Hot weather amplifies the urban dry island effect, especially in wetter climates

Atmospheric humidity is usually drier in cities than the surrounding rural areas, a phenomenon known as the urban dry island (UDI) effect. However, the response of atmospheric humidity to hot weather in urban versus rural settings remains unknown. Using long-term summer (June-August) observations at 1658 stations over 1961-2020, we find that China is dominated by drying trends in atmospheric humidity (i.e., increasing vapor pressure deficit [VPD]). These drying trends are aggravated on hot days and amplified by urbanization, i.e., the UDI effect is stronger in hot weather. This amplification of the UDI effect on hot days is more prominent in humid than in arid regions. Attributions show that the stronger VPD-based UDI effect on hot days is explained by increased contribution of air temperature in southeastern China, and specific humidity in North China. We suggest that adaptations are required to mitigate adverse combined effects of urban heatwaves and UDIs.