Surface Hoar:
Surface hoar is a fancy name for frost. When you have to scrape
your windshield in the morning, surface hoar grows on the surface
of snow—hence its name. It grows during clear, humid and
calm conditions and once buried, it is a particularly thin,
fragile and persistent weak layer in the snowpack, which accounts
for a number of avalanche deaths each season. Especially in
maritime climates, surface hoar accounts for most avalanche
accidents.
Surface hoar is an especially tricky weak layer because it can
form very quickly. One calm, clear night—sometimes just
a few hours—is enough time to deposit a thin layer on
the snow surface. And once buried, it is very thin and difficult
to detect, yet very weak. Also, it tends to form in a complex,
hard-to-predict distribution pattern on the terrain. For instance
it might form only above a certain elevation where the mountain
rises above the clouds. It might form below a certain elevation
where cold, humid air pools. It might form in a distinct elevation
band where thin clouds form a “bathtub ring” in
a confined mountain valley. It tends to form on open slopes
as opposed to in trees. Also, when deposited on the snow surface,
since it is so fragile, any small disturbance—especially
wind—can easily destroy the layer making it very “pockety”
i.e. you find it in one spot but not another. No wonder Canadian
research indicated that surface hoar accounts for most unintentional
human triggered avalanches triggered by professionals
Because surface hoar is so thin, it is also difficult to detect.
Often you can’t see it in a snow pit wall and it only
reveals itself when you get a clean shear and you look at the
bottom of the block and see the flat, feathery, sparkly crystals
glittering back at you. The best way to detect surface hoar
is to carefully pay attention to the snow surface each day.
Before the storm arrives, carefully make a mental map of where
surface hoar remains intact. You can typically find surface
hoar in basin bottoms and near creeks or lakes.
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How it forms:
During a clear sky, the snow in the shade or at night radiates
a tremendous amount of heat away and the snow surface becomes
very cold. Since we know from earlier in this chapter that warm
air holds more water vapor than cold air, the vapor from the
warmer air above the snow will condense onto the surface of
the snow, and voila, we have surface hoar. Surface hoar (frost)
is simply the winter equivalent of dew.
Note: in arctic latitudes, the mid-winter sun is so weak that
surface hoar grows all day long, even in the sun. You can grow
HUGE surface hoar in the north-country, especially in basin
bottoms and near streams.
Next, let's take a short lesson in the second ingredient for
surface hoar--humid air. Humidity, or relative humidity, is
the amount of water air can hold compared to the amount it actually
does hold. For instance, air at 50 percent relative humidity
contains only half the amount of water vapor it could if there
was an infinite supply of water around. How much water can air
hold? It depends on the temperature. Remember, warm air holds
much more water vapor than cold air. In other words, we can
change relative humidity two ways, first, by adding or taking
away water (humid air left over after a storm or humid air near
streams), and second by raising or lowering the temperature.
This second method, as it turns out, creates much, if not most,
of the humidity that forms surface hoar. As air cools down during
a clear, calm night, it becomes more humid. Often, this cold,
humid air pools up into the bottoms of mountain valleys and
basins, exactly where we find surface hoar.
Finally, we need the last ingredient, calm air. Too much wind
will destroy the fragile surface hoar crystals, plus, too much
wind doesn't allow the cold, air to pool and become humid. Actually
about 3 mph is best for surface hoar production because it's
just fast enough to bring a continuous supply of humid air to
the snow surface but not too fast to destroy it.
In summary, surface hoar forms in the following conditions:
| • Clear sky |
| • No direct sunshine, or very weak
sun |
| • Calm or light winds (about 3 mph
is best) |
| • Open slope exposed to a clear sky
(trees or clouds can radiate their own heat and disrupt
the process) |
| • Humid air |
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Distribution Pattern
of Surface Hoar:
With this knowledge of both radiation and humidity in mind,
let's where we are most likely to find surface hoar after a
clear, calm night. First, the snow must be exposed to a clear
sky. This means that surface hoar doesn't grow under evergreen
trees where the thick branches disrupt the back-radiation process.
However surface hoar grows just fine in a sparse grove of aspen
trees because they don't block much radiation.
And what about humidity? We know that cold air sinks and on
cold, clear conditions, cold air will pool in the bottom of
a valley or a mountain basin. When air cools it becomes more
humid, thus, surface hoar tends to form more at lower elevations
or especially in the bottom of mountain basins and not nearly
as much on mountain tops or ridges. We also find thick layers
of surface hoar near open streams because they provide such
a constant vapor source.
This is a tricky situation, because normally we expect more
avalanche danger the higher we go on a mountain because there's
more snow and more wind. But with surface hoar as a weak-layer
there's counter-intuitively more danger at lower elevations,
which commonly surprises people who aren't accustomed to surface
hoar.
