Atlanta summers follow a predictable cycle: muggy mornings, sweltering afternoons, stormy evenings. Think thunder rumbles here more than elsewhere? You’re not imagining things. Atlanta “births” storms frequently, according to an analysis of 26,000 Southeastern storm starts. The study, spanning 17 summers between 1997 and 2013, was published in the Quarterly Journal of the Royal Meteorological Society. Frequent storms—which can trigger flash floods, lightning, hail, and other dangers—should not be taken lightly, says Alex Haberlie, a doctoral student at Northern Illinois University (NIU) and the study’s lead author. “City planners, meteorologists, and citizens who live in or near large urban areas should be aware of the increased risk.” This is not the first time our storm patterns have received scientific scrutiny. Researchers Walker Ashley at NIU and J. Marshall Shepherd at the University of Georgia are among those who’ve examined Atlanta storm waves. Interest started in the mid-1990s to assess potential storm activity during the 1996 Olympics. We asked Haberlie about his contribution to this niche:
Why focus on Atlanta? As you know, Atlanta experiences a lot of thunderstorms. We were able to identify 26,000 starting points in a 17-year period. This is much better for scientific analysis than, say, Los Angeles, which experienced far fewer storms during that period. On top of that, Atlanta is very large. Previous studies suggested that a city needs to be expansive to have a noticeable climatological effect on thunderstorms. And Atlanta is away from oceans or large lakes, which could send breezes into the city and mute the effect of urban development on thunderstorms.
How does Atlanta rank for storminess? This study only focused on Atlanta, but a 2012 study by Walker Ashley compared Atlanta with cities in the Southeast, such as Birmingham. In general, thunderstorm activity increases as you approach the Gulf of Mexico. Therefore, Miami, Houston, and cities nearer the Gulf experience more thunderstorms than Atlanta.
What is unique about Atlanta is how quickly thunderstorm activity decreases as you move away from downtown—the city experiences the quickest drop-off (moving outward from the Capitol) compared to Birmingham or Memphis. This suggests that thunderstorm activity levels in rural areas around Atlanta should be the norm, but Atlanta’s development changes this. Compared to the rest of the U.S., Atlanta is certainly among the top cities as far as thunderstorm occurrence goes because of its proximity to the Gulf. The urban heat island only serves to increase this baseline risk.
We’ve had severe droughts. Sometimes we hear thunder or see lightning flashes, but it doesn’t rain. Did you compare storms with or without rain? Our dataset was weather radar. Basically, the animations of yellow and red shown by a TV meteorologist during the summer depict areas of very heavy rainfall. Our study tracked these rainfall clusters and noted when they started. We did not use lightning data. Likely though, if Atlanta experienced the rain we focused on, lightning occurred somewhere in the city. So if all you heard was thunder but didn’t experience rainfall, your location was not a spot in our study.
We suspect that drought can change the frequency of thunderstorms around Atlanta. This is a research question we have thought about looking into in the future.
likelihood that thunderstorms occurred in Atlanta rather than surrounding rural areas
Late afternoon and early evening
the most likely time for Atlanta storms
July and August
months when storms occur most often
Factors that might contribute to storm frequency in the city
Asphalt, concrete, and densely packed buildings increase heat and change atmospheric pressure, which can contribute to storm formation.
Tall buildings and choppier terrain could make air patterns shift, causing hot air to converge.
Might “enrich” thunderstorms. Storms are more likely on weekdays, when pollution is higher, than weekends.
This article originally appeared in our May 2015 issue under the headline “Big Bang Theory.”