Temperature and Moisture Conditions in Medium Low-exchange Nocturnal Radiation Periods 1992
The parameters temperature, water vapor capacity and relative humidity are determining elements for the characterization of local climates and are closely dependent on each other. The risk of excessive humidity is of additional significance as a bioclimatic aspect. The overwarming of the city compared to the surrounding countryside and also small-scale local climatic effects within the city show themselves more prominently than in long-term means in extreme individual cases. At night, with slight clouding and low wind weather conditions, the urban climatic changes can be especially clearly detected. Moreover at night some influences which exhibit tremendous local variations, like e.g. the shadow throw and/or solar illumination are no longer to be found. Thus a generalized display of climate parameters for the area becomes considerably easier.
The air temperature during the course of the day for the near ground atmosphere in cities and conurbations develops differently under low-exchange weather conditions according to the characteristics of the location, vegetation share, development structure and use. The maximums in heavily built-up city structures are moderately exceeded and shift somewhat in the afternoon hours. The nocturnal cooling is itself significantly delayed this way and does not reach the level of the climatically uninfluenced surrounding countryside. So on very hot radiation days the highest temperatures appear in Kreuzberg, at Alexanderplatz and in the high-rise settlement Hellersdorf. The temperature decrease in the night hours is more noticeable in Hellersdorf, influenced through the agricultural structured environment, than in other heavily built-up areas (see Fig. 1). Sparsely built-up and heavily greened residential areas like Zehlendorf exhibit more favorable all-day conditions. The night temperatures over the open Dahlemer Feld in the Grunewald sink in comparison to Kreuzberg more than 8 °C lower, while at the forest location a suppression of the daily temperature course occurs as expected.
In Figure 2, the temperature differences of different climate stations to the coldest location Dahlemer Feld for the same summer day are listed. At 4:00 a.m. CET the values in the station Leibnizstraße lie 9 °C over the values for Dahlemer Feld; at other residential locations the differences sink continuously to the low values in Zehlendorf (5.2 °C) and Frohnau (4.8 °C). The night temperatures are remarkably low within the Tiergarten located in the inner city.
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.
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.
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.
The humidity load is caused by different climate parameters. These include the vapor capacity of the atmosphere, the sunshine, the wind and the air temperature standing. Thus the equivalence temperature has been applied to determine the level of humidity. It consists of the air temperature and the latent heat which would be present at a given level of condensation from a specified vapor capacity.
Again using the example of the very hot summer day, it becomes clear that the inner city residential areas exhibit very high equivalence temperatures at noon and still in the evening hours (see Fig. 5). The station in Zehlendorf is, especially in the evening hours, already to be classified as essentially favorable. The outer areas like the Grunewald and especially the Dahlemer Feld are the least burdened during the course of the day.
On the basis of investigations in Berlin (Wedler 1970), health burdens are to be anticipated in areas in which the daily average equivalence temperature exceeds 50 °C. Other authors (Linke and Baur 1957) define the humidity limit at 56 °C. In health spa climatology, the limit has already been reached at 49 °C according to Jendritzky et. al. (1979). Based on these values, the number of hours in which the limits are exceeded in the inner city residential areas during the months of July and August is very high. This is a period in which the risk of excessive humidity is especially high (see Table 1). That applies also for the high-rise settlement in Hellersdorf on the outskirts of town. In the heavily greened residential areas, as well as at forest and field locations the number of the burdened hours is considerably lower. The absolute number of hours are subject to the annual variations. However the relative differences at the individual locations may be taken as representative.