As temperatures around the world reach record levels, urban areas are facing increased heat stress. Cities are generally hotter and drier than adjacent rural land. But in the Global South, there is an additional complicating factor: humid urban heat.
A new study, led by scientists at the Yale School of the Environment and published in Nature, investigated the combined effect of temperature and humidity on urban heat stress using observational data and an urban climate model calculation. The researchers found that the heat stress load depends on the local climate, and a humidification effect can erase the cooling benefits that would come from trees and vegetation.
“A widely shared view is that urban residents experience more heat than the general population due to the urban heat island phenomenon. This view is incomplete because it omits another ubiquitous urban microclimate phenomenon called the urban dry island: that urban land tends to be less humid than surrounding rural land,” says Xuhui Lee, the Sara Shallenberger Brown Professor of Meteorology, who led the study. “In dry, temperate, boreal climates, urban residents actually experience less heat stress than rural residents. But in the humid Global South, the urban heat island is dominant over the urban dry island, resulting in two to six most dangerous heat stress days per summer”.
Lee and YSE doctoral student Keer Zhang, the study’s lead author, say they were motivated to investigate the topic for several reasons: a large percentage of the world’s population lives in urban areas; many people in informal urban settlements do not have access to air conditioning; and the problem will get worse as temperatures rise and more people move to cities. About 4.3 billion people, or 55% of the world’s population, live in urban settings, with the number expected to rise to 80% by 2050, according to the World Economic Forum.
The researchers developed a theoretical framework on how urban soil modifies both air temperature and humidity and demonstrated that these two effects have the same weight in heat stress measured by wet bulb temperature, unlike other heat indices. , which weigh more than temperature. than humidity. Wet bulb temperature combines dry air temperature with humidity to measure moist heat. The study results, the authors note, raise important questions.
“Green vegetation can reduce air temperature through water evaporation, but it can also increase heat load due to air humidity. So the question is to what extent does this humidification effect erase the cooling benefit? that arises from temperature reduction.We hope to answer this question in a follow-up study, where we are comparing wet-bulb temperature observations in urban green spaces (with dense tree cover) and those in built-up neighborhoods says Lee.
Zhang says he hopes the study can lead to more research on how cities can mitigate heat stress.
“Our diagnostic analysis on the urban wet bulb island found that improving the efficiency of urban convection (the efficiency in dissipating heat and water) and reducing heat storage at night can reduce urban humid heat during the day. and night, respectively. We hope that our work will promote further research on optimizing urban forms and materials to improve thermal comfort,” he says.
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