Rain gauges are generally pretty accurate at measuring the amount of precipitation that has fallen at their location, but they can't be everywhere. This means that average rainfall figures for a region could be inaccurate, if considerably more or less rain has been falling in unmonitored areas. Cars, however, are just about everywhere that there are roads. With that in mind, researchers from Germany's University of Hanover are looking at using them to tell us how much water is coming from the sky.

When you're driving and it starts raining, what's the first thing that you do? That's right, you turn on the windshield wipers. The harder it rains, the faster the setting that you select. Scientists with U Hanover's RainCars project postulated that if select cars where outfitted with GPS and wiper speed-monitoring sensors, they could provide real-time data on how much rain was falling in a wide number of locations.

In order to test their theory, they set up an experiment in which a stationary car was placed under a sprinkler system, with a person inside the car. As water flowed from the sprinkler, at known output rates, the person turned up the speed of the wipers in order to maintain their view through the windshield. It turned out that there was a fairly consistent correlation between the speed of the wipers, and the amount of water coming from the sprinkler – in other words, the flow rate of the sprinkler could be roughly determined by measuring wiper speed.

One problem with this approach, however, is that not all drivers are equally fussy about how moisture-free they like their windshield. Therefore, the RainCars team believe that their system would work better on cars with optical sensors that automatically adjust the speed of the wipers – an increasing number of which are already on the roads.

Additionally, though, factors such as car speed, wind, and road spray from other cars also affect how much rain hits the windshield. The scientists are looking at ways of correcting for these variables. Even if they only have limited success in doing so, they believe that a large number of approximate rainfall figures gathered by cars will still provide a more reliable overall picture than more accurate readings from just a few rain gauges.

Field trials are already under way in the city of Hanover, with volunteer drivers and a taxi service taking part. A paper on the research was recently published in the journal Hydrology and Earth System Sciences.