An EF4 tornado is a devastating natural phenomenon characterized by extreme wind speeds and widespread destruction. Tornado intensity is classified after the event, using the damage left behind to estimate its power. Understanding this classification system is necessary to grasp the physical dimensions and behavior of a tornado rated at the EF4 level.
Understanding the Enhanced Fujita Scale
The intensity of a tornado is rated using the Enhanced Fujita Scale (EF Scale), which ranges from EF0 to EF5. This scale became operational in the United States in 2007, replacing the original Fujita Scale to provide a more accurate correlation between wind speed and the damage inflicted on structures. The EF Scale does not measure wind speed directly, as instruments are often destroyed by the storm’s power. Instead, it uses a post-storm assessment that relies on 28 different damage indicators (DIs) and their corresponding degrees of damage (DOD) to estimate the wind speed. These indicators account for variables like construction quality and building type, providing a more reliable intensity rating.
Defining the EF4 Tornado
The EF4 rating is given to tornadoes that exhibit wind speeds estimated to be between 166 and 200 miles per hour (mph). Tornadoes in this category are classified as “violent” and are responsible for a large percentage of tornado-related fatalities, despite making up only about two percent of all recorded tornadoes.
A tornado is rated EF4 when well-constructed frame houses are completely leveled, leaving behind only a pile of debris. Structures with weak foundations may be blown a considerable distance, and cars and trains can be thrown or rolled. The force of the wind is so great that large trees are often debarked by flying debris, and steel structures can be badly damaged. The damage assessment takes into account the highest wind speed that occurred at any point along the tornado’s track to assign the final EF rating.
The Width of an EF4 Tornado
The width of a tornado is highly variable and is not a factor used to determine its EF rating, which is based solely on estimated wind speed and resulting damage. However, there is a general correlation, where more intense tornadoes, such as EF4s, often exhibit larger physical dimensions than their weaker counterparts. The width of an EF4 tornado, defined as the widest extent of its damage path, can range from less than 100 yards to over a mile.
The average width of a tornado in the United States is approximately 500 feet, but EF4 tornadoes commonly exceed this average. For example, the 2021 Western Kentucky tornado, rated high-end EF4, reached a maximum width of 2,600 yards (about 1.5 miles). This immense size is often associated with “wedge” tornadoes, which appear wider than they are tall and can produce damage paths a mile or more across.
Atmospheric conditions and the maturity of the storm significantly influence the tornado’s physical size. A large, well-organized supercell thunderstorm is necessary to generate the rotating column of air required for a long-lived, high-intensity EF4 tornado. As the tornado matures, its vortex may broaden, increasing the width of the damage swath even if the core wind speeds remain constant. Local topography can also play a role, sometimes causing a tornado to temporarily contract or expand.
The actual measurement of a tornado’s width is determined during the post-storm survey by recording the distance between the outermost points of visible damage. While the EF scale focuses on intensity, the path width and path length are collected as continuous data points. These dimensions are strongly correlated with the final EF category and help climatologists analyze tornado power and the full scope of the event.
Movement and Duration
EF4 tornadoes tend to have longer path lengths and durations compared to weaker tornadoes. These powerful storms are often associated with supercell thunderstorms, which can travel for extended periods. The 2021 Western Kentucky event, a high-end EF4, remained on the ground for nearly three hours and tracked over 165 miles.
The forward movement speed, known as translational speed, of an EF4 can vary significantly depending on the speed of the parent thunderstorm. Many violent tornadoes travel at an average speed of 30 to 70 mph. The combination of extreme wind speed and a sustained, fast forward movement allows EF4 tornadoes to cause catastrophic destruction over a large geographical area.