F5 and EF5 tornadoes are the most powerful and violent weather phenomena on Earth. These storms are exceptionally rare but cause a disproportionately large amount of damage and fatalities. Understanding their frequency and categorization methods is necessary for assessing the true hazard they pose. The categorization system has evolved to more accurately reflect the immense power and damage caused by these high-impact events.
Historical Frequency of F5 and EF5 Tornadoes
The number of F5 and EF5 tornadoes recorded annually is remarkably small and highly variable. Since 1950, when reliable record-keeping began in the United States, the average frequency of these most intense tornadoes is about one per year. This average is based on 50 F5 tornadoes recorded between 1950 and 2007, plus the EF5 tornadoes recorded since the scale update.
The annual count is inconsistent, with some years experiencing multiple violent storms and others enduring long droughts. Major outbreaks, such as the Super Outbreak of 1974, accounted for a significant portion of the total F5 ratings in a single day. Conversely, over eight years passed between the last F5 in 1999 and the first EF5 in 2007. Only about 0.06% of all tornadoes are classified as F5 or EF5, compared to the roughly 1,250 tornadoes of all classifications that occur in the U.S. each year.
Defining the Most Powerful Tornadoes
The classification of these powerful storms uses a six-category system that has undergone significant revision. The original Fujita (F) Scale, developed in the 1970s, classified F5 tornadoes with estimated wind speeds ranging from 261 to 318 miles per hour. This scale had limitations, including a lack of detailed damage indicators and an overestimation of the required wind speeds.
The Enhanced Fujita (EF) Scale replaced the original scale in the United States on February 1, 2007, to provide a more accurate assessment of intensity. The EF5 category now represents estimated wind speeds exceeding 200 miles per hour, an adjustment made after engineering studies indicated lower wind speeds caused the damage previously attributed to an F5. The EF5 rating still signifies the highest level of damage, often described as incredible destruction. The EF Scale retains the six categories (EF0 to EF5) but refines the correlation between observed damage and estimated wind speed.
How EF5 Tornadoes Are Rated
The rating of an EF5 tornado is determined by a detailed examination of the damage left behind, not by real-time measured wind speeds. National Weather Service personnel conduct post-event damage surveys, comparing the destruction to a comprehensive list of 28 Damage Indicators (DIs). These indicators cover various structures and vegetation, such as buildings, trees, and mobile homes, each having specific criteria for damage.
Each Damage Indicator has several Degrees of Damage (DoD) that specify the severity of destruction, from minor visible damage to complete obliteration. To achieve an EF5 rating, the damage must correspond to the most extreme end of this spectrum, such as a well-constructed frame house being completely swept away, leaving only a bare foundation slab. This level of destruction often requires the complete removal of a structure built to above-standard construction, since damage to a typical frame home may only result in a high-end EF4 rating. The final EF rating is assigned based on the highest wind speed estimate derived from the damage indicators, ensuring the rating relies on physical evidence.