The urban moisture island phenomenon and its mechanisms in a high-rise high-density city

Previous studies tend to report the effect of rapid urbanization on temperature, but not on humidity. In this study, we aimed to address this research gap by examining diurnal profiles of temperature and humidity in Hong Kong. Our analysis revealed a more significant rising trend in humidity. In addition, our analysis of equivalent temperature revealed that latent temperature (i.e., moisture) contributes more to the aggravation of urban heat stress than does sensible temperature. The average urban air moisture concentration exceeds that in rural areas for both day and night, confirming the existence of an urban moisture island. A new lumped moisture model is developed to predict the hourly urban air humidity using weather data from the operational suburban weather stations, to better understand and quantify the urban moisture environment. The resulting coupled thermal-moisture model can capture the main characteristics of diurnal profiles of urban air temperature and humidity in Hong Kong. The modelling results demonstrate that weakened city ventilation tends to lead to an increase in urban air moisture. Furthermore, natural surfaces (vegetation and bare soil) reduce urban heat stress by evapotranspiration. It is noteworthy that although the increase in fractions of natural surfaces can lead to more potential moisture sources, the absolute humidity will start to decrease when the fraction of natural surfaces reaches a threshold. This decreasing moisture trend may be attributable to reduced evaporation owing to the decreased surface temperature, reduced anthropogenic moisture and increased ventilation. Besides, the large wall areas in the compact high-rise areas of Hong Kong function as both sinks and storage sites, contributing to a noticeable diurnal trend in the urban air humidity profile. In brief, the significant urban moisture island phenomenon in Hong Kong may result from enhanced evapotranspiration, restrained moisture sinks, intensive human activities and a weakened wind environment.