Raindrops keep falling on my windscreen

The most recent results of the RainCar project are now published in Hydrology and Earth System Sciences.

Rainfall can be very variable across different parts of a region such as Northern Germany. Conventional rain gauges are accurate, but are often distributed too sparsely to capture much of this variation.

‘If moving cars could be used to measure rainfall the network density could be improved dramatically,’ project-leader Uwe Haberlandt said in a statement.

To test their idea, they equipped a lab with a rain simulator to find out exactly how wiper speed relates to rainfall intensity.

Cars with different wiper systems were placed under the rain machine, which uses a sprinkler irrigation system with adjustable nozzles to simulate light to heavy rain.

In one set of experiments the team used the rain machine to test 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,’ said Haberlandt.

The team could also test the effects of car movement on the measurements by placing the sensors on a rotating device, which simulates car speed, under the rain simulator. By knowing how the readings are affected by car speed, they can correct for this effect when using moving cars to measure rainfall.

But speed is not all that can alter the rain measurements, said Ehsan Rabiei, Haberlandt’s collaborator and the paper’s lead author.

‘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.’

‘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,’ said Haberlandt.

In a Hydrology and Earth System Sciences study published in 2010, two of the team members showed that a high number of less accurate rain gauges gives more reliable rainfall readings than a low number of very accurate devices.

The researchers are now working on field experiments using cars to measure real rainfall in and around Hanover.