Some big cities, particularly that are hot and humid, spawn more thunderstorms than rural areas.That’s the conclusion of a , published in Quarterly Journal of the Royal Meteorological Society.The study also found that 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. Rural areas showed no significant weekday-weekend differences.The increased thunderstorm risks illustrate human-induced climate change at the local scale, said Alex Haberlie, a Northern Illinois University Ph.D. student in geography and lead author of the study.“City planners, meteorologists and citizens who live in or near large urban areas should be aware of the increased risk,” he said in a . “These storms can produce dangerous weather hazards, including lightning, hail, strong winds and flash floods, often with little or no warning.”

(MORE: )Using radar data, the researchers charted thunderstorm births over 17 years, from 1997 to 2013, in an area of the southeastern U.S. that included Atlanta. The findings would likely hold up in similar cities that experience warm and humid weather, including Nashville and Birmingham, Haberlie said.But heat and humidity are not the only factors to take into consideration. In Chicago, for example, the city’s effect on initiating storms could be muted by winds produced by Lake Michigan, which plays a large role in its localized weather.The researchers say cities contribute to a number of processes that might lead to more thunderstorm births or storm enhancement:The urban heat-island effect. A landscape of concrete, asphalt and densely packed buildings can enhance heat. Low pressure forms atop the urban area with higher pressure in surrounding rural areas. The scenario might cause low-level atmospheric convergence, which forces air up into thunderstorms.

Wind direction. A cityscape modifies wind direction and speed, which might lead to enhanced convergence and thunderstorm formation.Pollution. The science isn’t settled on this issue, but some researchers believe enhanced aerosol production caused by urban pollution can enrich thunderstorms. “We discovered that there may be an aerosol effect with initiation as well,” Ashley said, “because we had greater storm initiation events during weekdays compared to weekend days.”

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