During the daytime, green spaces tend to be cooler compared to the nearby urban environments; however, this cooling effect can be offset by higher humidity levels.
Urban green areas offer shade for city residents dealing with the increasing heat due to climate change, but how effective are they in alleviating the heat island effect when humidity is taken into account?
A recent study published in Nature Cities, co-authored by Yichen Yang, a doctoral student at Yale School of the Environment, and Xuhui Lee, the Sara Shallenberger Brown Professor of Climate Science, indicates that while temperature and humidity effects neutralize each other during the day, green spaces lead to a reduction in humid heat during nighttime.
“Humidity is just as important as temperature or shading when it comes to urban heat resilience,” Lee emphasized.
To investigate the combined effects of green spaces on moisture and heat in various urban areas worldwide, the research team utilized mobile smart sensors attached to bicycles. They measured conditions across 114 green spaces situated in 15 different cities. The results confirmed that while green spaces are cooler than the constructed environments around them during the day, this cooling is often offset by the accompanying rise in humidity.
Although trees can effectively lower temperatures, they also contribute to increased humidity due to evaporation. This balancing act between lower temperatures and elevated humidity frequently results in little change in the wet-bulb temperature, which indicates the stress caused by heat and humidity combined. Daytime green spaces may feel cooler, but the heightened humidity limits the actual comfort, the researchers explained.
During nighttime, green spaces can contribute to a decrease in both temperature and humidity due to lesser soil heat retention. This cooling effect is particularly pronounced in cities characterized by intermediate and humid climates, as highlighted in the study.
However, the findings do not imply that urban green spaces or tree planting offer negligible climate advantages, as asserted by Lee.
“Green spaces provide advantages throughout the day. They offer shade during the day and relief from humid heat at night,” he added.
The authors urged urban planners to refrain from applying a uniform approach in city design. To maximize the benefits of green spaces, designers should consider incorporating various cooling strategies.
The sensors used in this research were developed with funding from a Leitner Award aimed at promoting unique environmental collaborations. Field data collection received support through a grant from the Robert Wood Johnson Foundation.
