Urban heat island behaviors in dryland regions

Urban heat island (UHI) characteristics and mitigation strategies for dryland cities differ from those for wetter urban regions. Whereas the latter typically see daytime surface UHIs, the rapid heating and cooling of deserts surrounding arid cities often produces daytime 'urban cool islands' and nighttime UHIs. Degrees of aridness, extent of vegetation, elevation, latitude, humidity, topography, and typical building types are likely to influence dryland UHI dynamics. This study analyzes variations in thermal effects at multiple scales for 10 dryland urban regions representing varied geographies worldwide with an aim to establish a broader understanding of the spectrum of UHI patterns in dryland cities. We used GIS to assemble daytime and nighttime satellite imagery, determined land surface temperature and vegetation at a 30-meter scale, and analyzed typical neighborhood-scale examples of six land cover types in each region. The 10 regions showed large variation in thermal effects. We found a strong daytime surface UHI in only one. Nighttime heat islands were more pronounced. However, all regions showed strong small-scale variation in temperature, averaging a 12.3 °C difference between mean top-quintile and bottom-quintile surface temperatures. Samples of urban forest landscapes cooled daytime temperatures an average of 5.6 °C compared to metro averages. Irrigated lawn and multistory building land cover samples also had a substantial cooling effect. Xeriscaped landscapes amplified daytime heating. Our results indicate that UHIs for dryland cities are unlikely to be reduced by xeriscape strategies, but that shade-maximizing urban forestry and built form hold promise to reduce heat islands.