Counting raindrops

How to use mobile-phone networks for weather forecasting.

FORECASTING the weather requires huge quantities of data. Many of these data are collected by high-tech means such as satellites and radar, and then crunched by some of the world’s fastest supercomputers into predictions that are far more accurate than they were 20 years ago. But low-tech tools are important too—especially old-fashioned rain gauges, which are nothing more than tubes with funnels fixed to places such as rooftops.

Each technique has its upsides and downsides. Radar and satellites can cover swathes of land, yet they lack detail. Gauges are much more accurate, but the price of that accuracy is spotty coverage. Now, though, Aart Overeem of the Royal Netherlands Meteorological Institute and his colleagues reckon they have come up with another way to keep an eye on the rain. It offers, they believe, both broad coverage and fine detail. Best of all, it relies on something that is already almost omnipresent—the mobile-phone network.

Their scheme starts from the observation that rain can make it harder for certain sorts of electromagnetic radiation to travel through the atmosphere. Measure this impedance (and scrub out any other sources of variation) and you can measure how rainy it is. The researchers do not measure the strength of mobile-phone signals themselves. Instead, they piggyback on something that mobile networks already do, and measure the strength of the microwave links that base stations use to talk to each other.

The idea itself is not new, and there have been trials in recent years. But, as they report in the Proceedings of the National Academy of Sciences, Dr Overeem and his colleagues have successfully applied the technique to an entire country. Using data from around 2,400 links between base-stations belonging to T-Mobile, one of the Netherlands’ three mobile-phone operators, they were able to generate a rain map of the whole kingdom every 15 minutes.

Like all the best science, the idea is both technically elegant and practically useful. Dr Overeem points out that simply coming up with another way to measure rainfall is handy by itself, since it allows better cross-checking of existing methods. There are other advantages, too. Coverage is one. Even in rich countries with well-financed weather forecasters, there are likely to be far more mobile-phone base stations than rain gauges. That is truer still in poor countries, where rain gauges are scarce and radar often nonexistent, but mobile phones common. The GSM Association, a mobile-phone trade group, estimates that 90% of the world’s population lives within range of a base station.

Another boon is that network operators tend to keep a close eye on their microwave links. Although the researchers were able to obtain data only every 15 minutes, some firms sample their networks once a minute. That means rainfall could, in principle, be measured almost in real time, something that neither gauges nor radar nor satellites can manage.

The technology is not perfect: snow and hail are harder than rain for microwaves to spot, for example. And there are other caveats. Mobile networks are densest in urban areas, which are also the places most likely to have meteorological equipment already. Even in the rich, urbanised Netherlands, coverage outside cities was noticeably patchier. But that might eventually prove to be a boon, for if the technology becomes widespread then weather forecasters might contribute to the cost of installing base stations in coverage blackspots—something for which the 10% of humanity not yet within range of a mobile-phone mast might be thankful.

- The Economist