Strong contributions of local background climate to urban heat islands

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, Delta T). A common perception is that reduction in evaporative cooling in urban land is the dominant driver of DT (ref. 5). Here we use a climate model to show that, for cities across North America, geographic variations in daytime Delta 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 less efficient and urban warming occurs (and vice versa). This convection effect depends on the local background climate, increasing daytime Delta T by 3.06 +/- 0.3 kelvin (mean and standard error) in humid climates but decreasing Delta T by 1.5 +/- 0.2 kelvin in dry climates. In the humid eastern United States, there is evidence of higher Delta T in drier years. These relationship simply that UHIs will exacerbate heat wave stress on human health in wet climates where high temperature effects are already compounded by high air humidity(6,7) and in drier years when positive temperature anomalies may be reinforced by a precipitation temperature feedback(8). Our results support albedo management as a viable means of reducing Delta T on large scales(9,10).