With "snow" I am assuming you mean falling snow? I'd image you can pull this off with clouds. Get a nice fine-grain cloud layer, and mask it by another fine grain cloud layer (or better a image map shader with speckles) to add more space in-between speckles. Then just localize and adjust settings for a ball look.
I am guessing this approach based on my phytoplankton/water debris ideas which worked fairly well. The only issue is getting the scale right on the snowflakes.
