Rain sensing wipers and snow?

Does anyone know how these rain sensors work? Is it optical or some sort of moisture sensor? A combination of both?

I'd say for the system to detect snow reliably that it would depend on the snow and what type of sensor is used to detect it. A heavy wet snow should be easy to detect. I light dusting of 'dry' very cold snow would be more difficult to detect.

It's an optical sensor and it's also used by the auto light feature.
 
It was snowing up here all day yesterday, and I had to go out in it several times. The sensor just couldn't pick up the damn snow (I don't remember it being so bad about it last winter). So when I had a chance to park I finally turned off the auto-sensor and went with standard intermittent.
 
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