4.1.12 Day 4, 6AM: Depth hoar formation
Long, undisturbed periods of cold atmospheric temperatures and radiative loss at the top of the snowpack have a cumulative effect, with most of the snowpack steadily cooling. Since the ground remains relatively warm because it's insolated by the snow, the temperature gradient increases. If it gets large enough, depth hoar can form. Depth hoar is highly faceted, large, feathery crystals that grow on the edges of existing snow grains.
These electron microscopy images show depth hoar crystals, first at a very small size (a few tens of micrometers), then at a large, mature size. Notice that the depth hoar grows at the expense of the pre-existing snow grains.
Depth hoar typically forms over several days. Although the crystals are bonded, together they form a weak, brittle structure. If you try to pick up or move a layer dominated by depth hoar, it will disintegrate. Note that weak layers of depth hoar are a significant concern from an avalanche perspective, since they can be the source of fractures and slides.