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:
Looks
like:
Sparkly, larger grained, beginning and intermediate facets are
square 1-3 mm, advanced facets can be cup-shaped 4-10 mm.
Feels like:
Loose, runs through your fingers, granular, crunchy when chewed.
Smells like:
The ground. (because the rapid diffusion of warm, moist air
from the ground causes depth hoar)
Also
called:
Temperature Gradient (TG) (but this is an outdated term) sugar
snow, squares, sometimes incorrectly called "hoar frost"
by old, rural geezers.
Formed:
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.
Mechanical
Properties:
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.
Persistence:
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.
Distribution Pattern:
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.
Regional Differences: • 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.
Forecasting
considerations:
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.
Routefinding Considerations:
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:
Distribution Pattern:
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.
Forecasting Concerns:
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.
Routefinding Considerations:
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.
