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.
The ground. (because the rapid diffusion of warm, moist air from the ground causes depth hoar)
Temperature Gradient (TG) (but this is an outdated term) sugar snow, squares, sometimes incorrectly called "hoar frost" by old, rural geezers.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.
Best Stability tests:
Explosives tests, cornice drops, Rutschblock, compression test, test slopes.
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.
Depth Hoar Details:
We normally think of depth hoar as an early season phenomenon. It begins to form after the first snowfall as soon as temperatures get cold or more important, when the skies clear. As with surface hoar, radiation plays an extremely important role in snow surface temperature (Remember the discussion of radiation in the Weather chapter); in most climates, it plays a more important role than air temperature. Therefore, in mid latitudes, depth hoar grows primarily on the shady aspects, northwest, north, northeast and often east-facing slopes. Often in mid winter when the snow is deep enough that depth hoar quits growing, we find depth hoar on the slopes with thin snow such as west and south facing slope and near ridgelines where the wind has thinned the snowpack. In high latitudes such as Alaska and northern Canada, as well as in equatorial latitudes, if depth hoar grows, it tends to do so on all aspects nearly equally.
More than most other weak layers, the strength of depth hoar varies quite dramatically from one location to another, depending mostly on the depth of the snowpack. Remember: thin snowpack means a weak snowpack. Thicker snowpacks insulate the cold air from the warm ground, have a small temperature gradient and thus a stronger snowpack. For this reason, you usually don't find bad depth hoar under the thick layers of wind loaded snow near the ridgetops. It's usually much weaker at mid-slope and especially near the bottom of basins, where thin snowpacks combine with cold air pooling, and around rock outcroppings.
Depth Hoar and Climate:
In continental climates, depth hoar is extremely common; in fact, depth hoar often makes up nearly the entire snowpack until about February of each year, and then in thin snowpack years, large wet slabs fail on depth hoar in spring. Depth hoar accounts for most avalanche fatalities in continental climates and most snow stability and forecasting decisions revolve around it.
At the other end of the spectrum, in maritime climates, depth hoar usually forms only in the early season and quickly disappears after the first couple snowstorms bury it. In very warm maritime climates you can go several years without even seeing it.
In between these two extremes, in intermountain climates, depth hoar forms in the early season during most years and depth hoar instabilities commonly last until December or January, and is not much of problem after that. In bad depth hoar years, large wet slabs may release in release in spring when melt water saturates the old depth hoar layers.
Mechanical Properties of Depth Hoar:
Mechanically, depth hoar is one nasty dude. Depth hoar behaves like a stack of champagne glasses. Although it's stronger in compression than in shear, it can also fail in a catastrophic collapse of the layer. Fractures commonly propagate long distances and around corners. Almost all catastrophic, climax avalanches (involving the entire season's snow cover) fail on depth hoar.
A hard wind slab on top of depth hoar is double trouble. It's like laying a pane of glass on top of a stack of champagne glasses. It bridges a person's weight out over a larger area allowing them to walk on eggshells without breaking them until they either give it a hard thump, reach a place where the slab is thinner, or where the depth hoar is weaker, and then the whole slope shatters catastrophically. Fractures involving hard slabs commonly form above the victim, leaving very little chance for escape. Wind slabs on depth hoar exist throughout most of the season in continental climates, and when you add large populations to the equation it also means large numbers of fatalities. It's easy to see why Colorado leads the nation in avalanche fatalities.
As Canadian avalanche specialist Clair Isrealson once told me, "Depth hoar is like having your crazy aunt come for a visit. She stays forever and you just never know when she's going to snap."
Large-grained depth hoar persists longer than any other kind of weak-layer. And as long as it does, you just tiptoe around and accumulate gray hairs. Usually the larger the grain size, the more persistent the instability. The time-honored adage among experienced avalanche professionals is: "Never trust a depth hoar snowpack." In other words, it's always guilty until proven innocent.
Carefully watch each loading event all winter--especially the big ones. Then even after you think you've seen the last of it, percolating melt water in the spring will re-activate the depth hoar layer and produce large, wet slab avalanches. Yikes! The best stability tests for depth hoar listed roughly in the order of reliability: explosive tests, cornice drops, Rutschblock tests, compression tests (do lots of them in representative places), jump on test slopes and pay attention to recent avalanche activity. Weather isn't quite as reliable unless it's really obvious weather like a heavy loading event or rapid warming of a thin slab overlying depth hoar.
If you can't use active tests, use a thermometer and carefully measure the temperature gradient across the weakest layers. As soon as the temperature gradient drops below the critical level (about 1 degree centigrade per 10 centimeters) then it is gaining strength. But remember that depth hoar is quick to form but takes a long time to gain strength after the temperature gradient is removed. With no additional loading and with a weak-layer of -5 deg C or warmer, it takes several days to a week to stabilize. With cold weak layers and a lightweight overlying layer, it can take much longer.
In latitudes, say, from about the southern U.S. to the Canadian border, depth hoar usually exists only on shady aspects (NW, N, NE and often E facing slopes in the Northern Hemisphere). North of the Canadian border or In cold very climates, or during arctic outbreaks, sometimes it's just too cold to grow depth hoar very rapidly and it forms, instead, on sunny aspects where it's warm enough to grow faster. It grows best in snow temperatures between about -2 deg C and about - 15 deg C. And no matter what aspect with respect to the sun, you usually find depth hoar in thin snowpack areas such as wind-blown aspects.
At mid latitudes, people especially get in trouble in the early season with the first slab that forms on top of depth hoar. The sun melts the snow away on the sunny slopes so it forces people onto the shady slopes where the depth hoar lives and they take lots of ride in avalanches.
It's common to trigger depth hoar avalanches from long distances away. Many people, including a co-worker of mine, have been killed when they triggered the slope from the bottom, in this case, from a nearly quarter mile away. It's also easy to trigger a steep slope by standing on a flat slope above or to the side. Sometimes the steep slope below will pull a hard slab off a flat ridge above, like when a child yanks on the tablecloth hanging over the edge and dumps dinner onto the floor.
Victims usually trigger depth hoar avalanches from shallow (thus weaker) snowpack areas, like a wind blown ridge, a rock outcrop in the middle of a slope, or a spur ridge next to a steep slope. Most of the time we think of a rock outcrop in the middle of a steep slope to be an "island of safety" but in a depth hoar snowpack you should think of it as a trigger point.