You left out the higher entity that takes away a significant portion of the snow that was accumulated at regular intervals.
It doesn't need to ruin the metaphor, though:
The sun could do that job. It could also explain the fact that the portion that's taken differs depending on where on earth you are creating the snowball.
It doesn't need to ruin the metaphor, though: The sun could do that job. It could also explain the fact that the portion that's taken differs depending on where on earth you are creating the snowball.