Anyone who lives in a city is familiar with the excruciatingly unbearable heat that creeps in midsummer and doesn’t let up until the first fall breeze rolls around. But you’ll probably be glad to know it’s not just your imagination—the mercury does indeed rise when you step inside big-city limits.The phenomenon is called the urban heat island (UHI) effect, and you can thank it for contributing to your untamable summer frizz, and even ruining your would-be perfect Instagram shots.Now, a study published July 10 in the journal Nature provides new insight into the main driver of these stifling hot temperatures: the natural heat transfer process called convection.Researchers at the Yale School of Forestry & Environmental Studies analyzed 65 North American cities and found that daytime UHI is caused by a city’s impaired ability to release heat upward into the lower atmosphere. In plainer English: Buildings don’t diffuse heat as well as trees do.“The ‘rougher’ surfaces of the vegetation trigger turbulence, and turbulence removes heat from the surface to the atmosphere,” says Lei Zhao, a doctoral student at F&ES and lead author of the study. The smooth surfaces of buildings and other man-made structures, on the other hand, dampen convection and trap heat.And to make matters worse for those in Nashville, Atlanta, and other similarly “wet” or humid cities, “The effects of impaired ‘convective efficiency’ are particularly acute.” In fact, researchers found that overall, in wetter climates, urbanization reduces convection efficiency by 58 percent. These effects of inefficient convection can contribute a 3-degree C rise in average daytime temps.UHI’s stifling effects not only make urban areas totally uncomfortable, but can also increase health risks for city folk—specifically, increasing heat wave stress on the body—in areas where there is already high humidity.Sounds like a good excuse to us to escape the city more often during the hottest summer days and post up poolside with a cool, refreshing drink (or perhaps a healthy, homemade popsicle) in hand.