How does the weather forecast get on the mobile phone? Quite clearly: via the corresponding app. But where does the app get the data from? So far, they come from the major meteorological services - in Germany, for example, from the Deutscher Wetterdienst (DWD). These services work with far-flung networks of meteorological observation stations and weather radars. But it would also be much easier. Because the mobile phone networks themselves provide the data. The reason: the performance of the network is strongly influenced by the weather; rain can significantly reduce the spread of radio waves.
This might annoy the telecommunications companies, but for meteorological research this is a stroke of luck: From the interaction between weather events and technology, researchers have developed a completely new method for measuring rain. "If a mobile network is available, we do not need a new infrastructure or additional meteorological staff," says Harald Kunstmann from the Institute of Meteorology and Climate Research - Atmospheric Environmental Research (IMK-IFU) of the Karlsruhe Institute of Technology (KIT). Together with scientists from the University of Augsburg, his team has now succeeded in carrying out the first area-wide rain measurement with the new method in Germany. They were able to generate rain maps with high temporal resolution from the weakening of the radio link between several thousand mobile phone masts caused by precipitation. A comparison with the measured values of the weather data provider Deutscher Wetterdienst (DWD) showed a high degree of agreement (Fig 1).
The determination of precipitation was possible due to the directional radio antennas used in mobile phone masts for transmission over long distances. "A frequency of 15 to 40 gigahertz is used here, whose wavelength corresponds to the typical size of raindrops", explains Christian Chwala from the University of Augsburg. "The more precipitation falls, the weaker the signal with which the transmission masts exchange information". For their investigations, the researchers measured the current attenuation of 4000 microwave links throughout the country every minute for a year. This resulted in a data set that is unique due to its resolution and size.