City Heat is Worse When It's Wet: The Cause of the Urban Heat Island

It turns out that urban heat isn't a myth. Scientists have quantified for the first time the primary causes of the "urban heat island" (UHI) effect, which makes the world's urban areas significantly warmer than the surrounding countryside.

Scientists have long recognized the primary causes of the UHI effect. In addition to changes in convection efficiency and evaporative cooling, these include the tendency of buildings, pavement and other structures to store more heat than vegetation and soil. In order to examine this heat a bit more closely, the scientists used satellite data of land surface temperatures and vegetation cover from cities in the United States and Canada to calculate the mean temperature differentials between urban centers and their rural surroundings.

In the end, the researchers found that there was a three-degree C rise in average daytime temperatures in cities. In addition, the researchers found that, regardless of local climate, the release of heat stored in human-built structures is the dominant contributor to UHI during the nighttime. During the daytime, though, the dominant factor was convection. The smooth surfaces of buildings and other human-made features are far less conducive to heat, and in wetter climates urbanization reduces convection efficiency by about 58 percent.

The "rougher" surfaces of vegetation trigger turbulence, which removes heat from the surface. But where there is a smoother surface, there is less convection and heat is trapped in the surface.

"There is a synergistic relationship between climate conditions and the urban heat island," said Xuhui Lee, one of the researchers, in a news release. "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."

The findings are published in the journal Nature.