Ask Chicagoist: Can the Windy City Get THAT Windy?

By Thales Exoo in Miscellaneous on Sep 28, 2006 5:38PM

Hi Chicagoist,

Not Dorothy

Dear Auntie Em,

Last week's sirens and warnings had Chicagoist wondering the same thing. We think there's something horrifying and yet oddly fascinating at the prospect of a tornado hitting downtown. Maybe it's just good ammo for next summer's run of disaster movies, and maybe it's just because tornadoes seem meant for open fields, desolate areas, Munchkinland, and mobile home parks, but the thought of a twister making its way down Michigan Ave. really freaks us out, no matter how many tornado drills we had to deal with growing up in Indiana.

Some people insist that our tall buildings shield us from such horrifying disaster, and our urban pocket in the midst of rural Illinois helps discourage tornadoes from forming. The theory is that any modern large city creates a heat island, due to the fact that cities are about ten degrees (Fahrenheit) hotter than the surrounding areas, mostly because of excess dark surfaces (pavement and buildings) and pollution (cars, buses, and air conditioners), and fewer green surfaces (parks, trees, and vegetation). It's this temperature increase that is theorized to protect us from the small tornadoes, because turbulent rising air caused by the tall buildings disrupts their formation.

However, it seems best to not completely rely on this phenomenon and get too big for our britches, as there are a number of recorded cases of tornadoes hitting major cities. And some scientists insist that we've just been lucky, or that it's a coincidence because the area of the city with all the tall buildings is relatively small. Plus, not too many actual tornadoes ever hit in any given area, so it may be similar to asking why a tornado hasn't ever hit at the corner of Main and 1st in Anytown, USA (assuming a much larger area than just a street corner, of course).

Plus, this heat island effect wouldn't protect us from large tornadoes. A "monster" tornado that formed outside of the city, for example, wouldn't be swayed off course by tall buildings, and in fact might even be more damaging because once it got in the city it would be likely to spin faster. But again, it's probably best not to get too worked up by the possibility, as "the probability of a violent tornado in the downtown area of any large city is about once in a thousand years." Remember it's possible, but we don't think it should cause you anxiety. There's enough other stuff going on in the world that can do that.

That's not to say there's never been a tornado here. Within the official city limits, there have been twelve tornadoes since 1870. One in particular happened in 1967, when a tornado "skipped across the South Side of the city, crossing the Dan Ryan Expressway at rush hour, and hitting the lakefront near 79th Street." But the loop itself with its tall buildings (which we think is the area most people think about when discussing tornadoes and cities), has not seen a tornado since May 6, 1876. And since downtown Chicago isn't quite the same as it was in 1876, it hardly seems to count.

The general Chicagoland area, however, has seen more than its share of tornadoes over the years. The western suburbs have even seen an increase in tornadoes since the 1960s, possibly caused when "cooler winds from Lake Michigan collide with hot air coming up from the southwest, giving extra lift to thunderstorms in a corridor from Aurora to Joliet and south toward Kankakee." Exactly.

If you want to read more about significant tornadoes in the Chicago area and their intensities and locations (and really, who wouldn't?!), check out the National Weather Service Weather Forecast Office.

Image via hyku

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