Berlin Environmental Atlas

04.04 Temperature and Moisture Conditions in Medium and Low-exchange Nocturnal Radiation Periods (Edition 1993)

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Relative Humidity

The relative humidity (degree of saturation of the air with water vapor) emerges from the air temperature and the vapor pressure (water vapor capacity of the air) available in the atmosphere. The water vapor capacity is increased above all through the evaporation process (surfaces of waters, vegetation), respiration, particularly of plants (perspiration) and vapor inputs from industry, small business and households. Conversely the vapor capacity is reduced through decreases in the evaporation potential e.g. due to lowered groundwater levels and ground sealing.

The atmosphere's vapor capacity and with it the city landscape are subject to very multi-faceted peripheral conditions. The vegetation deficit in the heavily built-up centers must not necessarily lead to a reduction of the air's vapor capacity. Anthropogenic sources provide not only a balance but often produce a higher level. This phenomenon is very apparent in the summer months (see Fig. 3). Kreuzberg, Alexanderplatz and also Hellersdorf display slight increases compared to heavily greened Zehlendorf. In the pine stocks of Grunewald the vapor pressure lies higher than in the treeless location Dahlemer Feld. This is in part because of the exposed sands and the interspersed vegetation only little vapor can be produced there in the course of the summer.

Fig. 3: Mean Vapor Pressure and Relative Humidity at 2 m Height from June to September 1991 at Various Locations in Berlin

[Statistical base of this Figure is also available as Excel-File (MS-Excel is required).]

Altogether the vapor pressure differences in the area of the city are comparatively slight, so that the relative humidity is inversely proportional to the air temperature. The relative humidity in the built-up areas lies on average around 4 to 11 % lower than in the Grunewald. At Dahlemer Feld, the especially low air temperature in the night hours leads to an equalization with the other locations despite low vapor pressure.

In the course of an especially warm day differences in the vapor pressure from location to location are more significant than for the monthly average (see Fig. 4). In contrast, the course of the day for a given location is only distinctly noticeable in the outer areas of the city and with respect to the relative humidity. On this day there is the formation of a normal double wave in the vapor pressure which is marked by a minimum in the early morning hours (dew formation and reduction of perspiration) and by an interruption of the maximum during the noon - and afternoon hours on account of increased air exchange. Due to the high temperature on this day a high moisture deficit is produced over the well ventilated Dahlemer Feld. At night a moisture saturation of more than 90 % is reached here through the strong cooling already at 2 m height to so that at the floor dew formation occurs. The forest location Grunewald also at exhibits a relatively large day-time amplitude.

Fig. 4: Daily Course of Vapor Pressure and Relative Humidity on a Hot Low-exchange Radiation Day (8 July 1991) at Various Locations in Berlin

[Statistical base of this Figure is also available as Excel-File (MS-Excel is required).]

With a proven nocturnal moisture deficit of over 20 %, the built-up city areas are relatively dry at night in contrast to the locations Dahlemer Feld and Grunewald. During the day occasionally a higher value at night than can be found at the Dahlemer Feld and the Grunewald. At the urban locations Kreuzberg and Alexanderplatz anthropogene sources contribute to the comparatively high vapor pressure.

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