Air humidity characteristics of local climate zones: A three-year observational study in Nanjing

Atmospheric humidity in the urban canopy layer is relevant to various fields, such as bioclimatology and public health. However, considerably less research has been conducted about urban effects on humidity than on temperature. This study aims to elucidate the spatiotemporal characteristics of humidity at the neighborhood scale. The humidity differences of 8 local climate zones (LCZs) in Nanjing, China, were analyzed based on three-year hourly observations. LCZ D (low plants) was used as the reference site. Differences in temperature (ΔTX−D), relative humidity (ΔRHX−D), and humidity ratio (ΔWX−D) were investigated. The relationships of ΔW with condensation (dew and frost) and precipitation events, meteorological parameters, and ΔT were examined. The results show that variations in ΔRHX−D exhibited a close inverse relationship with variations in ΔTX−D. Greater negative ΔRHX−D occurred during the nighttime when strong heat islands were present. Meanwhile, diurnal and seasonal patterns of ΔWX−D were clearly observed. Greater and more frequent negative ΔWX−D (moisture deficit) were observed during the daytime than during the nighttime and in warm months than in cold months, whereas the opposite trend was observed for positive ΔWX−D (moisture excess). The frequency of urban moisture excess increased during the nighttime with heavy condensation. Precipitation events significantly increased the frequency of urban moisture excess, and this effect could last until 24 h after precipitation. ΔW was influenced by meteorological parameters, but the trends were not clear. No relation was observed between ΔW and ΔT. The findings can improve understanding of urban humidity and provide valuable information for humidity-related studies.