On second thoughts...

I retract my comment on binocular vs parallax depth perception, at least partially. Parallax makes slightly less sense, considering that wind and other random forces make the relative motion of snowflakes much less than uniform, so trying to deduce distance from that would be considerably more complicated. I thought maybe there might be something to do with the way different layers (meaning vertical layers at differing distances) of snowflakes move as units, but right now there's no wind, and the layers are rarely so easily divided anyway. Also, I tried out the binocular thing again. Maybe it's that I'm wearing glasses or that my left-eye vision is suffering from the dimness or that the lack of wind today is causing there to be much less structure in the motion of the flakes (perhaps I should say simpler structure), but now my monocular depth perception seems less successful. The interesting thing is that it manifests not as a picture of a flat plane of falling snow but instead in a noticeable deficiency of awareness regarding the smaller flakes, presumably the ones which would be better fixed in place with the right-eye's aid...