Large-grained, faceted, cup-shaped crystals near the ground. Depth hoar forms because of large temperature gradients within the snowpack.
Depth Hoar–faceted snow near the ground:
Contrary to popular belief, as long as the ground has an insulating blanket of snow, the ground is almost always warm–near freezing–even with very cold air temperatures. Snow is a wonderful insulator and even with very cold air temperatures it’s common for the snow near the ground to remain damp for most of the season. The only exception to this is in permafrost areas (very high elevations at mid latitudes or arctic latitudes) or in areas with a very thin snow cover combined with very cold temperatures.
The top of the snow surface, on the other hand, can become extremely cold–especially when exposed to a clear sky–thus creating one of the most common temperature gradient conditions. Especially in the early winter, cold temperature often combines with a thin snowpack making the perfect breeding conditions for the dreaded faceted snow near the ground, which we call depth hoar.
Depth Hoar Summary:
Sparkly, larger grained, beginning and intermediate facets are square 1-3 mm, advanced facets can be cup-shaped 4-10 mm.
Loose, runs through your fingers, granular, crunchy when chewed.
Temperature Gradient (TG) (but this is an outdated term) sugar snow, squares, sometimes incorrectly called “hoar frost” by old, rural geezers.
From large temperature gradients between the warm ground and the cold snow surface. Usually requires a thin snowpack combined with a clear sky or cold air temperature. Grows best at snow temperatures from -2 deg C to -15 deg C.
Behaves like a stack of champagne glasses. Relatively stronger in compression than in shear. Fails both in collapse and in shear. Especially nasty when it forms on a hard bed surface. Commonly propagates long distances, around corners and easily triggered from the bottom–your basic nightmare.
Extremely persistent in the snowpack from several days to several weeks, depending on temperature. The larger the grain, the more persistent. Percolating melt water in spring often re-activates large-grained depth hoar. Depth hoar is guilty until proven innocent.
At mid latitudes, mainly on shady aspects (NW-NE). In very cold climates, forms on warmer slopes (sun exposed, near fumaroles, non permafrost areas). At arctic and equatorial latitudes, it shows much less preference for aspect.
• Continental climates: extremely common throughout the season. Often makes up the entire snowpack until about February.
• Intermountain climates: Common before about January.
• Maritime climates: Rare and usually in the early season.
Never underestimate the persistence of faceted snow as a weak layer. Makes large and scary avalanches. Carefully measure temperature gradients across the weak layer. Large gradients mean the snow will remain weak, small gradients mean the snow is gaining strength but it takes several days to several weeks depending on temperature.
Easily triggered from the bottom of a slope or from an adjacent flat area. Pay attention to what your slope is connected to. Depth hoar avalanches usually triggered from a shallow snowpack area–avoid rocks outcropping in the middle of a slope.