Snow accumulation

Rules

Weather	It must be raining in the world, and the spot must be cold enough for the rain to fall as snow.
Temperature	The temperature at the spot must be below 0.15, in a biome that has precipitation.
Light	Block light at the spot must be below 10. Sky light does not count.
Where it lands	Snow lands on top of the highest block of a column. The space there must be Air or Snow, on a block that Snow can rest on.
Layer limit	The Snow accumulation height game rule sets the limit: 1 layer by default, 8 at most, and 0 for no accumulation.
Growth	Each time snow lands on a spot, it places Snow there or adds 1 layer to Snow already there, up to the limit.