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See Inside October 2010

Real Skyscrapers: How Cities Affect the Path of Hurricanes

Cities' coarse coasts cause cyclonic course corrections



Matt Collins

One of my favorite things about New York City is our almost complete lack of earthquakes and hurricanes. Most reviews of the city don’t start with its lack of natural disasters, but I also love spaghetti because it doesn’t have any bones. Of course, New Yorkers do suffer the occasional small tem­blor, but those are indistinguishable from the shaking caused by subways, truck traffic or incredibly powerful bass notes coming from car stereos blocks away. And we haven’t had a decent hurricane in my lifetime, although the October 1991 weather event that became known as the Perfect Storm was impressive: because of the driving rain, every stick in the Tri-State area, whether on the ground or flying through the air, served as an infallible divining rod.

Records show that a good-size hurricane hits the New York area every 75 years or so, meaning I may get lucky and see one, or I may get luckier and not see one. But maybe the odds of seeing one are slightly better than I thought. Because, as I just learned, computer models suggest that the very city I love for its lack of hurricanes—along with any other city on a coast—­may actually attract monster storms.

Cities, it turns out, are rough. Sure, they’re also tumble, but the roughness in this case is a measure of their topography. Farmland is nice and smooth. Forests have some roughness, with all those trees sticking up and out. But a big city will leave a nasty rug burn on, say, any giant prehistoric lizard, awakened from hibernation by a nuclear explosion, that slides across it.

Here’s how that urban roughness gives hurricanes the come-hither. When a hurricane starts to sample the land, the friction of jagged cities slows down the leading edge more than any adjacent smoother surface does (with all other factors, such as available moisture, being equal). The back of the hurricane hasn’t gotten the news yet, so there’s a pileup.

The squeezed air goes up, condensing its water vapor and giving off heat. Which feeds energy back into the nearby part of the hurricane, making it move faster and pulling the rest of the storm in that direction.

Or, as Johnny Chan and Andie Au-Yeung of the City University of Hong Kong put it in a paper that will appear in the Journal of Geophysical Research (Atmospheres): “Higher roughness induces stronger convergence and hence increases the vertical ad­vec­tion term in the potential vorticity tendency distribution over the rougher area. Hence, a TC (tropical cyclone) tends to move towards a region with higher roughness”—that is, toward my house. Sure, it’s not just toward my house. But as Yossarian, the hero of Joseph Heller’s novel Catch-22, notes when he complains that the enemy is trying to kill him, and his fellow fliers respond that the enemy is trying to kill all of them, “What difference does that make?”

Something else that settles into cities more than into the surrounding countryside is heat. The well-known “urban heat-island effect” is ticking off more people, because (a) more people than ever live in cities, neighbor cheek by jowl with annoyed neighbor, and (b) it sure has been hot. It’s been so hot that french fries in Germany are expected to be half an inch shorter than usual because the potatoes aren’t growing as large. Conversely, Italian ices everywhere have been going through the roof.

Speaking of roofs, they could be put to work to cool down cities, according to a recent article in Physics World. The review cites work by the Urban Heat Island Research Group at Lawrence Berkeley National Laboratory, which modeled what Los Angeles would be like if the albedo (the ratio of light reflected versus that received) of its buildings and road surfaces were 30 percent higher. Bouncing the light away better translates into the town being a full two degrees Celsius cooler. Two degrees in New York City could be the difference between staying inside with the air conditioner running and going outside. Where we could dissuade hurricanes by smoothing things out with the neighbors. 

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