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.