Joshua Wurman, president of the Center for Severe Weather Research in Boulder, Colo., whips up a response.

The glib answer for why tornadoes don't strike urban areas that often is: Cities are small.

If you take a look at Google Maps and see what percentage of the U.S. urban and suburban areas cover, it's a pretty small fraction. The regions where you have peak tornado frequencies—from Texas up through Kansas, and even east toward Atlanta and the Southeast—are open country, so that's where most tornadoes spend the overwhelming fraction of their lifetimes.

It's very rare that one encounters a city, as happened in Atlanta last weekend. In 1999 there was a tornado that hit Oklahoma City and killed about 40 people. It was a long-track tornado that lasted about an hour—but most of its lifetime was spent over pretty open country. It crossed two subdivisions, and that's where most of the fatalities happened.

The Atlanta twister has not been characterized as a violent tornado. Tornadoes are rated using the Enhanced Fujita (EF) Scale, which scores the damage caused by a particular twister on a scale from 0 to 5. Violent tornadoes are classified as EF4 and EF5, significant ones EF2 and EF3. EF5 damage is typically quite catastrophic: Houses are not only just destroyed but destroyed down to their foundation—no walls left standing--and the tornado might cause structural damage to larger, well-engineered buildings that are designed to survive even very intense winds. The tornado that went through Atlanta, although it broke lots of windows, didn't cause major damage to any downtown buildings. I believe that tornado has been rated an EF2.

As to whether global warming will increase the number of tornadoes—making it more likely that they will encounter cities—we have no clue.

Climate change predictions are uncertain, even on a global scale. One could be confident that the global temperature is going to go up; however, the error ranges on what's going to happen locally—whether the temperature is going to go up in Atlanta, or here in Boulder—are much wider.

In the case of hurricanes, there's a reasonable, if unproved, hypothesis that the storms might get more intense if global temperature rises and the surface temperature of the earth's oceans rises—after all, in terms of the physics, warm water is key to creating a hurricane. With tornadoes, you can't even make that basic statement because the effect of an increase in local temperature on tornado frequency or tornado intensity is unclear. Brazil is pretty hot, but it doesn't have a lot tornadoes. Oklahoma and Texas are really hot in July and August, but that's not the peak of their tornado season; spring is when those states see the most tornadoes.

So it is possible that climate change could shift the tornado season to earlier in the year as spring creeps into winter. Perhaps it will move the distribution of where the stronger tornadoes occur. A warming climate does not necessarily make for more tornadoes; it could cut off tornadoes completely or could cause their incidence to double.

5 Comments

1. 1. Hugh Jones 08:52 PM 3/23/08

   By implication it's the change itself that most likely will cause the increase in tornadoes and not because the earth is becoming warmer.

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2. 2. Debra F 11:04 PM 4/6/08

   "EF5 damage is typically quite catastrophic: Houses are not only just destroyed but destroyed down to their foundationno walls left standing--"
   
   If that's the case then Lafayette, TN was hit by an EF5 2-5-08. Brick churches and homes completely wiped out. and they claim it was an EF2 - EF3. Get real, I still have to drive through the destruction everyday.

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3. 3. whatbox 05:50 AM 4/11/08

   Is there "ANY" way that these high wind events could be put to a useful purpose?

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4. 4. Julius St Swithin 02:46 AM 5/22/09

   Two comments:
   1. Tropical storms are caused by warm moist air over seas rising. As the air rises it draws in more warm air which feeds the storm. As the hot air rises it also cools and condenses. When it condenses it gives up the latent heat of vaporisation which gives more energy to the storm. When storms reach land, or cold water, they lose energy as the conditions necessary to reinforce them are no longer present. Most cities are not close to the sea.
   2. There has been no long-term trend in hurricane severity though the evidence of the last 150 years suggests that they may be more frequent when temperature is higher. See:
   http://www.climatedata.info/Impacts/impacts.html
   and click on "Tropical cyclones".
   

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5. 5. John089057 08:48 AM 10/6/09

   The reason is that there are fewer rednecks in the city. You see Torandos only exist to smite the crackers and to take away the only thing they love most. Their trailer and pickup truck.

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