Does a rain-sensing windscreen wiper physically detect raindrops? Or is it magic? Here’s everything you need to know about how a rain-sensing windscreen wiper works.

Most new cars these days will be fitted with rain-sensing windscreen wipers or, at least certain variants of new models will be. But how do they work? Let’s take a few steps back before answering that question because here at Practical Motoring we want you to know the full picture. Let’s go.

History of the windscreen wiper

The windscreen wiper on vehicles wasn’t a thing until the early 1900s when Mary Anderson allegedly caught a street car in New York, which is a bit like a tram, during a rain storm. The story goes that the driver was struggling to see where he was going and ended up opening the side window and stuck his head.

Once Anderson arrived home and dried off she set to work designing a manual system whereby a series of wooden and rubber levers could be operated by a driver to clear water from a windscreen. It was a genius idea but she wasn’t the only one working on the idea and several other inventors in different countries had similar ideas and patents applied at the same time in 1903. Then, in 1917, Charlotte Bridgwood patented a design for an electric windscreen wiper. And then, William M. Folbeth, in 1922, patented a vacuum-based system and this automatic windscreen wiper system which ended up being used by just about every car maker up until the 1960s when the intermittent wiper took over.

Father of the intermittent windscreen wiper

The intermittent windscreen wiper as we know it was the brainchild of Robert Kearns (but there were several other designs floating around from as early as 1923 and Lucas Industries filed a patent in the UK for electric intermittent wipers in 1961). Kearns was an ex-US Army intelligence officer and mechanical engineer. The story gets weird. After being blinded by a popping Champagne cork on his wedding night, Kearns told the story that the constant movement of the vacuum-powered windscreen wiper on his car used to irritate his ‘good’ eye because of the smearing it caused in ‘et-dry’ situation, and so he conceived an idea based on the ‘movement’ of the human eye which blinks every few seconds rather than like clockwork.

In his patent for the system, confirmed in 1964, he wrote: “Under certain conditions, such as light rain or splash-back produced by other vehicles on wet roads, the condition of the windshield is often in what may be termed a wet-dry condition. Continuous windshield wiper operation with such a windshield condition may cause smearing to obscure the vision of the driver. The wiper element also may be inadequately lubricated, causing undue Wear on the wiper blade.

“To overcome these problems, it is desirable to provide an intermittent operation in which the wiper dwells for an interval of time after a wiping operation and then automatically begins another cycle of operation. During these intermittent dwell periods, the wind-stream against the windshield due to vehicle motion can be employed advantageously to dry and dissipate the thin film of vision obscuring moisture which is created by wiper operation over a wet-dry windshield. The intermittent operation also affords relief from monotony of wiper motion and prolongs the life of the wiper blades, the wiper motor and the wiper linkages, not only because of the intermittent rather than constant cycling, but also because the accumulation of moisture during dwell periods acts as a lubricant for the wipers during the next cycle of operation”.

Kearns took his idea to the Big Three, GM, Ford and Chrysler and all three knocked him back. However, the companies set their engineers to using Kearns idea to build their own intermittent windscreen wiper systems from about 1967 prompting Kearns to release his lawyers. In 2008, Kearns’ battle with Ford was turned into a film, called Flash of Genius.

The next step…rain-sensing windscreen wipers

The rain-sensing part of the intermittent wiper is an add-on rather than an entirely new form of windscreen wiper. If you’ve got a newer car with a rain-sensing wiper you’ll notice a sensor when you look at the windscreen where centre-mounted rear vision mirror is mounted. Of course, quite often active safety systems use sensors mounted up here too and in that case, they’re all usually bundled together. Some vehicles now do away with the sensor and rely on cameras to detect if the windscreen is wet.

On my car, which is from 2011, the sensor in the main photograph manages the rain-sensing wipers and what it does is fire a beam of light at the windscreen from behind. If the glass is wet then less light makes it back to the sensor and the light that does it make it back is scattered and more or less scattered depending on the amount of rain. Thus the windscreen wipers are triggered. Simple. If your vehicle has rain-sensing wipers then, on the wiper stalk will usually be an ‘Auto’ setting. Once activated, the wipers will operate either faster or slower depending on the amount rain, or, rather, the light making it back to the sensor.