The urban heat island (UHI) induces a weak inflow of air from the rural surroundings. In Göteborg, 0.7 million inhabitants on the Swedish west coast, this urban heat island circulation (UHIC) is fairly well investigated. The UHIC in Göteborg is channelled along three valleys in the east while there is an outflow along the river estuary towards the sea. The wind speed is only 0.5 ms-1. During these nights an elevated inversion is capping the area - 50 metres in the valleys and 90 metres in the centre. Thus, it is possible to calculate the advection. If the average UHI is 2 degreesC, then the advection gives a cooling potential of 0.3 degreesCh-1 or 9 Wm-2 in the city centre.
In the winter the nighttime urban net radiation loss is 1-3 Wm-2 lesser than in the rural area during clear and calm weather according to investigations in cities in Canada and Japan. Then there is a cooling need in the urban area to compensate the lower radiation loss. The advection seems to overcompensate this loss but there is also the antropogenic heat flux, which must be balanced.
During clear and calm summer nights the humidity of the air is higher than in the rural area (an urban moisture excess - UME). The UME develops as a result of different combinations of rural and urban condensation and/or evaporation. The UME is usually 1-3 hPa. In the winter the UME is small or nonexistent.
Since there is a capping inversion, rising vapour pressure indicates evaporation and decreasing condensation. However, the advection of drier air from the rural area gives a dilution of the urban air. In order to estimate the urban latent heat flux the urban vapour pressure change must be corrected for the advective influence. If the UME is 1 hPa then the advection will diminish the urban vapour pressure with about 0.1 hPa·h-1. The advection shall be added to the observed vapour pressure change is rising and subtracted if it is decreasing.
Calculations for three typical cases changed the observed rates of -0.34, 0.12 and 0.28 hPa·h-1 respectively to -0.13, 0.50 and 0.44 hPa·h-1. Thus, the effect of the advection is considerable. After the correction the urban-rural differences in latent heat flux ranged between -3 and -25 Wm-2. The latent heat flux difference gives in all calculations a negative feedback on the UHI.
The summer-night urban net radiation loss is also lesser compared with the rural area (typically about 15 Wm-2). The advection of sensible heat (9 Wm-2) and the advection corrected latent heat flux (up to 25 Wm-2) are sometimes not enough to balance the radiation deficit. In other cases there will a deficit implying a cooling potential of -0.3degreesCh-1compared to rural areas, which storage heat flux may balance.