Humidity metric choice shapes our understanding of urban heat stress and its underlying mechanisms

As heat stress increasingly threatens urban livability worldwide, it is essential to improve our understanding of its drivers and mitigation. While temperature has historically been the focus of heat stress research, the role of humidity is gaining recognition due to its contributions to both heat stress and mitigation. However, there are conflicting findings on the relative role of humidity, with some studies suggesting that drying can offset the warming effect of air temperature increases, and others suggesting that projected moistening will exacerbate humid heat. This uncertainty is compounded by the complex nature of humidity, both as a physical variable and in its measurement through diverse metrics. In this study, we seek to elucidate 1) global patterns of urban-rural moisture differences and 2) the relative roles of humidity and temperature contributions to urban humid heat. We use urban climate simulations from the Community Earth System Model and observations from a global set of urban-rural weather stations. We derive physics-based, theoretical expressions to quantify the relative roles of humidity and temperature towards humid heat stress. Our findings reveal that humidity metric choice fundamentally shapes 1) our understanding of urban-rural humidity differences and 2) our ability to differentiate between humidity and temperature contributions. Consequently, our understanding of the impacts of urbanization on humidity and humid heat stress are strongly influenced by metric selection. These results provide insight into the mechanisms of urban heat stress and can inform climate-sensitive urban planning to mitigate future urban heat risks.