Diurnal and Seasonal Patterns of Urban Dry Islands Quantified with a Global Dataset

Research quantifying urban-rural differences in humidity, the so called urban dry or moisture islands (UDIs, UMIs), has been mostly confined to case-studies of single cities or regions. An analysis of the typical diurnal and seasonal patterns of UDIs at larger scale is still missing even though changes in humidity can impact human well-being, building energy consumption, and urban ecology. In this study, we use a large data set (1089 stations) of globally distributed near surface air temperature and humidity measurements to quantify the typical diurnal and seasonal patterns of UDIs, which developed due to rapid urbanization in many parts of the world, using a time for space substitution. We distinguish between “relative” and “absolute” UDIs quantified as the urban-rural difference in relative and actual humidity measurements, respectively, to account for differences in their diurnal and seasonal patterns.

We find that absolute UDI is largest during daytime with the highest humidity decrease in the late afternoon hours, while relative UDI is generally largest at night. Peak relative humidity decrease occurs during the late evening hours with magnitudes of around -10 to -11% between 20-00 local time in summer. Both relative and absolute UDI are largest during the warm season. Separating the contribution of actual humidity decrease and change in temperature to the formation of relative UDI, we find that relative UDI is mostly caused by absolute UDI during daytime and by temperature, i.e., urban heat island (UHI), during nighttime. The quantification of UDIs, as presented here, is crucial for subsequent impact analyses of urbanization on outdoor thermal comfort, urban ecology, and building energy consumption.