Big city, bright lightning

Hot, dirty conurbations are thunderstorm magnets.
4 July 2002
TOM CLARKE


Large cities are lightning factories. A new study of Houston, Texas, the fourth largest city in the United States, finds that it attracts 40% more bolts-from-the-blue than the surrounding countryside.

Is the conurbation being smitten like a modern-day Sodom or Gomorrah? Do tall buildings attract more lightning? No, say meteorologists. City heat and air pollution generate more cloud-to-ground activity.

Using twelve years' data from the National Lightning Detection Network (NLDN), Scott Steiger of Texas A&M University in College Station and colleagues mapped lightning strikes within 300 km of Houston1. Once they began analysing the data, "the anomaly was highly visible", says Steiger.

On average, Houston and the area immediately downwind experience seven lightning strikes per kilometre per year - other areas get around two. The difference is most pronounced around midday in summer and autumn - when Houston is struck over 70% more frequently than surrounding areas. Cloud-to-cloud, or 'sheet' lightning, was not measured.

Knowing where and when lightning might strike is valuable. In lightning-prone cities, stricken transformers are a major cause of power cuts. Using their lightning map, "power companies will know where to put their transformers", Steiger says.

In fact, the NLDN - a network of radio receivers tuned to zero-in on the frequencies that lightning naturally produces - is financed partly by power companies.

Furthermore, cloud-to-ground lightning is always accompanied by heavy rainfall. Cities could therefore be disrupting local rainfall that is important for reservoirs and agriculture. "It's just more evidence that we're disturbing the natural environment," says Steiger.

On strike

Cities' effects on weather had been noticed before. Early data from the NLDN hinted at increased lightning and rainfall around large Midwestern towns. But the latest work is the most persuasive. "Here the urban influence on lightning is clearly defined," says Nancy Westcott, the meteorologist at the Illinois State Water Survey in Champaign who first documented the effect.

Urban heat islands - pockets of warm, rising air produced by concrete-clad cities reflecting the Sun's rays - probably drive the phenomenon. The heat adds energy to storms over cities, turning a passing rainstorm into a full-blown thunderhead.

Here the urban influence on lightning is clearly defined
Nancy Westcott
Illinois State
Water Survey



The effect over Houston is most pronounced because of its size, location and heavy air pollution. "There's a lot of concrete in Houston," Westcott says. And the city lies in America's lightning-prone subtropical zone, close to the warm, wet Gulf of Mexico.

Also, the area hosts half of US oil refineries. These produce tonnes of airborne pollution. Pollutant particles cause clouds to form from smaller droplets, making them more electrically active. Steiger's group plans to probe these processes further using airborne sensors.

Despite Houston's influence on the surrounding climate, it is still not lightning capital of the United States. That title goes to Tampa Bay in Florida, where air currents from Atlantic and Gulf waters converge.

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Houston gets 40 percent more lightening strikes than the surrounding countryside. © S. Steiger
Twelve-year (1989 - 2000) mean annual lightning flash density in flashes km-2 yr-1, centered on
Houston, Texas (outlined in white), at a spatial resolution of 5 km. Galveston Bay is located to the southeast of the
Houston urban area.


References
Steiger, S. M., Orville, R. E. & Huffines, G. Cloud-to-Ground lightning characteristics over Houston, Texas: 1989-2000. Journal of Geophysical Research - Atmospheres, 107, Published online doi:10.1029/2001JD001142 (2002).


© Nature News Service / Macmillan Magazines Ltd 2002