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Assessing the Impact of Urban Green Spaces on Heat and Humidity

Urban green spaces are increasingly recognized for their ability to mitigate the effects of rising temperatures in cities, largely driven by climate change. However, their effectiveness in combating the heat island effect, particularly when humidity levels are taken into account, has been the subject of recent research.

A study published in Nature Cities, co-authored by Yale School of the Environment doctoral student Yichen Yang and Professor Xuhui Lee, sheds light on this issue. The research investigates how temperature and humidity interplay within urban green areas and their overall impact on discomfort due to heat.

“In the context of urban heat resilience, humidity is as significant as temperature and shade,” explained Lee, emphasizing the multifaceted nature of heat stress in cities.

The research team employed mobile smart sensors attached to bicycles to collect data on temperature and humidity across 114 green spaces in 15 cities worldwide. This innovative approach allowed them to analyze how these factors interact in both humid and arid urban environments. The findings revealed that while green spaces successfully lower temperatures during the daytime compared to surrounding urban areas, the accompanying rise in humidity often negates this cooling effect.

Trees and other vegetation effectively cool the air through shade and evapotranspiration. However, this same evaporation process can lead to increased humidity levels, which diminishes the perceived reduction in heat. The research noted that while green spaces provide relief, the effect on wet-bulb temperature—a measure of combined heat and humidity stress—remains minimal during the day.

Interestingly, the study found that at night, urban green spaces can contribute to lower both temperature and humidity. This phenomenon occurs due to the decreased retention of heat in the soil during cooler nighttime hours. The impact tends to be most pronounced in cities that experience intermediate or wet climates.

Lee was keen to clarify that these findings do not undermine the climate benefits provided by green spaces. He stated, “Urban green spaces play a crucial role throughout the day, offering shade and reducing humid heat during nighttime hours.”

In light of these insights, the authors advocate for a more nuanced approach to urban planning. They suggest that city designers should implement tailored green space strategies and complement them with additional cooling techniques to maximize their benefits.

The development of the sensors used in the study was facilitated by a Leitner Award for Uncommon Environmental Collaborations, while field observations were supported by a grant from the Robert Wood Johnson Foundation.

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