A new Yale-led study quantifies for the first time the primary causes of the “urban heat island” (UHI) effect, a common phenomenon that makes the world’s urban areas significantly warmer than surrounding countryside and may increase health risks for city residents.
In an analysis of 65 cities across North America, researchers found that variation in how efficiently urban areas release heat back into the lower atmosphere — through the process of convection — is the dominant factor in the daytime UHI effect. This finding challenges a long-held belief that the phenomenon is driven principally by diminished evaporative cooling through the loss of vegetation.
The effects of impaired “convective efficiency” are particularly acute in wet climates, the researchers say. In cities such as Atlanta, Georgia, and Nashville, Tennessee, this factor alone contributes a 3-degree C rise in average daytime temperatures, according to the study, published July 10 in the journal Nature
The phenomenon could have profound impacts on human health in cities worldwide as mean global temperatures continue to rise — and as more and more people move into cities — said Xuhui Lee
, the Sara Shallenberger Brown Professor of Meteorology at the Yale School of Forestry & Environmental Studies (F&ES) and one of the study’s authors.