on the severity of the destruction left in the tornado’s path. The F-Scale became the historical benchmark for understanding the power of tornadoes.
The Last Tornado Rated F4
The final tornado to be officially assigned an F4 rating under the original Fujita Scale occurred on September 22, 2006, striking near Crosstown, Missouri. This violent storm was part of a broader outbreak across the Midwest. The tornado reached its maximum intensity near Crosstown, where estimated winds peaked near 210 miles per hour.
The path of destruction resulted in an F4 classification from the damage survey team. Along the track in Perry County, Missouri, over sixty homes were completely destroyed, while dozens more sustained major damage. The force of the wind was powerful enough to toss numerous vehicles and snap hundreds of trees and utility poles. The tornado eventually weakened to an F2 rating as it crossed the Mississippi River into Jackson County, Illinois.
Defining the F4 Rating
The F4 rating on the original Fujita Scale represented “Devastating” damage, indicating intensity just below the maximum F5 classification. This rating correlated with estimated wind speeds ranging from 207 to 260 miles per hour. The criteria required the leveling of well-constructed houses, meaning the structure was completely reduced to rubble or swept away.
Damage surveys would also document structures with weak foundations being blown away for some distance. The wind speeds were sufficient to generate large missiles from debris and throw cars significant distances. This damage level signified that only the most robust structures could withstand the force, with most residential buildings facing near-total destruction.
The Transition to the Enhanced Fujita Scale
The original F-Scale possessed inherent scientific limitations that prompted a need for a more accurate system. A primary issue was the overestimation of wind speeds, particularly in the higher categories, discovered through engineering studies. The scale’s wind speed correlations were largely educated guesses rather than being rigorously verified against actual wind measurements.
A major concern was that the F-Scale did not account for the quality of construction. This meant a poorly built house could be destroyed by a weaker tornado and still receive a high rating, inaccurately inflating the estimated wind speed. To address these flaws, the Enhanced Fujita Scale, or EF-Scale, was developed by a team of meteorologists and wind engineers. The EF-Scale was officially implemented in the United States on February 1, 2007, replacing the original system.
The methodology of the EF-Scale marked a significant improvement by incorporating 28 different damage indicators (DIs) and varying degrees of damage (DoD). This change allowed for a more detailed analysis of the damage based on the specific type of structure affected, such as a mobile home, school, or well-built residential house. For instance, the estimated wind speed range for an EF4 tornado was adjusted downward to 166 to 200 miles per hour. The EF-Scale provides a more standardized and scientifically sound assessment of a tornado’s intensity.