Long-term Development of Selected Climate Parameters 2013

The Potsdam climate station is the only meteorological station worldwide that exhibits such a comprehensive measurement programme without gaps over a time period of more than 100 years without a change in the site or the measurement and observation methods. It is therefore called a "_long-term meteorological station_". Much further historical and current information about the station is available here: Long-Term Meteorological Station Potsdam-Telegrafenberg (accessed March 15, 2015).

The station lies in the southwest of Potsdam approx. 600 m outside the built-up zone on the elevation of the so-called Telegraph Mountain (_Telegrafenberg_). It is located at a height of 81 m above sea level on the edge of an expansive forest area. The immediate surroundings of the measurement field are flat and surrounded by trees of 15 – 20 m height at a distance of approx. 50 – 100 m. The measurement meadow has a size of 1,200 m² and adjoins the building of the former Meteorological Observatory, on whose tower both the radiation and wind measurements and, due to the free view in all directions, the ocular observations are carried out (PIK 2015).

Long-term mean values

Table 7.2 gives an overview of the long-term monthly mean values of selected climate parameters at the Potsdam station in the 30-year period 1981 to 2010 under consideration.

Figure 7.1 shows the climate diagram for the Potsdam station. It shows an averaged history of temperature and precipitation for the long-term period 1981 to 2010. Most of the precipitation occurs in the summer months, as is typical of the predominant climate. With 9.34 °C, the average temperature is about half a degree Celsius higher than in the entire measurement period 1893 to 2013.

Developments, trends

Figure 7.3 shows the history of the annual average temperature since climate records began on January 1, 1893, up to the end of 2013. In addition to some natural fluctuations, it can also be discerned that the annual mean temperature rises significantly over the course of the measured last 120 years. Whereas in the early measurement years the annual mean temperature was around 8 °C , in recent years average values between 9.5 and 10.5 °C were usually reached. This temperature rise is also illustrated by the trend line.

With an average temperature of 8.85 °C over the entire period, the Potsdam station exhibits the lowest temperature of all stations compared. Besides the peripheral location, which was not subject to significant changes due to construction in its surroundings, a reason for this lies in the long measurement series of 120 years, which also includes temperature values that have not yet been influenced by anthropogenic climate change.

Figure 7.4 shows the histories of the threshold days summer day, hot day and tropical night. The individual trend lines for summer days and hot days illustrate the increase in threshold days in the measurement period. Whereas the number of summer days was 34 per year in 1893, in the trend it increased by 16 to 50 days per year in 2013.

In the development of the hot days, the increase in the case numbers is naturally slowed down and amounts to 3 days in the trend over the entire measurement period. However, here, too, significant fluctuations are observed in the course of the 120 measurement years. With 26 days, the year 1947 recorded the most hot days. With respect to the tropical nights, a clustering in the current decade can be observed.

If only the 30-year period from 1981 to 2010 is considered in Figure 7.5, it becomes apparent that it was primarily in this last quarter of the entire measurement time that most of the increase in the summerly threshold days occurred (in the case of the summer days about 70 % of the total increase).

Under stable summerly high-pressure weather conditions with low winds, an enduring rise in air temperatures may occur over several days or over weeks. Figure 7.6 shows the frequency of consecutive threshold days in the long-term period 1981 to 2010. In the "hundred-year summer" of 2003, daily maximum temperatures were at least 25 °C throughout about four weeks in the period from July 13 to August 12; on 17 of these days the criterion of a hot day was even fulfilled (temperature maximum 30 °C or more).

Due to climate warming, the number of summerly threshold days is increasing, whereas the number of winterly threshold days is decreasing. Figure 7.7 shows the histories for the threshold days frost day and ice day for the entire period 1893 to 2013. The linear trend illustrates that in the last 120 years the number of frost days at the Potsdam station decreased from the initial level of about 100 frost days by 16 days (of which about 9 days in the current 30-year period 1981 to 2010; cf. Figure 7.8); the number of ice days shows a decrease by about 8 days per winter half-year (of which approx. 3 days in the period 1981 to 2010).