MorganS:
Snow is mostly air. Fresh snow moreso. It won't reflect sound like a solid surface.I would use an optical sensor. Maybe a bit more expensive but it "sees" what you see.
Dear MorganS,
Thank you a lot for that experience. I am planning to use both sensors, i.e. ultrasonic and light now, for my project. There is a Swiss company that produces ultrasonic snow sensors:
This principle seems to work for ultrasound but I am not 100% convinced. Sadly, there's not yet snow here to check.