How tornadoes are measured
Damage from the aftermath of a tornado that hit Alabama in March 2019. Photo: Elijah Nouvelage/Getty Images
Since February 2007, the National Weather Service has used the Enhanced Fujita Scale to measure the intensity of tornadoes. After the tornado strikes, meteorologists survey the damage it caused and assign a "rating" based on estimated wind speeds corresponding with the damage.
The backdrop: The EF scale is an updated version of the Fujita Scale, which was first introduced in 1971. The enhanced scale takes into consideration how winds affect certain types of structures in the tornado's path. It looks at how those structures are designed and how the design might impact the destruction they face.
Details: When damage from a tornado is assessed, NWS uses a list of "damage indicators" and "degrees of damage" to estimate likely wind speeds and then assigns the tornado a rating between EF-0 to EF-5.
- Per the NWS website, the goal is to "assign an EF Scale category based on the highest wind speed that occurred within the damage path."
- EF-0: 65-85 mph (Light damage)
- EF-1: 86-110 mph (Moderate damage)
- EF-2: 111-135 mph (Considerable damage)
- EF-3: 136-165 mph (Severe damage)
- EF-4: 166-200 mph (Devastating damage)
- EF-5: Over 200 mph (Incredible damage)
How it works: First, the NWS will assess a variety of structures impacted by the tornado. Each structure, or damage indicator, has its own list of degrees of damage, which helps an evaluator determine a range of wind speeds and whether it is higher or lower than expected. This process is done for several structures before the overall tornado is rated.
- For example, damage indicator 1 is "small barns, farm outbuildings" and damage indicator 2 is "one- or two-family residences." Other damage indicators include "motel," "strip mall," "school - 1-story elementary" and "tree-softwood."
- Each damage indicator includes a description of its typical construction (to ensure a structure is being classified correctly).
- Each damage indicator also has specific degrees of damage.
- Oftentimes, NWS experts will conduct aerial surveys of the tornado-affected region to get an overview of the damage path as well.