Minecraft wiki entry for Snowfall, associated text: Snow actually falls one block into the void (Layer -1). This can be seen by digging a vertical shaft down in Creative mode, removing the bottom layers of Bedrock, and flying down into the Void. Note that no particles are emitted from the snow, due to the absence of a block below it.
Snow still falls above the clouds. Notch's explanation is that the gray above the clouds during a storm is another layer of clouds and the origin of the snow.[3]
There is a never ending snowfall above a certain Y value, which turns into rain then rain turns into nothing.[needs testing]
At extremely high Y values, snow appears to fall more slowly and the texture of the snow particles becomes vertically stretched. The slowed falling speed becomes noticeable at around Y=200000, while the vertical stretching effect becomes noticeable around Y=[[1]]