Abstract:Urban/rural humidity differences were analysed by means of a climate station pair in Krefeld (51°20 N, 6°33 E), Germany, during the period from 11/2001 to 10/2002, on the basis of hourly averages of water vapour pressure. Focus was put upon on the examination of frequency, timing and duration of the Urban Moisture Excess (UME) ( e u−r > 0 hPa). It was found that the urban site was more humid (0 hPa < e u−r ≤ 0.5 hPa; weak UME) in 31.4% of the cases investigated and was only rarely significantly more humid ( e u−r > 0.5 hPa; intense UME) in 4.6%. Weak and intense UME occur during every month of the year with different frequencies per month. A diurnal course of UME was found for summer but not for winter. Weak and intense UME events show frequency maxima in the second half of the night. Most of them are characterized by durations of 1 hour, in few cases several hours duration were observed for weak (up to 14 h) and intense UME (up to 12 h).The main reason for formation of UME events might be that the surface dew point at the rural station was reached more often, earlier and lasted longer in comparison to the urban environment.