Behind the Forecast: How tornadoes are rated

LOUISVILLE, Ky. (WAVE) - Tornadoes are some of the most destructive storms on the planet. They can range from just a few yards to more than a mile wide. Some may be nearly stationary while others can travel at higher than highway speeds. A tornado's size isn't necessarily indicative of its strength. In their aftermath, the National Weather Service's (NWS) meteorologists are tasked is to rate these potentially deadly and devastating storms.

Tornadoes are classified based on their estimated wind speed and the damage they cause.

For more than 30 years before 2007, the F-Scale, created by Dr. Theodore (Ted) Fujita, was used worldwide to estimate a tornado’s strength and wind speed. The Enhanced F-Scale (EF Scale) is now used to rate tornadoes. The Enhanced Fujita (EF) Scale was created by a group of meteorologists and engineers assembled by the Wind Science and Engineering Research Center at Texas Tech University. The EF Scale uses estimates of three-second gusts at the point where the damage occurred. The estimates can vary with height and exposure to the elements. Keep in mind that the three-second wind gusts are different from the standard measurements taken by weather stations; those use a directly measured “one-minute-mile” speed.

The EF Scale has a more accurate range of wind speeds compared to the F-Scale, according to the National Weather Service.

The EF Scale incorporates 28 damage indicators (DI) when assigning a rating to tornadoes, including building type, structures, and trees. Each of the 28 damage indicators has 8 degrees of damage (DOD) ranging from the start of visible damage to utter destruction.

The actual wind speeds in most tornadoes are not known, and the wind needed to do similar-looking damage can vary greatly, even from building to building, according to the National Weather Service. Due to this, wind speeds on the EF Scale are acquired from engineering guidelines and are only estimates. The NWS is the only federal agency that can provide an official tornado rating.

On a damage survey, meteorologists must uncover where the tornado initially touched down, its width, strength and where it lifted. They must also determine whether the damage was caused by a tornado or straight-line winds. The NWS says that a survey team is typically equipped with a GPS unit, a cell phone, a laptop with damage survey software, a digital camera, an atlas or gazetteer, and a notebook. While damage is usually surveyed by the ground, sometimes an aerial survey is conducted.