Well, this is very nice! You could almost feel the coldness. How you created the fog is VERY well done. You see it behind and before the tree, yet you are still able to see the tree. I like how you can't see anything else due to the fog's thickness.
Okay, sooo....there's only two things that could be improved. The first is the tree's shadow. It's too blunt looking in my oppinion due to there being just the blue. I think it'd look nice if some faint streaks of white were added into the shadow, just like the patches of snow along the bottom edge. The second thing, isn't necessarily a problem, but the snow level just cuts off directly at the tree, making me think it is right on the edge of a cliff. If that's intended, then that's great, just don't make the tree so straight. But if it's not on the edge of a cliff, just push the snow leve/horizon a bit further behind the tree.