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 (Delta TX-D), relative humidity (Delta RHX-D), and humidity ratio (Delta WX-D) were investigated. The relationships of Delta W with condensation (dew and frost) and precipitation events, meteorological parameters, and Delta T were examined. The results show that variations in Delta RHX-D exhibited a close inverse relationship with variations in Delta TX-D. Greater negative Delta RHX-D occurred during the nighttime when strong heat islands were present. Meanwhile, diurnal and seasonal patterns of Delta WX-D were clearly observed. Greater and more frequent negative Delta WX-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 Delta WX-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. Delta W was influenced by meteorological parameters, but the trends were not clear. No relation was observed between Delta W and Delta T. The findings can improve understanding of urban humidity and provide valuable information for humidity-related studies.