Some big cities, particularly those located in hot and humid environments, actually spawn more thunderstorms than surrounding rural areas.

That's the conclusion of a new study by Northern Illinois University researchers, which was published in Quarterly Journal of the Royal Meteorological Society.

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The scientists found thunderstorm "births" were significantly higher on weekdays compared with weekend days, suggesting that increased pollution levels related to industry and commuting may play a role. The urban heat-island effect, in which dry, exposed surfaces such as roofs and pavement absorb more solar energy than less-developed areas with tree cover, farm fields or bodies of water, may also play a role.

Typically, a city might experience two to three additional thunderstorms per year compared to the countryside. It's the first time than anyone has showed a difference between birthing of thunderstorms in urban versus rural areas.

"City planners, meteorologists and citizens who live in or near large urban areas should be aware of the increased risk," Alex Haberlie, a Ph.D. candidate in geography who is the study's lead author, explained in a university press release.

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"These storms can produce dangerous weather hazards, including lightning, hail, strong winds and flash floods, often with little or no warning."

With the help of radar data, the the researchers charted thunderstorm births over a 17 year period from 1997 to 2013, in an area of the southeastern U.S. that included Atlanta. But the results probably would be similar in cities with a similarly warm, humid climate, including Nashville and Birmingham. In a place such as Chicago, where urban pollution's effect on storms might be muted by winds from Lake Michigan, the effect might not be so pronounced.

A previous study in 2012, by a team of researchers that included Northern Illinois meteorology professor Walker Ashley, found that large cities in the southeastern United Sates had a much higher risk of heavy rainfall during summertime thunderstorms than cities elsewhere.

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This post originally appeared on Discovery News. It has been republished with permission. Image by Diegojaf22 via Discovery News.