As temperatures around the world reach record highs, urban areas face increased heat stress. Cities are generally warmer and drier than adjacent rural country. But in the Global South there is another complicating factor – urban humid heat.
The new study, led by researchers at the Yale School of the Environment and published in Nature, examined 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 burden of heat stress depends on the local climate, and the humidifying effect can wipe out the cooling benefits that would come from trees and vegetation.
“A widely held view is that urban dwellers suffer more heat stress than the general population due to the urban heat island phenomenon. This view is incomplete because it ignores another ubiquitous urban microclimate phenomenon called the urban dry island – that urban soil tends to be less humid than the surrounding rural country,” says Xuhui Lee, the Sara Shallenberger Brown Professor of Meteorology, who led the study.
“In dry, temperate, boreal climates, urban dwellers are actually less heat-stressed than rural dwellers. But in the humid global south, the urban heat island is dominant over the urban dry island, resulting in two to six additional dangerous heat stress days in the summer.”
4.3 billion people, or 55% of the world’s population
Lee and YSE doctoral student Keer Zhang, lead author of the study, say they were motivated to explore the issue 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 environments, and that number is 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 air humidity, and showed that the two effects have equal weight to heat stress as measured by wet bulb temperature, unlike other thermal indices that weight temperature more . than humidity.
“Green vegetation can reduce the air temperature by evaporating water, but it can also increase the heat load by air humidity. The question then is to what extent this humidifying effect cancels out the cooling benefit from the temperature reduction. We hope to answer this question in a follow-up study where we compare the observations wet-bulb temperatures in urban green spaces (with dense tree cover) and built-up neighborhoods,” says Lee.
Zhang says he hopes the study can lead to further research into how cities can mitigate heat stress.
“Our diagnostic analysis of the urban wet bulb island found that increasing the efficiency of urban convection (efficiency in dissipating heat and water) and reducing heat accumulation at night can reduce daytime and nighttime urban humid heat. We hope our work will encourage further optimization research urban shapes and materials for better thermal comfort,” he says.
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