How To Measure Rainfall Accurately: Scientists Tag GPS Enabled Moving Cars To Increase Measurement Points

By Ajit Jha on November 29, 2013 3:57 PM EST

Windshield Wipers in the Rain
Scientists have determined that tracking windshield wiper use in GPS-enabled cars may be the most accurate way to figure out rainfall. (Photo: Photo courtesy of >a href=&)

A simple observation inspired researchers from the University of Hanover in Germany to come up with "rain cars" that could help in tracking the amount of rainfall year to year. The scientists noticed that drivers on a rainy day regulate the speed of their windshield wipers according to rain intensity. So, they decided that they could use any GPS-equipped moving cars to help measure rainfall.

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Rain gauges, though pretty accurate at measuring the amount of rainfall where they are, cannot be everywhere the precipitation has taken place. In other words, average rainfall figures could turn out to be quite inaccurate because no one would know the amount of rainfall in unmonitored areas. Cars are pretty much everywhere there are roads, which gave the University of Hanover researcher's an idea to more accurately measure rainfall.

The project leader of the study, Uwe Haberlandt, is upbeat that "network density could be improved dramatically," if moving cars were used to measure rainfall. This idea emerged, during a brainstorming session between geoinformatics researchers and hydrologists. Over 40 million cars in German, according to study authors, can successfully track rainfall with the help of wiper speed.  

The project team tested their idea using a sprinkler system that aimed water at a stationery car at known output rates. The person inside the car turned the speed of the wiper to maintain visibility through the windshield. A fairly consistent correlation was established between the amount of water striking the car and the speed of wiper. In other words, wiper speed could be interpreted to determine the flow rate from sprinkler.

"The experiments have shown that the front visibility is a good indicator for rainfall intensity," said Ehsan Rabiei, Haberlandt's collaborator and the paper's lead author. However, a measurement dependent on hundreds of people adjusting their wipers might not always give accurate data, since different drivers might have different feelings on how much or often to use their wipers. So, the team conducted another set of experiments, this time using the optical sensors that many newer cars come fitted with to automate wipers. Turns out that sensor readings offer very accurate information on the amount of precipitation. "The optical sensors measure the rain on the windshield in a more direct and continuous manner so, currently, they would be the better choice for rain sensors in cars," Haberlandt said.

The team also went on to test the effects of car movement on the measurements by placing the sensors on a rotating device, which simulates car speed. This would help them to identify how readings are affected by the speed of vehicle and subsequently correct the effect.

There are other possible factors that could impact rain measurements. "Our experiments so far were carried out in an ideal and controlled environment. In nature there are external effects like wind, spray from other cars or shielding trees that can affect the readings, and rainfall characteristics are different from the rain simulator," said Rabiei, a team member and lead author. However, Haberlandt calrified that it is more important to have a higher number of measurement points than higher accuracy to get more reliable rainfall readings - and this innovation should raise those measurement points significantly. 

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