Urban-rural vapour pressure differences and their role in the development of urban heat islands

Data for a 4-year period from Göteborg, 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.