Complex sentence: 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.
Split into simpler sentences: Cities usually have a lower relative humidity than the surrounding air because cities are hotter. 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.