LETTER doi:10.1038/nature13462 Strong contributions of local background climate to urban heat islands Lei Zhao 1,2 , Xuhui Lee 1,2 , Ronald B. Smith 3 & Keith Oleson 4 The urban heat island (UHI), a common phenomenon in which sur- face 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, DT). A common perception is that reduction in evaporative cooling in urban land is the dominant driver of DT (ref. 5). Here we use a cli- mate model to show that, for cities across North America, geographic variations in daytime DT 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 sur- rounding 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 DT by 3.0 6 0.3kelvin (mean and standard error) in humid climates but decreasing DT by 1.5 6 0.2 kelvin in dry climates. In the humid east- ern United States, there is evidence of higher DT in drier years. These relationships imply that UHIs will exacerbate heatwave 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 pos- itive temperature anomalies may be reinforced by a precipitation– temperature feedback 8 . Our results support albedo management as a viable means of reducing DT on large scales 9,10 . The conversion of natural land to urban land causes several notable perturbations to the Earth’s surface energy balance. Reduction of evap- orative cooling is generally thought to be the dominant factor contrib- uting to UHI. Anthropogenic heat release is an added energy input to the energy balance and should increase the surface temperature. Energy input by solar radiation will also increase if albedo is reduced in the process of land conversion. Buildings and other artificial materials can store more radiation energy in the daytime than can natural vegetation and soil; release of the stored energy at night contributes to night-time 1 Yale-NUIST Center on Atmospheric Environment, Nanjing University of Information Science and Technology, Nanjing 210044, China. 2 School of Forestry and Environmental Studies, Yale University, New Haven, Connecticut 06511, USA. 3 Department of Geology and Geophysics, Yale University, New Haven, Connecticut 06511, USA. 4 National Center for Atmospheric Research, Boulder, Colorado 80305, USA. b a d c 0 500 1,000 1,500 0 <0 1.0 3.5 6.5 <0 1.0 2.5 4.5 2 4 6 Annual-mean precipitation (mm) Annual-mean daytime ΔT (K) Annual-mean night-time ΔT (K) 10 4 10 5 10 8 10 7 0 2 4 6 Population y = (0.0052 ± 0.0009)x – 1.3 y = (0.64 ± 0.0013)x – 6.0 Figure 1 | Precipitation and population influences on MODIS-derived annual-mean UHI intensity. a, Map of daytime UHI (shown in K by symbol type/size). b, Dependence of daytime UHI on precipitation (r 5 0.74, P , 0.001). c, Map of night-time UHI. d, Dependence of night-time UHI on population (r 5 0.54, P , 0.001). Red, green and blue symbols denote cities with annual mean precipitations less than 500 mm, between 500 and 1,100 mm, and over 1,100 mm, respectively. Lines in b and d are linear regression fits to the data. Parameter bounds for the regression slope are the 95% confidence interval. 216 | NATURE | VOL 511 | 10 JULY 2014 Macmillan Publishers Limited. All rights reserved ©2014