Long-term Development of Selected Climate Parameters 2013

Results of the station evaluations

The main measurement data at the stations under consideration are first considered in combination in order to analyse differences and common features of the measurement sites. The focus of the considerations is on the period from 1981 to 2010, as climate data are available for most of the stations under consideration in this period and these 30 years are regarded as a current period for considering "long-term climate conditions".

The individual data are provided in relation to the figures in a combined table for all stations (with reference to the summer and winter characteristic quantities) as well as individual station tables (including all daily values since records began). However, they are also available for download in a combined archive (zip; 110 MB).

Comparison of the measurement series of the Berlin stations Tempelhof, Tegel, Dahlem, Alexanderplatz and Grunewald as well as the Potsdam station

The history of the air temperature of all five Berlin measurement stations as well as the Potsdam station is illustrated in Figure 1.1. The data for the stations Berlin-Alexanderplatz and Berlin-Grunewald were available only up to the end of 2012, for all other measurement sites up to the end of 2013.

Measurements going back to 1893 are available for the Potsdam measurement station; hence a trend line (dashed in dark blue) and a mean value across all measurement years (light blue) are additionally shown for this station. These clearly indicate the rise in air temperature within the period under consideration. The trend line shows a warming of more than 1 K between 1893 and 2013. While the temperature values of the Potsdam station were significantly below the mean value when records began, they are consistently above the mean value line for the last 20 years. It can also be seen that the Alexanderplatz station consistently exhibits the highest values in comparison to the other stations. The Berlin-Tempelhof station with its inner-city peripheral location also shows higher temperature values than the stations in peripheral locations or in the surroundings.

Fig. 1.1: History of the annual mean temperatures at a height of 2 m of all stations under consideration in the respective measurement period up to the end of 2013; Berlin-Alexanderplatz and Berlin-Grunewald stations up to the end of 2012

Figure 1.2 shows the history of the summer days in the Berlin area. A summer day is a day on which the maximum air temperature is at least 25 °C (T_max ≥ 25 °C). Here, too, a trend line for the Potsdam measurement station was added. It shows an increase in the number of summer days by about 15 days in the period from 1893 to 2013. Note the year 1947, in which 71 days were recorded in Potsdam on which the air temperature reached a value of 25 °C or more. This value is even higher than the one for the "hundred-year summer" in 2003. The Berlin-Grunewald station consistently records the fewest summer days due to its peripheral forest location.

Fig. 1.2: History of the number of summer days of all stations under consideration in the respective measurement period up to the end of 2013; Berlin-Alexanderplatz and Berlin-Grunewald stations up to end of 2012

The history of the hot days is similar to that of the summer days. On a hot day, the maximum air temperature is 30 °C or above (T_max ≥ 30 °C). In this investigation, hot days are simultaneously also counted as summer days. In years with particularly many summer days, the number of hot days recorded is usually also above average (Fig. 1.3). Here, too, the year 1947 stands out, in which 26 hot days were recorded at the Potsdam station. But many hot days were also recorded in the years 2003 and 2006, with the year 2006 featuring prominently also with respect to the tropical nights (T_min ≥ 20 °C) (cf. Fig. 1.4).

In comparison to the summer days, the two years 2003 and 2006 stand out more prominently in the history of the hot days. At the Berlin-Alexanderplatz station, at which the temperature is inherently higher than at the other stations under consideration due to its inner-city location, 18 hot days were recorded in 2003, compared with 25 corresponding days in 2006.

Fig. 1.3: History of the number of hot days of all stations under consideration in the respective measurement period up to the end of 2013; Berlin-Alexanderplatz and Berlin-Grunewald stations up to the end of 2012

The history of the tropical nights in the region also clearly shows the influence of urban structures on the air temperature (Fig. 1.4). The Berlin-Alexanderplatz station consistently shows — apart from years with missed measurements (1993/94) — the highest number of tropical nights. The thermal effects of the nocturnal energy flows from the surrounding building structures have a particular effect here. A high proportion of impervious cover and the dense roadside development with its large heat storage capacity form a comprehensive reservoir for sustained heat radiation in summer nights. According to the German Meteorological Service, the Berlin-Alexanderplatz station is in first place out of all German climate stations with an average of 5 tropical nights per year. However, in years with pronounced summers the number of tropical nights can lie far above the average value, as the number of 21 corresponding nights in 2006 shows.

Fig. 1.4: History of the number of tropical nights of all stations under consideration in the period 1981 to 2013; Berlin-Alexanderplatz and Berlin-Grunewald stations up to the end of 2012

The history of the frost days (minimum temperature under 0 °C) (cf. Fig. 1.5) shows a downward trend on the basis of the Potsdam station, which correlates well with the upward trend of the summerly threshold days (Figure 1.1 to 1.4). A decrease in the number of frost days by close to 15 days over the entire period can be observed. The Berlin-Alexanderplatz station registered the fewest frost days per year over the entire history.

