Heat waves in urban areas: impacts and mitigation

Cities are well known to be hotter than the rural areas that surround them; this phenomenon is called the “urban heat island” (UHI). Heat waves are excessively hot periods during which the air temperatures of both urban and rural areas increase significantly. However, whether urban and rural temperatures respond in the same way to heat waves remains a critical unanswered question. In this study, a combination of observational and modeling analyses of a heat wave event over the Baltimore-Washington urban corridor indicates synergistic interactions between urban heat islands and heat waves. That is, not only do heat waves increase the regional temperatures, but they also intensify the difference between urban and rural temperatures. As a result, the added heat stress in cities will be even higher than the sum of the background urban heat island effect and the heat wave effect. Our results also attribute this added impact of heat waves on urban areas primarily to the lack of surface moisture. Given that heat waves are projected to become more frequent and that urban populations are substantially increasing, these findings underline the serious heat-related health risks facing urban residents in the 21st century and the necessity of implementing mitigation strategies to reduce these risks. The effectiveness of cool and green roofs as UHI mitigation strategies are thus further evaluated at city scales, during heat wave episodes. These strategies are shown to reduce urban surface temperatures by about 5 K and urban air temperatures by about 1 K. These reductions are most significant in the dense urban cores of Baltimore and Washington, but they are not sufficient to fully offset the UHI effect.