In this study, we tended to fill in the gap of less-explored urban humidity. Our analysis revealed a more significantly rising trend in humidity in Hong Kong. A new lumped moisture model is developed to better understand and quantify the urban moisture environment. The modelling results demonstrate that a weakened city ventilation tends to increase urban air moisture. Besides, natural surfaces (vegetation and bare soil) contribute to reducing urban heat stress through evapotranspiration. It is noteworthy that the absolute humidity will start to decrease when the increment of natural surfaces reaches a threshold. On the other hand, large wall area in Hong Kong performs both sinks and storages for urban air moisture.