Fig. 1.5: History of the number of frost days of all stations under consideration in the respective measurement period up to the end of 2013; Berlin-Alexanderplatz and Berlin-Grunewald stations up to the end of 2012

The history of the ice days (maximum temperature under 0 °C) at the measurement stations in the Berlin area also shows a decrease. With about five days for the trend line of the Potsdam station, this is much less pronounced than in the case of the frost days. In analogy with the analysis of the hot days, the frost days are also being counted as ice days here.

Here, too, the Berlin-Alexanderplatz measurement station plays a special role in the collective of stations due to its inner-city location: As in the case of the frost days, it exhibits the fewest ice days. All other stations show a similar history.

Fig. 1.6: History of the number of ice days of all stations under consideration in the respective measurement period up to the end of 2013; Berlin-Alexanderplatz and Berlin-Grunewald stations up to the end of 2012

In the following, the evaluation of selected climate parameters at the individual stations is represented using tables and graphs. The current long-term time interval of the 30 years from 1981 to 2010 was selected as a reference period, as far as data were available. The evaluations of the mean wind direction distributions refer to the 10-year time interval from 2001 to 2010. Hourly values of wind direction and wind speeds were available for the Berlin measurement stations Tegel, Tempelhof, Dahlem and Grunewald.

• Berlin-Alexanderplatz
• Berlin-Dahlem
• Berlin-Grunewald
• Berlin-Tegel
• Berlin-Tempelhof
• Potsdam

Summary

Development of the air temperatures in summer in the Berlin area up to 2013

The temporal development of the annual mean temperatures was already portrayed in the chapters Methodology and Results of the station evaluations.

Here the essential results of the extensive evaluations shall be summarised once more. For this purpose, Figure 8.1 juxtaposes the main climatological threshold days in their frequency per station.

Fig. 8.1: Mean frequencies of summer days, hot days and tropical nights in the long-term period 1981 to 2010 at the climate stations under consideration in the Berlin area (time series for the Grunewald station: 1988-2012)

• The maximum air temperatures of the individual measurement sites do not differ significantly, whereas the nocturnal minimum values on summer days sometimes exhibit marked differences. Typically, temperature differences between urban and rural measurement sites are less pronounced with respect to the daily maxima than with respect to the nocturnal minimum temperatures. The temperature maxima occurring in the afternoons of summer days differ much less, since due to the good mixing throughout summer days temperature differences can more easily be equalised during daytime. At night, the urban buildings only gradually release the stored heat; the increasing stability of the temperature stratification in the lower air layers and the subsiding of the wind reduce equalisation processes.
• The average nocturnal minimum temperatures on summer days clearly show the described influence of the surrounding buildings and thus the urban heat island effect. The highest nocturnal temperatures were recorded at the inner-city Alexanderplatz station, whereas the lowest nighttime values were measured at the peripheral or more rural sites Dahlem and Potsdam. In relation to the Dahlem and Potsdam sites, the nocturnal temperature level at the Grunewald station increases overall in the course of the recording period. The Tegel and Tempelhof sites take a middle position in their temperature level.
• For the highest temperatures of a summer, the location of the measurement sites cannot definitely be linked to the relative temperature level that the stations exhibit on average. While the Dahlem and Tempelhof stations are often found in the lower range of the maximum temperatures, the highest maximum values were often reached at the Potsdam station. The temperature level of the inner-city Alexanderplatz station is also at a very high level. The highest temperature of the measurement series under consideration was reached with 38.6 °C on August 9, 1992 at the Potsdam station.
• The mean diurnal variation on summer days, like the average minimum temperatures, reflects the location of the stations in the city area. The stronger the influence of the surrounding buildings, the smaller the average amplitude of the diurnal variation. The Dahlem and Tempelhof stations exhibit an increase in the mean diurnal variation on summer days. For the Tegel and Potsdam stations, the diurnal variation remained nearly constant in the mean, whereas at the inner-city Alexanderplatz station the amplitude of the diurnal variation showed a decreasing tendency. Increases and decreases in the amplitude of the diurnal variation can be caused through local changes of the heat island effect in the surroundings of the measurement site.
• An increase in the summer days occurring is exhibited by all stations. Due to the relatively homogeneous maximum temperatures in summer within the city area, the number of summer days at the different stations does not differ markedly. The histories of the individual stations are sometimes hard to distinguish, as the frequencies of the summer days only differ insignificantly. It is notable that not only the temperature level but also the frequency of summer days at the Potsdam station is relatively high up to the early 90s in comparison to the stations in the Berlin city area.
• As expected, larger differences occur in the number of tropical nights, as the nocturnal minimum temperatures more closely reflect the degree of development of the surroundings. Here the inner-city Alexanderplatz station stands out due to its significantly higher number of tropical nights.