There's a commonly-held belief that if your house is on a hillside or in a valley, you're less likely to be affected by a tornado, and a bit of research coming out of the University of Arkansas has apparently caused some controversy about this belief.
Let me start off by saying that this is one of the most common myths about tornadoes. Some believe that a tornado will skip over a valley, or that a tornado will weaken if it heads downhill or through a valley. However, whereas that may have happened in a few cases, the only generalization about tornadoes that these cases support is that tornadoes have an unpredictable and chaotic nature.
One thing to point out about the study is that it only looked at two tornadoes — the Tuscaloosa–Birmingham tornado that happened on April 27th, 2011, and the Joplin tornado from May 22nd, 2011. That's not enough to make general statements about tornadoes, and from an interview with UA professor Panneer Selvam, the research doesn't seem to be trying to make general statements, regardless of what the reaction to the story has been.
Another point is that the press release from the University of Arkansas may be overstating the conclusions of the study. In the particular case of these two tornadoes, the researchers found that more damage was caused as the tornadoes moved up a hill, with the greatest damage at the crest of the hill, and less damage was caused on the lee side of the crest and as the tornado moved down the other side of the hill. The research isn't saying that the tornado was increasing in intensity while going up the hill and decreasing in intensity going down the other side, but instead it was looking at the very small scale effects, at ground level, as the tornado passed over a hill.
Also, although the press release seemed to be making a general statement about how tornadoes can skip over valleys, Prof. Selvam specifically said in the interview: "You cannot make the general statement: you are surrounded by hills, you are fine. No. Valleys are a complicated issue."
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Just to make it clear that a valley or lee side of a hill won't necessarily offer you any significant protection against a tornado, in a paper examining tornadoes from 2008, in the Boston Mountains of northern Arkansas, some tornadoes actually increased in strength by going into a valley. This is because the tornado funnel stretches and gets thinner. Just like how a spinning figure skater will spin faster when they pull their arms in close to their body, a tornado funnel getting thinner means that the winds in the funnel will rotate faster, which increases the strength of the tornado.
Professor Selvam wants to continue the research, looking at more tornadoes and their damage paths as they moved over hilly terrain, and it will be interesting to see if they can actually find any kind of general trends. For now, though, this is just a preliminary work and no one should be using it to form (or support) any beliefs about tornadoes.
(Photo courtesy: Marvin Gentry/Reuters)
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