Urban heat — not a myth, and worst where it’s wet

A press release from the Yale School of Forestry and Environmental Studies: 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.

 “There is a synergistic relationship between climate conditions and the urban heat island,” Lee said. “This relationship suggests that the urban heat island will exacerbate heat wave stress on human health in wet climates where temperature effects are already compounded by high humidity. This is a huge concern from a public health perspective.”

...Their results reaffirmed the consensus view that, regardless of the local climate, the release of heat stored in human-built structures is the dominant contributor to the heat island phenomenon during the nighttime.

But during the daytime, convection is the dominant factor, researchers found — particularly in “wetter” cities of the southeastern U.S. In those places, the smooth surfaces of buildings and other human-made features are far less conducive to heat diffusion than the densely vegetated natural areas that surround them, researchers say. Overall, in wetter climates urbanization reduces convection efficiency by 58 percent....

A NASA image showing the temperature differential between Providence, Rhode Island, and surrounding areas