Counting raindrops using mobile-phone towers

NO ONE knows exactly how many people died in a series of mudslides that happened in and near Freetown, the capital of Sierra Leone, on August 14th. The upper estimates are more than a thousand. The areas swept away had not been evacuated partly because no one knew how much rain had actually fallen beforehand, laments Modeste Kacou, a rainfall expert at Félix Houphouët-Boigny University in Abidjan, in nearby Ivory Coast. Rain gauges are sparse in Sierra Leone. Satellites detect rainfall in the tropics, but estimates for small areas are often inaccurate. Worse, these numbers are calculated hours after the fact. Many countries therefore use cloud-scanning ground radar to measure precipitation as it is happening, but Sierra Leone has no such radar.

Nor do many other poor countries. Ivory Coast has double the GDP per person of Sierra Leone, but like most of west Africa, it also lacks precipitation radar. Indeed, maintenance costs mean that…Continue reading

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