|
Myth or Misconception #3 .... Tornadoes never strike big cities.
This misconception has a small kernel of possible truth at its heart. Before we get to that possible bit of truth, we first have to make a number of things clear. When one thinks of a "big city", the image of sky scrapers and large office or apartment buildings come to mind. In actuality, if you were to compare the downtown where these buildings occur with the rest of the city, it would comprise a rather small percentage of the city's area.
Let's look at a map:
The area that is considered "the city" and consists of tall buildings is filled in with blue. The boundaries of the city are outlined in green. The entire city is also surrounded by suburbs, which are outlined in purple. If you think of the city as just the area filled in with blue, then "the city" is a very small target. That more "cities" aren't struck by tornadoes is probably more coincidence than anything else. There are very few "big cities" with skyscrapers in Tornado Alley. In fact, there are only a dozen, and one of them, St. Louis, has a long history of tornadoes in its central area.
The downtown areas of "big cities" have had tornadoes on occasion. This past spring, a tornado passed through Miami, Florida before it moved out to sea, disproving the idea that they can't form in cities.
The St. Louis, Missouri and East St. Louis, Illinois areas have had more than their share of tornado strikes to their downtowns. The first time was on March 8, 1871, when a tornado that did F-3 damage moved from the west bank of the Mississippi River, across the river, to the Brooklyn-Venice area of Illinois. Ferries and steamers were torn apart and their fragments carried as much as 30 miles. Most of the damage was to the six railroad depots that were destroyed. Eight of the nine deaths occurred in the railroad yards.
A tornado struck again on May 27, 1896, killing 255 people in the two states. Touching down on the NW edge of Tower Grove Park, the storm widened into a mile-wide, complex combination of tornado and downburst winds. It struck the Eads Bridge, just as the windstorm had in 1871. Seven people were killed in a building about where the I-55 sign is on the map to the right. A photo of that building, and another photo of the East St. Louis damage can be seen here. About 1000 people were injured as the storm collapsed or swept away portions of houses, factories, saloons, hospitals, mills, railroad yards, and churches. The storm was at its maximum intensity as it crossed the Mississippi into East St. Louis, and it killed 118 people there, 35 of which were in the Vandalia railroad freight yards.
The third time St. Louis was struck was on September 29, 1927. The tornado began at the south edge of Webster Grove and as it passed through the middle of the city, its path widened from 100 to 600 yards. Over 200 city blocks were torn apart, and 72 people were killed. In Illinois, 6 people were killed when a crucible of molten metal was overturned. Debris from the tornado was carried up to 50 miles.
In the past 40 years, the city of St. Louis and the surrounding suburbs of St. Louis County have been hit 22 times, although none of them were in the tiny skyscraper heart of the city. There are three possible reasons for that. First, the central city may produce a "heat island" in which turbulent rising air disrupts the formation of small tornadoes(keep in mind that most tornadoes are small). The second possibility is that the "roughness" created by the skyscrapers causes turbulence that disrupts the formation of small tornadoes. The third, is, of course, the idea that tornadoas are rare, and the central city is very small. So it is a matter of coincidence.
Professor Fujita of the University of Chicago suggested that the "heat island" effect takes hold for small tornadoes when a city reaches a population of about 1,000,000. There seems to be a lack of small tornadoes in the central cities of Chicago, Tokyo, and London. These are the only three cities that have been carefully studied over a long time.
None of this applies to intense tornadoes. They are just too rare to assume that they avoid central cities. There are thousands of small towns all across Tornado Alley that have never been hit by an intense tornado. If you have ever seen video of the Wichita/Andover, Kansas tornado, it should seem ridiculous to you to think that this monster, at the bottom of an 8-mile-high rotating column, would be bothered by the presence of a few 300-foot-tall buildings. Perhaps some time in the next century, a central city will be in the path of a violent tornado, and we will learn what will happen. The probability of a violent tornado in the downtown area of any large city is about once in a thousand years.
It is possible that a tornado could actually intensify even more after it forms outside of town and moves into the central city. One speculation has it that the friction of the buildings will slow down the inflow of air into the funnel. This would deprive the funnel of air. The pressure would drop, causing the funnel to shrink in diameter, and spin even faster. So central city tornadoes that began outside the city could be more damaging than average. As you can see, there is a lot more to this than the simplistic idea that heat and roughness keeps tornadoes away. The rarity of intense tornadoes and the fact that St. Louis has been hit by three of them is an interesting curiosity, but that is another puzzle for another day.
_________________ FavreFan wrote: Im pretty hammered right now.
|
Login
Register
FAQ
Search
, he has you Darkside... 
