A new study by Yale School of the Environment has found that humidity can significantly impact the cooling benefits of urban greenery. Researchers investigated the combined effect of temperature and humidity on urban heat stress, using observational data and urban climate modelling. The results of the study, published in the journal Nature, showed that humid air can cancel out the cooling benefits of trees and vegetation.
The study’s lead author, Xuhui Lee, explained that in the humid Global South, the urban heat island effect, which is caused by the absorption of heat by buildings and roads, is more dominant than the urban dry island effect. The urban dry island effect refers to urban land being less humid than surrounding rural areas. Lee said that the incomplete view of urban heat stress does not consider the urban dry island phenomenon.
The researchers developed a theoretical framework that showed the equal weight of air temperature and air humidity in heat stress, measured by the wet-bulb temperature. Wet-bulb temperature combines dry air temperature with humidity to measure humid heat.
The study raised important questions about the benefits of urban greenery. Green vegetation can reduce air temperature via water evaporation, but it can also increase heat stress because of air humidity. The researchers hope to answer this question in a follow-up study by comparing the wet-bulb temperature in urban green spaces with dense tree cover and those in built-up neighbourhoods.
The study also suggests that enhancing urban convection efficiency and reducing heat storage at night could reduce daytime and nighttime urban humid heat, respectively. Doctoral student Keer Zhang said that the work could promote research on optimising urban shapes and materials for better thermal comfort.
As temperatures globally reach record-level highs and urban areas face increased heat stress, understanding the impact of humidity on urban heat stress is increasingly important. The study’s findings can help cities develop better strategies for managing urban heat stress and improving the quality of life for residents.