单项选择题

Forecasting the weather requires huge quantities of data, mainly collected by high-tech means such as satellites and radar, but low-tech tools are important too—especially old-fashioned rain gauges (雨量器).
Each technique has its strengths and weaknesses. 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 watch the rain carefully. It offers, they believe, both broad coverage and fine detail. Best of all, it relies on something that is already almost everywhere—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 resistance and you can measure how rainy it is. The researchers do not measure the strength of mobile-phone signals themselves. Instead, they rely 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. Like all the best science, the idea is both technically elegant and practically useful, since it allows better cross-checking of existing methods. There are other advantages. Coverage is one. Even in rich countries with well-financed weather forecasters, there are probably 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. 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. Besides, mobile networks are densest in urban areas, which are also the places that probably have weather-forecasting equipment already. Even in the rich, urbanized Netherlands, coverage outside cities is noticeably irregular. The purpose of writing the passage is to ______.