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Study Turns Cell Phone Towers Into Rain Gauges

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These days, many people use their cell phones for purposes other than actually making calls, such as sending emails, taking photographs, using social media networks, and downloading an ever-expanding array of Apps. According to a new study published Monday in Proceedings of the National Academy of Sciences, there is another use for cell phone technology, and it is one that could bring vital information at low cost to water resource managers, farmers, and climate researchers, particularly in the developing world.

The study, by researchers based in the Netherlands, demonstrated that it is possible to use cellular telephone networks — namely the electromagnetic radio waves that pass between microwave antennas located atop cell phone towers — to estimate the amount of rainfall that falls between those two points. In fact, national rainfall maps can be generated by tapping into data gathered throughout a country’s cell phone network. The study did that by producing a 12-day rainfall map of the Netherlands that  closely mirrored measurements from weather radars and surface stations.

Rainfall attenuates the electromagnetic signals transmitted from the circular antenna of one cellular communications tower to another (top left panel). Map of the Netherlands with the locations of the employed 1,751 link paths (right panel).
Credit: Image courtesy of PNAS

Previous studies had used commercial microwave link data for measuring rainfall intensities, but the new study is based on an unprecedented number of microwave links, and provides the first national-level rainfall maps using this technique.

Monitoring rainfall through cell phone towers promises to have the greatest benefit in nations that lack a robust weather monitoring infrastructure, particularly in Africa, where the World Meteorological Organization has been working to try to improve weather and climate monitoring. Climate studies have shown that as the climate has warmed, extreme precipitation events have become more likely in many parts of the globe, since warmer air and ocean temperatures add more moisture to the atmosphere. This adds a sense of urgency to the task of accurately monitoring precipitation, since a scarcity of observations in Africa and other areas hinders researchers' abilities to measure and predict climate change-related impacts.

Cellular communication-based rainfall monitoring is made possible by the fact that rain droplets absorb and scatter, or attenuate, the signal sent from one telephone tower to another. “By measuring the decrease [in the signal] during rainy weather . . . and knowing the signal level during dry weather, we can estimate the average rainfall intensity between the antennas of telephone towers,” said lead author Aart Overeem of Wageningen University and the Royal Netherlands Meteorological Service.

Overeem said the number of microwave telecommunications links in a country tends to be much larger than the number of rain gauges, which means that cellular rainfall monitoring could help supplement ground-based measurements and serve as a way to verify their accuracy. For example, in the Netherlands, there are about 325 daily rainfall gauges and about 32 rain gauges that report data every 10 minutes, the study said. That compares to at least 8,000 microwave-link paths contained in the country's cellular communications network. The data also comes "shovel ready," since cell phone companies already monitor the loss of signal strength between their cell phone towers in order to keep tabs on their reliability, the study said.

As a follow-up study, Overeem said he and his colleagues plan to use cell phone towers to obtain long-term rainfall maps, such as an annual rainfall map, which could improve the accuracy of weather radar data. “We hope to obtain quite accurate rainfall maps for a long period and then to adjust radar data using those data to come up with an improved radar rainfall product,” Overeem said.

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