INTERNATIONAL JOURNAL OF CLIMATOLOGY Int. J. Climatol. 19: 989–1009 (1999) URBAN – RURAL VAPOUR PRESSURE DIFFERENCES AND THEIR ROLE IN THE DEVELOPMENT OF URBAN HEAT ISLANDS BJO  RN HOLMER* and INGEGA  RD ELIASSON Laboratory of Climatology at Physical Geography, Department of Earth Sciences, Go ¨teborg Uniersity, Box 460, S -405 30 Go ¨teborg, Sweden Receied 4 May 1998 Reised 14 December 1998 Accepted 17 January 1999 ABSTRACT Data for a 4-year period from Go ¨ teborg, Sweden was used to investigate the urban – rural humidity differences during clear and calm nights. The purpose was to analyse the impact of the Urban Moisture Excess (UME) on the Urban Heat Island (UHI) with focus on the longwave radiation balance and the latent heat flux. The statistical analysis demonstrated that a hot summer with reduced precipitation increased the mean UME to 3 hPa (‘normal’ summer conditions demonstrated an UME of about 1 hPa) and during some nights the UME was as high as 7 hPa. The analysis also demonstrated a positive correlation between UME and UHI, with the maximum UHI preceding the maximum UME by between 2 and 5 h. An analysis of the nocturnal development of UME demonstrated that the UME could develop from three combinations of urban and rural vapour pressure changes. The initial level of the rural vapour pressure, and thus the conditions for dewfall proved to be important. The influence of an UME of 3 hPa on the downward longwave radiation was calculated to about 3 W m -2 , which is 12% of the urban–rural radiation difference giving a weak enhancing effect of the UME on the UHI. Advection of dry air from the UHI circulation was considered to influence the urban vapour pressure change. After correction of the advective effect on the urban vapour pressure change, the urban – rural difference in the latent heat flux ranged from -5 to -25 W m -2 , which demonstrates that UME also has a considerable negative feedback on the UHI development. Copyright © 1999 Royal Meteorological Society. KEY WORDS: high-latitude city; Sweden; statistical analysis; humidity; longwave radiation; latent heat flux; advection 1. INTRODUCTION 1.1. Background This study has emerged from empirical observations made during the last few decades in Go ¨ teborg, Sweden. A considerable variation in the heat island intensity has been observed, even when apparently identical weather conditions prevailed, i.e. clear and calm nights. One of the field hypotheses is that a considerable part of this variation depends on processes related to air humidity. The question of the role of air humidity in Urban Heat Island (UHI) development is certainly not new. Multiple regression analyses demonstrate a positive correlation between nocturnal heat island intensity and vapour pressure (Sundborg, 1951; Emonds, 1954; Lindqvist 1970). These authors used vapour pressure in their calculations, but their equations did not include urban – rural humidity difference. However, several studies demonstrate that urban – rural humidity differences exist (Chandler, 1967; Ackerman, 1971, 1987; Kopec, 1973; Hage, 1975; Adebayo, 1991; Lee, 1991; Ja ´uregui and Tejeda, 1997). According to these studies, the urban – rural humidity difference displays both diurnal and seasonal variations. In general, the diurnal pattern demonstrates an Urban Moisture Excess (UME) at night and an urban moisture deficit during daytime. Winter values from temperate climates have been shown to be * Correspondence to: Laboratory of Climatology at Physical Geography, Department of Earth Sciences, Go ¨ teborg University, Box 460, S-405 30 Go ¨ teborg, Sweden; tel.: +46 31 7731955; fax: +46 31 7731986; e-mail: bjorn@gvc.gu.se CCC 0899–8418/99/090989 – 21$17.50 Copyright © 1999 Royal Meteorological Society