Surface runoff
Cities usually have a lower relative humidity than the surrounding air because cities are hotter, and rainwater in cities is unable to be absorbed into the ground to be released into the air by evaporation, and transpiration does not occur because cities have little vegetation. Surface runoff is usually taken up directly into the subterranean sewage water system and thus vanishes from the surface immediately. Better understanding of urban temperature and water vapor contributions and/ or loss will reveal the reasons for lower relative humidity within cities, especially since relative humidity is dependent on temperature changes.