Analysing the Urban Moisture Excess (UME) in a Midlatitude City

Urban moisture excess investigations have been conducted in Krefeld (240,000 inhabitants.; left-hand side of the river Rhine; western part of Germany) during 11/2001 – 10/2002 taking the hourly mean meteorological values of a suitable urban/rural climate station pair with a data availability of 86 %. Air humidity was calculated as water vapour pressure (e), urban/rural differences were expressed as De = e_u - e_r. Statistical data for De (in hPa) are: x (average) = -0.51, abs. max. = +2.61, abs. min = -4.56 and standard deviation = 0.53. The frequency distribution is right hand skewed and unimodal. During 90 % of time the rural environment was more moister than the urban area (De < 0.0 hPa), in 10 % of the cases the urban station was more moister (De > 0.0 hPa, coinciding with low atmospheric exchange conditions). No significant dependences of the De – values to wind direction, wind velocity and urban heat island effect could be detected. During winter months the average range of differences was smaller than in summer. No structured daily course in winter was evident. In summer the late afternoon values reached maximum negative differences, threefold to night values so that the rural environmentis more moist moister than the city.

Refering to a strong limit of positive urban/rural differences (De > 0.50 hPa) the city was more moister between April and November, mainly during the second half of the night (0 h – 7 h). During this time the duration of UME was in 9 of 25 cases one hour, in 8 cases between 2 and 4 hours, and in 8 cases, too, between 5 and 7 hours. The reason for the urban environment being occasionally more moister compared to than the rural environment might be that the surface dew point at the rural station has been will bereached more often, earlier and lasted longer than that in the urban environment. The results presented here are giving a first contribution to the question about the frequency and duration of UME which are discussed only scarcely in current literature very scarcely. Further research is neededhas to be done to gather more informationget a general statement toabout UME for cities in different climatic zones.