Urban areas are experiencing an increase in heat stress due to rising temperatures globally. Cities are known to be hotter and drier than rural areas, and this is particularly true in the Global South.
A recent Nature study led by scientists at the Yale School of the Environment explored the impact of temperature and humidity on urban heat stress, using observational data and urban climate models. The research found that the degree of heat stress depends on the local climate and that the cooling advantages provided by trees and vegetation can be offset by increased humidity.
Heat stress in urban areas
Using temperature data and urban climate models, they found that the degree of heat exposure depends on the local climate, and the wind chill benefits of trees and vegetation can be negated by humidity.
Xuhui Li, Professor of Meteorology, the urban heat island phenomenon, which refers to rising temperatures in urban areas compared to surrounding rural areas, causes city residents to experience more severe heat than the General population.
While it is not clear whether another urban microclimate phenomenon, urban humidity, is also responsible, it is known that urban areas tend to be less humid than rural areas. In dry, temperate, and boreal climates, rural residents experience more heat stress than urban residents.
However, in the humid Global South, the urban heat island effect is dominant, generating an additional two to six days of extreme heat per summer season.
A recent study also found that the combined effects of temperature and humidity on urban heat stress vary depending on the local climate, and that humidity can counteract the wind chill benefits of vegetation. Li’s comments were made in the context of a discussion on the impact of heat stress on urban populations.
Li said vegetation has the potential to lower air temperatures through evaporation, but it can also increase heat through increased air humidity. He is left with the question to what extent the humidifying effect compensates for the cooling effect caused by the decrease in temperature.
Study suggests ways to alleviate urban heat
The study compared humidity temperature observations in urban green spaces or areas with dense tree cover with those in surrounding areas.
Zhang expressed hope that the study will spur more research on ways to alleviate the effects of urban heat.
The urban wet bulb island study found that improving urban water and heat dissipation efficiency and reducing heat accumulation at night can decrease urban humid heat during the day and night, respectively.
He also expressed his hope that this study would inspire further research to optimize urban design and materials for better thermal comfort.
Urban heat: Risks and impacts
According to estimates by the World Economic Forum, by 2050, 80% of the world’s population will live in urban areas and will be at risk of further impacts from heat. Heat waves are responsible for numerous deaths each year in the US and other countries. The Centers for Disease Control and Prevention warn that prolonged exposure to hot air can cause heat exhaustion and even stroke.
While sweating can help humans and some animals cool down, higher levels of humidity can reduce the rate of evaporation from the skin, making it harder to cool down. In fact, a day with a temperature of 95 degrees and a relative humidity of 50% can feel like 107 degrees, resulting in discomfort and dehydration. Summer heat can also lead to irrational behaviour and an increase in violence, and studies show that violent crime tends to increase during the summer months.
University of Alabama psychologist and professor Dr. Josh Klapow says that people can feel uncomfortable even before they begin to experience the physical impacts of heat, and this discomfort can lead to distress. The researchers hope that further studies will provide guidance to cities on how to mitigate the effects of heat stress and combat climate warming.
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