Long-term Development of Selected Climate Parameters 2013

The Dahlem climate station has long been regarded as the Berlin reference station both for long-term observation and for current weather reporting. The station’s location changed several times — albeit within narrow confines — under the influence of the establishment and the initially varying locations of the Institute of Meteorology of the FU Berlin founded in 1948. Due to its significance for Berlin climate observation, its various sites since 1908 are documented in photographs below.

The measurement station of the Free University of Berlin is located about 13 km southwest of the city centre of Berlin in the Dahlem district of the borough Steglitz-Zehlendorf on the grounds of the Botanical Garden encompassing more than 43 ha (cf. Photo 3.1). The surroundings are characterised by mansions with spacious gardens and some smaller parks. To the west, the Dahlem district borders on the Berlin forest of Grunewald. The wind measurement station is located around 750 m to the northeast on the building of an FU institute on the Fichtenberg (Carl-Heinrich-Becker-Weg 10).

Long-term mean values

Table 3.2 gives an overview of the long-term monthly mean values of selected climate parameters at the Dahlem station in the 30-year period 1981 to 2010 under consideration.

The climate diagram of the Berlin-Dahlem station is shown in Figure 3.1. It shows the averaged history of temperature and precipitation over the individual months over the 30-year period 1981 to 2010. August is the month with the most rainfall; the least precipitation occurs in the transitional seasons.

The sunshine diagram follows the temperature curve, as expected (cf. Fig. 3.2).

The following figures show wind direction distributions at the Berlin-Dahlem measurement station in the period from 2001 to 2010. In addition to the mean long-term wind direction distribution, distributions for low-wind conditions with speeds up to 2 m/s as well as conditions with wind speeds above 2 m/s were evaluated. The wind recordings occur at a height of 26 m (cf. Tab. 3.1). At this measurement height, low-wind conditions only occurred in 8.5 % of the cases in the period under consideration. Calms (windless conditions) were observed in 0.1 % of the situations. Wind speeds above 2 m/s occurred in 91.5 % of the evaluated period. The red ring lines in the diagrams indicate the mean frequencies of occurrence of the wind direction in the respective period under consideration in percent. The distance between ring lines corresponds to 1 %.

Developments, trends

Figure 3.5 shows the history of the annual mean temperature of the years 1950 to 2013. The trend is increasing over the course of the measurement series, as for all other stations. In Berlin-Dahlem, this increase amounts to more than one Kelvin. A period of significantly lower temperatures between the mid-1950s until around 1970 can be discerned.

Figure 3.6 shows the history of the threshold days occurring in summer during the entire observation period 1950 to 2013. A positive trend can be observed both for the summer days and for the hot days. The number of summer days per year increased by around 16 days, the number of hot days by about five. No trend can be discerned for the tropical nights. This number is very low in comparison to the other measurement stations. The location of the station in surroundings with a high proportion of green spaces plays a significant role in this regard. Whereas buildings and sealed surfaces store much heat energy upon high solar irradiation during the day, which they release at night, this occurs to a significantly lower extent in the case of areas characterised by vegetation.

In considering the frequency, it is to be noted that the number of summer days occurring with a frequency of a duration of only one day is the lowest at the Berlin-Dahlem measurement station among all measurement sites. The location on the very open site on a meadow in the Botanical Garden may contribute to this. On the other hand, longer periods with maximum temperatures of 25 °C and above occur less frequently in Berlin-Dahlem than at other stations. The longest time span of 26 consecutive days is also shorter by five days than at the remaining stations. Tropical nights occurred on two consecutive days only in one case (July 27 and 28, 2013) (cf. Fig. 3.7).

The decrease in the number of frost days and ice days occurring (cf. Fig. 3.8) is probably also a contributing effect of global warming. The number of frost days decreased by around eight days on average in the period 1950 to 2013, the number of ice days by around five days.