forum #42247, snow is for snow that is visible on the ground. snowing is for the visual effect of white stuff falling from the sky or possibly snowflakes falling from the sky. It's in the wikis for both tags.
Snow and snowing are two visually distinct elements of an image that can commonly be depicted together. Implicating one to the other would make it difficult distinguishing between the three types of images that can exist with these two visual elements (snow on the ground but not snowing, snowing but no snow on the ground, and snow on the ground and snowing).
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