I've Been Thunderstruck

When people hear the term “Tornado Alley,” they tend to think of the area from mid-Texas up through the heartland that spawns a greater number of tornadoes annually than any other area of the country. However, recent research by Michael Frates of the University of Akron, reported on MSNBC, suggests that there are actually four regions of active tornado development in the US, and the original Tornado Alley is not the most active!

Michael Frates, a graduate assistant at the University of Akron in Ohio, devised the new boundaries and a more nuanced set of “Tornado Alleys” by analyzing the spatial distribution of F3 to F5 tornadoes with tracks greater than 20 miles in the Central and Eastern U.S. from 1950 to 2006. The output of that work is spread across a grid of more than 3,000 cells across the region.

Each cell was then given a different “frequency value” depending on the frequency of tornadoes with intersected the unit, and out of this process came “major spatial patterns, which served as the basis for delineating new tornado alleys,” as shown on his map, above.

“Results from this analysis indicate that Dixie Alley has the highest frequency of long-track F3 to F5 tornadoes, making it the most active region in the United States,” Frates concluded. Dixie Alley had a frequency value of 2.92, followed by Tornado Alley (2.59), Hoosier Alley (2.37) and Carolina Alley (2.00).

When Frates’ data is presented on a map, it gives the regions indicated below as the four Tornado Alley regions:

Four Tornado Alley Regions

This new data should help the National Weather Service understand better where to focus tornado predicting technologies, and where to concentrate research efforts. This years spring tornado season, while delayed likely due to the El Nino effect, has been particularly active.

Given that tornado season appears to be in full swing now (a recent tornado killed 11 in Mississippi), I thought it would be a good time to describe how tornadoes are rated. From time to time you might hear the weather reporter talk about an “F0″ or “F3″ tornado, and you might not know what they mean. I’m hear to set you straight!

Tornadoes are measured using the Fujita Scale, which was developed by Ted Fujita of the University of Chicago back in 1971. The scale Fujita developed is used to determine the intensity of a tornado after it has passed through an area and done its damage on human-based structures and vegetation. After a tornado has gone through an area, meteorologists and engineers will do a ground and/or aerial damage survey. If possible, they will also take into account things like eyewitness accounts, ground-swirl patterns, radar tracking and media reports and imagery. After everything has been considered, a rating of F0 to F5 is assigned to the tornado. Here is the criteria that they use to determine the rating:

Keep in mind that the rating cannot be applied until after the tornado has gone through and done its damage…the rating depends entirely on a guess of wind speed, and a measure of the damage done to structures and vegetation.

All of the world uses the Fujita Scale except the United States, which moved to the Enhanced Fujita Scale in 2007. It was revised to reflect better examinations of tornado damage surveys, so as to align wind speeds more closely with associated storm damage. Better standardizing and elucidating what was previously subjective and ambiguous, it also adds more types of structures, vegetation, expands degrees of damage, and better accounts for variables such as differences in construction quality.

The Fujita/Enhanced Fujita Scales are good ways to gauge just how bad a tornado was…not that you have to tell the people who were in them!