Why is it that small towns seem to suffer the brunt of tornado destruction while big cities -- even those located in at-risk regions -- rarely get hit?
There is a common myth that densely built areas and skyscrapers somehow break up a tornado's momentum (I remember hearing this in a college geology course).This may be comforting but it's just not the case. Thunderstorms and the tornadoes they spawn are typically five to 10 miles high. One-thousand-foot-tall buildings, even many of them, would be of little consequence to a tower of 300-miles-per-hour winds. There is even some preliminary evidence, based on the 2008 Atlanta tornado, that the "heat island" of urban areas could possibly exacerbate tornadic weather patterns. For the most part though, any effect that intense human development has on tornadoes is not well understood, and the reason is that such events are so rare, there's just not enough data to study them. Which brings us back to the original question: Why are they so rare?
In a word: statistics. For as many people as they hold, cities take up a tiny amount of space. Oklahoma's two counties with populations density greater than 1,000 people per square mile (Oklahoma County, which includes Oklahoma City, and Tulsa County) make up, collectively, just 1.86 percent of the state's square mileage (not counting water). At the same time, tornadoes, for all their destructive power, are like a pin-prick to the Earth's surface. Yesterday's tornado, which by all accounts was one of the largest ever, is said to have been about a mile wide and traveled for 20 miles. The chance that those 20 square miles cross paths with any given location are slim -- that the location is a major city is even more unlikely yet.
Tornado Alley, the region of the U.S. where severe tornadoes occur most frequently, also happens to have some of the most open country around. (If you do this same math for the country's most population dense state, New Jersey, you find that more than a third of the state -- 36 percent -- lies in counties with population density greater than 1,000 people per square mile. Though New Jersey may get an average of two tornadoes a year, it has had exactly zero rated as F4 or greater.)
In addition to this lanscape, tornadoes -- especially ones this powerful -- are rare, as Alexis noted yesterday. As he put it:
It is also worth noting, however, that even in the very center of Tornado Alley, it is rare for any particular area to be hit by a tornado. As you can see in the graphic below, any location in the reddest part of the map could expect that a violent storm like the one today would touch down within 25 miles only four times per century.
Over the long term though, even statistical unlikelihoods will occur, and they have. In 1913 an F4 tornado struck downtown Omaha on Easter Sunday, killing more than 100 people. St. Louis has been struck by major tornadoes multiple times, including an F4 in 1896 that killed 255. An F5 tornado went right through downtown Waco in 1953.