INTERNATIONAL JOURNAL OF CLIMATOLOGY Int. J. Climatol. 27: 2005–2015 (2007) Published online 27 June 2007 in Wiley InterScience (www.interscience.wiley.com) DOI: 10.1002/joc.1558 Urban/rural atmospheric water vapour pressure differences and urban moisture excess in Krefeld, Germany Wilhelm Kuttler,* Stephan Weber, Judith Schonnefeld and Alexandra Hesselschwerdt Department of Applied Climatology and Landscape Ecology, Institute of Geography, University of Duisburg-Essen, Campus Essen, D-45141 Essen, Germany 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. Copyright  2007 Royal Meteorological Society KEY WORDS atmospheric humidity; water vapour pressure; UME; UHI; urban climate Received 26 September 2006; Revised 6 February 2007; Accepted 5 April 2007 INTRODUCTION In the course of an increased number of basic research studies in the field of urban climatology, a growing interest in the behaviour of humidity within the urban atmosphere can be observed. A detailed knowledge about the behaviour of humidity within the urban canopy layer is not only important in order to better quantify and understand the dynamics of the urban energy balance (e.g. Weber and Kuttler, 2005) but also for the formation of the urban heat island (UHI) (Lee, 1991), urban radiation fog (Sachweh and Koepke, 1995), as an important factor and control parameter for the human energy balance and thermal comfort (Mayer, 1993) and in the context of emission of precursor gases for the development of photochemical smog by evaporation of dew in the morning (Yaalon and Ganor, 1968; Rubio et al., 2002). Furthermore, knowledge of urban dew properties is important in the assessment of deposition rates of trace gases since wet surfaces result in larger deposition velocities and therefore increased absorption of gases in comparison to dry surfaces (e.g. Mulawa et al., 1986). Urban areas are generally characterized by lower air humidity in comparison to the non-built surroundings * Correspondence to: Wilhelm Kuttler, Department of Applied Clima- tology and Landscape Ecology, Institute of Geography, University of Duisburg-Essen, Campus Essen, D-45141 Essen,Germany. E-mail: wiku@uni-due.de (Chandler, 1967; Landsberg, 1981). However, especially during clear and calm summer nights with weak wind speeds higher urban humidity levels are temporarily observed (e.g. Ackerman, 1987). Those positive water vapour pressure differences, indicating the urban area to be moister than the rural surroundings, are defined as Urban Moisture Excess (UME) (e.g. Holmer and Elias- son, 1999) or Urban Moisture Island (UMI) (Richards, 2005). In this paper we stick to the term UME which is widely used within scientific publications on urban/rural humidity differences. Different reasons for the development of UME are dis- cussed in literature, however, only few authors quantified those processes they identified for UME formation. There is consensus that the following processes are directly or indirectly related to UME formation (Hage, 1975; Nunez and Oke, 1977; Shreffler, 1978; Tapper, 1990; Grimmond and Oke, 1999; Richards, 2005; Fortuniak et al., 2006): - Absent, reduced or delayed dewfall in the city com- pared to the rural environment, - promoted evapotranspiration in the city during night and inhibited condensation associated with the noctur- nal UHI, - premature snowmelt in cities compared to rural areas and - anthropogenic sources of water vapour in the city (combustion, traffic, households, power plants). Copyright  2007 Royal Meteorological Society