Urban moisture and dry islands: spatiotemporal variation patterns and mechanisms of urban air humidity changes across the globe

Urbanization-induced atmospheric moisture changes, embodied as urban moisture island (UMI) and urban dry island (UDI) effects, are not as thoroughly understood as the urban heat island (UHI) effects, despite their significant influence on human comfort and well-being. This paper offers the first systematic review and quantitative meta-analysis of global urban–rural humidity contrasts, aiming to advance our comprehension of the mechanisms, intensity, patterns, and implications of urban humidity changes. The meta-analysis compiles observational data from 34 studies across 33 cities. It reveals that mid-latitude cities predominantly exhibit moderate UMI and UDI effects, and cities with low mean annual precipitation and distinct dry/wet seasons, however, exhibit extreme UMI and UDI effects. The diurnal cycle analysis presents more pronounced UMI effects at night, largely due to increased evapotranspiration and delayed dewfall linked with UHI. On a seasonal scale, UDI effects dominate in spring, while UMI effects peak in winter for mid-latitude cities and in summer for low-latitude cities. In addition, city characteristics such as topography, morphology, and size significantly shape urban–rural humidity contrasts. Coastal cities are subject to sea-breeze circulation, importing moisture from sea to land, whereas mountainous cities can accumulate humidity and precipitation due to geographical barriers and vertical airflow. High-density urban areas generally experience heightened UMI effects due to restricted airflow and ventilation. Larger cities with higher populations contribute to increased UMI effects, particularly in winter, due to stronger anthropogenic moisture sources. This paper also discusses multi-dimensional humidity impacts and strategies for humidity-sensitive urban planning in the context of climate change. It identifies critical gaps in current research, paving the way for future exploration into urban humidity changes.