The Weather Notebook: The A-Scale of Tornado Damage

The A-Scale of Tornado Damage
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Dave Thurlow, Host

Ted Fujita
Hi, I'm Dave Thurlow and this is The Weather Notebook. Thirty years ago, the F-scale was nothing more than a musical key. Then the late Ted Fujita created his tornado damage F-scale. Scientists now use the F-scale to rate each tornado's intensity, from F0 to F5. The numbers correspond to the intensity of the damage.

For example, a tornado that carries a strong frame house off its foundation and destroys it would be rated an F5. It appears that the wind must be at least 260 mph to cause this kind of damage. But what happens if a big twister plows across a wheat field without hitting any trees or buildings? That makes it an F0, no matter how strong the winds are, because there's no structural damage to give it a higher rating.

Two forecasters at the NOAA Storm Prediction Center, Richard Thompson and Michael Vescio want to fix this problem. They've come up with a tornado damage area scale, or A-scale. Each twister is rated on the A-scale by the amount of land it tears across. Our imaginary wheat-field twister might be rated an A5 if it had a wide track and a long path, even if there's no damage to buildings.

Thompson and Vescio have also developed a way to rank entire tornado outbreaks. It's called the Destruction Potential Index, and it combines the information from the F scale and the A scale for every tornado in an outbreak. Using both the F AND A-scales should provide meteorologists with a better distinction between 'weak' and 'significant' tornadoes.

Thanks to contributing writer Bob Henson. For more information on the A scale, visit our website at mountwashington.org. The Weather Notebook is underwritten by Subaru, with major support provided by the National Science Foundation.