Once settled onto the ground, snow acts as an insulating blanket and traps heat. The temperature gradient that developes as a result, "warm" near the ground
and cold near the surface, promotes a form of recrystalization in the snow. Snow will begin to degrade immediately upon hitting the ground and the form it
degrades to depends on the conditions. This releases water vapor that rises until trapped by a layer of denser snow or ice. If the temperature gradient is high
enough, the trapped moisture will form large, iconic crystals called depth hoar. You can see these layers as a courser textured snow if you look in the
snowpack.
Above, we can see the several layers of snow with the new, fine grained stuff near the surface, and the larger depth hoar in the, uh, depths.
And below, we can see what the depth hoar looks like up close. Notice the clearly defined crystal structure.
Eventually, the snow will do one of two things, depending on conditions. It may stick around, slowly compacting and recrystalizing into firn, snow-ice,
and then eventually ice-ice. This is the basis of glacier formation. The other thing it does is melt away. Generally, well managed programs want to know
when the snow is going to melt so they can deal with the affects on the hydrosphere, or roads, for example. A relatively straight-forward formula can
give us a good idea when the melt will happen.