The potential impact of cool roof technologies upon heat wave meteorology and human health in Boston and Chicago

Heat is the greatest weather-related killer in Boston and Chicago, as well as in other large urban areas. Our goal is to determine whether increasing urban solar reflectance, through the use of reflective roof products, would lessen the intensity of extreme heat events and save lives during such events. We use a synoptic climatological approach that places days into air mass categories encompassing a wide variety of individual weather metrics including air temperature and dew point. The dry tropical (DT) and moist tropical plus (MT+) air masses are the most oppressive and deadliest. We identify and perform an air mass classification for four actual heat events in Boston and Chicago to determine whether a 0.15 and a 0.25 increase in roof surface reflectance would alter weather conditions during heat waves. These reflectance modifications are achievable in cities adopting reasonable urban heat mitigation strategies. For Boston and Chicago, reflective roofs reduce temperatures and dew points enough to generate actual changes in air mass type from DT and MT+ to more benign air masses that are not harmful to human health. In Boston, using the 0.25 reflectance increase, our modeling indicates that 12 lives would be saved during the four extreme heat events. For Chicago, we find that 42 lives would be saved using the same reflectance increase. Considering that 10 to 15 such heat events could occur over a decade, we suggest that the use of reflective roofing products could potentially save hundreds of lives per decade during excessive heat events in each city.