Urban and rural airport surface weather observations in a 13-year period of rapid city growth are used to document city effects on absolute and relative humidity in a dry climate at fairly high latitudes. The city is found to be dry at all hours (relative humidity) and dry by day but moist at night (absolute humidity) in all but winter months. Some but not all of the major features of the humidity differences conform to those found by Ackerman for Chicago. In winter, relative and absolute humidities are high in the city at all hours because of vertical mixing and combustion sources. Maximum differences in absolute humidity at night occur in March and August. The former is attributed primarily to urban snowmelt on occasions when rural temperatures are below freezing. The August peak occurs near sunrise and is attributed mainly to rural dewfall. The times of maximum cooling and maximum absolute humidity in the city on clear hights in summer are strongly dependent on wind speed. For this reason it is argued that interaction of advection processes and vertical flux divergence (radiative plus turbulent) seems to be essential for realistic simulation of urban cooling rates at night. Moisture differences appear not to play a crucial role in heat island development.