Moving cars & rainfall

A car being tested under a rain simulator.

DRIVERS on a rainy day regulate the speed of their windshield wipers according to rain intensity: faster in heavy rain and slower in light rain. This simple observation inspired researchers from the University of Hanover in Germany to come up with “RainCars”, an initiative that aims to use GPS (Global Positioning System)-equipped moving cars as devices to measure rainfall. The results of the project were published in Hydrology and Earth System Sciences.

Conventional rain gauges are often distributed too sparsely to capture much of the rainfall variation across different parts of a region. “If moving cars could be used to measure rainfall, the network density could be improved dramatically,” explained Uwe Haberlandt, the project leader. In a laboratory equipped with a rain simulator, the researchers were able to put their idea to test. They placed cars with different wiper systems under a rain machine, which uses a sprinkler irrigation system with adjustable nozzles to simulate light to heavy rain, to find out exactly how wiper speed relates to rainfall intensity.

“The experiments have shown that the front visibility is a good indicator for rainfall intensity,” said Ehsan Rabiei, the paper’s lead author. The team used the rain machine to test the optical sensors that are installed in many modern cars to automate wipers. The sensors use a system of infrared laser beams that detect when drops of rain accumulate on the surface of the device. Each sensor reading corresponds to a specific amount of water, with more frequent readings corresponding to more intense rainfall. “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 [than manually controlled wipers],” said Haberlandt.

The experiments were carried out in an ideal and controlled environment. However, car speed and other factors, such as wind, spray from other cars, and shielding trees, can alter the rain measurements. “The value of using moving cars to measure rainfall is not about a higher accuracy of rainfall measurements but about a much higher number of measurement points,” clarified Haberlandt. In a paper three years ago, two of the team members showed that a high number of less accurate rain gauges gave more reliable rainfall readings than a low number of very accurate devices. The researchers are already working on field experiments using cars to measure real rainfall in and around the city of Hanover. “There are some volunteers, a taxi company and a car company involved in the field experiments.”