Cell towers send signals one to the next via microwaves. In dry conditions those signals drop off as distances get longer. But, when it rains, they drop off faster (and differently). That observation led Dutch researcher to try to use signal strength as a way to track rainfall in roughly real time.
There are roughly 8000 microwave links in the Netherlands, and the team was given access to data on about 2400 of those, with signal strength recorded every 15 minutes – enough to get a snapshot of rainfall across the whole country. By contrast, the meteorological institute has just 32 rain gauges that take a reading every 10 minutes.
The team mapped rainfall between June and September 2011 and found that readings derived from cellular data tended to agree with those from the traditional combination of rain gauges and radar.