Long-term Mean Precipitation Distribution 1991 - 2020

The mean precipitation distribution provides key information on the development of precipitation in an area. In individual events, however, the precipitation distribution may deviate greatly from the mean precipitation distribution. This applies in particular to heavy rainfalls, as they are usually confined to smaller areas and not evenly distributed (cf. Spektrum.de online 2016, only in German). They occur during the summer months as a result of convective air flows reinforcing themselves. Although physiographic characteristics may contribute to the development of heavy rain cells, their formation is highly coincidental.

Heavy rainfalls can thus contribute to small-scale changes in the annual or semi-annual means. Considering the long-term period of 30 years, however, the impact of individual events is rather insignificant, as heavy rainfalls are quite rare and random in their distribution.

The weather conditions in an area are greatly influenced by its topography (surface structure of the earth). Mountain ranges, smaller hill chains, and even lower ridges already have an impact on the amount of precipitation. Other influencing factors include forests, lakes, fields and such (cf. Flohn 1954, only in German). Furthermore, cities with their agglomerations of buildings influence the quantity and distribution of precipitation once they reach a certain size.

Increased precipitation within a confined area, due to, for example, relief rainfalls are mainly caused by the influence of soil friction, i.e. the surface roughness parameter. Soil friction slows down the lower air layers, causing subsequent air masses to accumulate and rise. Adiabatic cooling may effect the formation of clouds and precipitation. Additionally, aerosols tend to accumulate above urban areas more often. As condensation nuclei, they influence the formation of clouds and precipitation. In addition, the heat emanating from urban areas may contribute to convective precipitation, if additional constraints come into play.

The present evaluations are based on grid data provided by Germany’s Meteorological Service (DWD). The DWD’s REGNIE data was used to analyse the reference period between 1981 and 2010. This data set, however, has been discontinued and will no longer be updated. The current update was therefore based on the precipitation data of the HYRAS-DE-PRE data set (only in German). The DWD’s scientifically improved HYDRAS-DE-PRE data set succeeds and replaces REGNIE entirely.

As the data base changed, the Environmental Atlas’ results of the long-term mean precipitation distribution from 1981-2010 may only be compared to those of the current period to a limited extent.

Precipitation is an essential component of nature. Animals, plants, and indeed us humans cannot survive without it. To analyse the effects of precipitation, however, a differentiated approach is needed. On the one hand, precipitation causes a cleaning process of the air. On the other hand, highly impervious surfaces and how they are used cause precipitation to flush out a variety of pollutants. The latter are then able to enter rainwater drains and combined sewerage systems and eventually make their way into our bodies of water, too.

A lack of precipitation affects animals and plants alike, which may lead to permanent damage. This is especially true if dry periods occur increasingly, as has been the case in recent years. The simultaneous increase in heavy rainfalls, however, does not compensate for the lack of precipitation in the water balance. Soils, especially when dry, cannot absorb these copious amounts of precipitation, or only to a small extent. The precipitation therefore largely runs off superficially and does not contribute to the regeneration of the soil water store. What is more, heavy rain may also cause soil erosion. Flash floods resulting from heavy rain also pose a danger to people, animals and property.

Berlin is located in an area of transition, where the climate shifts from continental to predominantly oceanic. This is reflected in Berlin’s precipitation conditions on a regional scale. Berlin is one of the drier regions compared to the rest of Germany. Incidentally, the annual mean precipitation for Germany was 782 mm per square metre for the international standard reference period from 1991 to 2020, whereas it was 579 mm per square metre for Berlin for the same period (long-term mean of the calendar years, cf. Fig. 1).

In addition to the above-mentioned factors, the global climate change is also expected to influence our regional water supply more and more in the future. Over the past 10,000 years, changes in climate have altered the geographical distribution of precipitation dramatically. Forecasts of potential developments largely depend on future greenhouse gas emissions and are being studied by authorities such as the DWD (cf. DWD 2020a, only in German). According to the DWD, the annual precipitation is predicted to increase slightly (+6%) in Germany by the end of the century. The winter months and the transitional seasons are expected to see an increase in precipitation. For the summer, precipitation predictions range from slight increases to decreases, depending on the scenario.