After yet another round of deadly tornadoes yesterday, this time in Joplin, Missouri, Reading, Kansas, and Minneapolis, Minnesota, I began to think about the frequency and severity of tornadoes in the United States. This year it seems we’ve had one severe outbreak of tornadoes after another. It makes you wonder if the climate really is changing. Is this the “new normal?”
As it turns out, the answer to the question, “Are tornadoes more frequent now?” is not a simple one. We don’t have hundreds of years of tornado data to study. Tornadoes in the past weren’t as well-reported as they are now. Even today, it is likely that many, many weak tornadoes are never reported. They may appear in isolated areas, last but a few minutes and then disappear without having been seen by anyone.
Of course, there are a lot more people in the United States now, so the likelihood of an impact on populated places is greater than in the past. These are the ones that are seen and are reported, often in great detail.
There simply aren’t any clear-cut answers concerning frequency. Maybe there were more tornadoes, and more violent ones at that, back before 1492, but the native Americans were so scattered that they were rarely affected by them, or if they were, they didn’t leave records. We just don’t know.
Then there is the question of severity. Are storms more severe now? Again, it’s difficult to tell. There were different ways of measuring storm severity, even in recent times. We’ve moved from the Fujita Scale to the Enhanced Fujita Scale in 2007, which makes it a bit difficult to compare storms’ severity, even in the Twentieth century. Before the implementation of the Fujita Scale in 1971, there was no standard measurement for tornado strength. I guess they referred to them as “bad tornadoes,” “really bad tornadoes,” and “killer tornadoes” before the Fujita Scale. Since scientists always want to give things a numeric value, it must have been pretty frustrating for them.
We probably won’t know if tornadoes are actually more frequent and/or more severe until more time has passed and more data has been gathered. There just isn’t a standard or average to compare present activity against.
But where are tornadoes most likely to occur? All 50 states experience tornadoes, (yes, even Hawaii). Oklahoma, Texas, and Kansas report the most twisters on a consistent basis, but then all three of those states are relatively large, so statistically speaking, this wouldn’t be all that unusual. Just about everyone knows that the greatest number of tornadoes tend to occur in the middle of the country, between the Rockies and the Appalachian Mountains, an area known as “tornado alley.”
Here’s a map of “tornado alley” from Wikipedia.
Here’s another map from NOAA that depicts “tornado alley” a little bit differently. I’ll bet you’d have a hard time convincing the people in Mississippi and Alabama that they’re not living in “tornado alley.”