Strong contributions of local background climate to urban heat islands

Climate modelling is used to show that for cities across North America, geographic variations in daytime urban heat islands—that is, the temperature differences between urban and adjacent rural areas—are largely explained by variations in the efficiency with which those areas convect heat to the lower atmosphere. Why it's hotter in the city It is often warmer in a city than in the surrounding rural areas, sometimes by up to a few degrees. This urban heat island effect is commonly explained as a consequence of a lower rate of evaporative cooling in urban areas. But here Xuhui Lee and colleagues use climate modelling to show that for cities across North America, the daytime urban heat island effect varies with the efficiency of heat convection between the land surface and the lower atmosphere. The convection effect varies with climate regime, causing significant urban warming in wet climates but cooling in dry climates. Aerodynamics also play a part, and if urban areas are aerodynamically smoother than surrounding rural areas, urban heat dissipation is less efficient and warming occurs. The health impact of heatwaves means that mitigation of the heat island effect may be beneficial. The authors suggest that aerodynamic spoilers — a city-wide increase in building height for instance — may be impractical. But efforts to increase urban albedo, by installing reflective roofs for instance, might be worth pursuing. The urban heat island (UHI), a common phenomenon in which surface temperatures are higher in urban areas than in surrounding rural areas, represents one of the most significant human-induced changes to Earth’s surface climate 1 , 2 . Even though they are localized hotspots in the landscape, UHIs have a profound impact on the lives of urban residents, who comprise more than half of the world’s population 3 . A barrier to UHI mitigation is the lack of quantitative attribution of the various contributions to UHI intensity 4 (expressed as the temperature difference between urban and rural areas, Δ T ). A common perception is that reduction in evaporative cooling in urban land is the dominant driver of Δ T (ref. 5 ). Here we use a climate model to show that, for cities across North America, geographic variations in daytime Δ T are largely explained by variations in the efficiency with which urban and rural areas convect heat to the lower atmosphere. If urban areas are aerodynamically smoother than surrounding rural areas, urban heat dissipation is relatively