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Phoenix666 writes:

The basic insight is straightforward enough: rain weakens electromagnetic signals. Many mobile-phone towers, especially in remote areas, use microwaves to communicate with other towers on the network. A dip in the strength of those microwaves could therefore reveal the presence of rain. The technique is not as accurate as rooftop rain gauges. But, as Dr Kacou points out, transmission towers are far more numerous, they report their data automatically and they cost meteorologists nothing. He runs the Ivory Coast operations of Rain Cell Africa, an effort paid for by the World Bank, the UN Foundation, a charity, and the Institute for Development Research, which is based in France, to map rainfall in parts of Africa using data donated by Orange, a big telecoms firm, and Telecel Faso of Burkina Faso, a small one. Had the system been running in Sierra Leone, he reckons evacuations could have been carried out in time.

Rich countries are interested, too. A pilot project in the Netherlands a few years ago produced promising results, but it has not yet been followed up. This month officials in Gothenburg, Sweden, began to study rainfall maps derived from data collected every ten seconds from 418 mobile-phone towers owned by a firm called Hi3G. The hope is this will provide more accurate estimates of rainwater about to slosh into the municipal waterworks, helping managers to limit flooding and sewage overflows. Until now, the city has relied on 13 rain gauges, backed up by radar sweeps of the sky that are neither sufficiently frequent nor sufficiently precise, says Jafet Andersson, a hydrologist behind the scheme at the Swedish Meteorological and Hydrological Institute. Satellite data on rain in northern latitudes are so poor the agency does not bother using them at all.

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