Tornado classification determines a tornado’s intensity based on the assessment of the damage it leaves behind. This retrospective analysis allows meteorologists and engineers to assign a rating that reflects the storm’s power. The primary system used in the United States and Canada is the Enhanced Fujita (EF) Scale, which provides a standardized way to categorize the destructive nature of tornadoes.
The Evolution of Tornado Rating Systems
The foundation for modern tornado classification began in 1971 with the introduction of the original Fujita Scale, or F-Scale, developed by Dr. Tetsuya Fujita. The F-Scale was the first widely accepted method to categorize tornadoes, using damage to estimate the wind speeds that caused it.
A major limitation was the F-Scale’s reliance on estimated wind speeds that were determined to be too high, particularly in the upper categories. Engineering analysis showed that common structures could be destroyed by lower wind speeds than those assigned to the F-Scale. This often led to an overestimation of a tornado’s actual wind speed. The need for a more accurate system led to the development of the Enhanced Fujita Scale, which became operational in the United States in February 2007.
Understanding the Enhanced Fujita Scale Methodology
The EF Scale correlates damage more closely with the wind speeds required to cause destruction. The scale relies on an analysis of the damage left in the tornado’s path, not direct wind speed measurements. The methodology revolves around two primary components: Damage Indicators (DIs) and Degrees of Damage (DODs).
There are 28 different Damage Indicators, representing specific structures or objects a tornado might encounter, such as residences, shopping malls, barns, and various types of trees. These indicators allow evaluators to account for differences in construction quality. For each DI, there are defined Degrees of Damage (DODs) that describe the extent of the destruction, ranging from minor issues to total obliteration.
The final EF rating is determined by comparing the observed DOD on a specific DI to a table that estimates the three-second wind gust needed to inflict that damage. This process is repeated across multiple damaged objects to determine the most appropriate wind speed estimate and the final classification.
The Six Classification Categories
The Enhanced Fujita Scale uses six classification categories, ranging from EF0 to EF5, each corresponding to a specific range of wind speed and a descriptive level of damage.
An EF0 tornado has estimated wind gusts between 65 and 85 miles per hour. This level causes light damage, which may include peeling surface off some roofs, minor siding or gutter damage, and breaking off tree branches.
An EF1 tornado is assigned when wind speeds are estimated to be between 86 and 110 miles per hour, resulting in moderate damage. Mobile homes may be overturned or badly damaged, roofs can be severely stripped, and exterior doors may be lost.
The next level, EF2, involves considerable damage from wind speeds of 111 to 135 miles per hour. Well-constructed homes may have their roofs torn off and foundations shifted, while large trees are snapped or uprooted.
Tornadoes classified as EF3 exhibit severe damage, with wind estimates ranging from 136 to 165 miles per hour. Entire stories of well-built houses can be destroyed, large buildings like shopping malls suffer severe structural damage, and heavy cars may be lifted off the ground.
An EF4 tornado has estimated wind speeds between 166 and 200 miles per hour. At this level, entire frame houses are leveled and reduced to piles of debris, and cars can be thrown significant distances.
The highest classification is the EF5, where wind speeds are estimated to be greater than 200 miles per hour, resulting in incredible damage. These rare tornadoes can sweep strong frame houses clean off their foundations, leaving only a bare slab. Vehicles can be shredded and hurled for miles, and the ground may be scoured of dirt and vegetation.
The Post-Event Survey Process
The task of officially classifying a tornado falls to meteorologists and engineers from the National Weather Service (NWS). They conduct a thorough damage survey immediately after the tornado has dissipated to document the event’s path, width, and life cycle.
The team physically inspects the damage along the tornado’s track, identifying the appropriate Damage Indicators and matching the observed destruction to the Degree of Damage. The final EF rating assigned to the entire tornado is based on the highest wind speed estimate found anywhere along its path.