Nice job, Carol. But I hope you don't mind me outlining an alternative method that I find more convenient - doing it in post using PSP. I use a free plugin that generates the snow randomly for you on a separate transparent layer. You can vary the size and number of flakes, and like your multiple cylinders, I use several layers at different sizes and densities to achieve the effect. I sometimes also use motion blur on just a couple of the layers at fractionally different angles to one another to simulate the effect of wind-blown snow. And because they are all on transparent layers, it is easy to delete any unwanted flakes from areas that should be in shelter from the weather or look unnatural too close to the ground.