But what happens if the air in the valley bottom becomes so
humid it turns into fog? Remember the snow surface has to be
exposed to a clear sky to form surface hoar. So if the fog is
thick enough, it prevents surface hoar from forming. But with
a thin fog, surface hoar grows like crazy. Now let's say the
fog is thick, perhaps 300 m (1000 vertical feet) which is probably
thick enough to prevent surface hoar from forming on the valley
floor, it still forms along the top of the fog layer where we
still have the perfect conditions for surface hoar. So like
a bathtub ring, in the morning we often see a thick layer of
surface hoar along the top of the fog layer. Often you see this
same bathtub ring effect along the top layer of stratus clouds
that are low enough for the mountaintops to rise above the clouds.
Once formed, surface hoar is very fragile, and even a light
wind can either blow or sublimate it away. Because the wind
can remove surface hoar from some areas and leave it in others,
once buried, it can be devilishly difficult to detect. A snowpit
in one place might show nothing suspicious while one 10 feet
away may show a very fragile layer. We don't find as much surface
hoar on mountain tops not only because of the aforementioned
humidity differences but because the wind blows more on mountaintops
and ridges than in valleys.
Surface hoar forms much more commonly in maritime climates than
continental climates because it needs humid air. In high latitudes
such as Alaska and northern Canada, surface hoar grows all day
long since the sun is so weak in mid winter. I have seen widespread
areas of eight inch thick surface hoar crystals in Alaska, in
the bottom of mountain basins.
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Mechanical properties
of surface hoar:
Surface hoar makes perhaps the perfect avalanche weak-layer.
It's thin, it's very weak, it's notoriously persistent and
it commonly forms on hard bed surfaces, which are also slippery.
Finally, thin weak-layers tend to fail more easily because
any shear deformation within the snowpack is concentrated
into a small area.
Surface hoar can fail either by collapse or in shear. It can
fail in collapse if the new snow is added slowly, the surface
hoar crystals remain standing up, like columns, and when critically
loaded, just one thump and all the columns collapse catastrophically,
like the old college trick where you can stand on an upright,
empty beer can without crushing it, but one tap of a finger--and
crunch!--ready for the recycle bin. In fact, this is probably
the most common scenario for surface hoar, as well as other
persistent weak layers: often the first or second storm on
top of a surface hoar layer doesn't weigh enough to overload
it, but the third or fourth storm finally adds up to the critical
weight. Whamo! Just like the college beer can experiment.
Surface hoar can also fail in shear when the first snowfall
lays the surface hoar crystals over on their side; they remain
as a paper-thin discontinuity in the snowpack with very poor
bonding across that layer. These laid-over crystals, however,
tend to bond up more quickly than the ones that remain standing
on end.
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Types of surface hoar:
This is getting a little fancy for mere mortals, but there different
kinds of surface hoar crystals and some are more dangerous than
others. I call these: needles, feathers and wedges. Different
combinations of temperature and relative humidity form each
kind. The take home point here is that the danger and persistence
of surface hoar goes in the order of: needles, feathers and
wedges--wedges being the worst.
| Type
of surface hoar |
Conditions
formed under |
Looks
like |
Forecasting
considerations |
| Needles |
Very cold temperatures < -21 deg
C. |
Tiny needles |
Less persistent, doesn't form thick
layers |
| Feathers |
Normal temperatures |
Feathers |
Persistent, but is laid down more easily
than wedges |
| Wedges |
Normal temperatures |
Wedges |
Very persistent and tends to remains
upright |
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Forecasting Considerations:
Surface hoar crystals are notoriously persistent in the snowpack.
Instabilities commonly last for a week or two. In the cold
snowpacks of Montana and Wyoming, I have seen avalanches on
a surface hoar layer four months after it was first deposited.
The best way to deal with surface hoar is carefully map it
every time it forms BEFORE new snow covers it up. Any time
we have surface hoar on the snow surface and I know we have
a storm on the way, I will dutifully march around and carefully
notice where it still exists and where either the sun has
melted it away or the wind has destroyed it, and I will document
it for future reference. As you can imagine, this information
literally takes on life and death importance during each successive
loading event.
Another tricky situation with surface hoar: During a snowstorm,
it might be snowing and cloudy when you go to bed, and still
snowing and cloudy when you wake up. But during the night,
unbeknownst to you, the winds die and the sky clears for a
few hours, and voila, a thin layer of surface hoar forms--and
you didn't even notice it. The next day, you will notice sensitive
soft slab avalanches within the new snow and you expect them
to calm down after a day like usual, but instead, they last
for several days. You dig to investigate and find the culprit.
Darn that sneaky surface hoar!
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Depth Hoar Summary:
Forms:
clear sky, light to calm wind, humid air.
Looks like:
Sparkly, flat, feather-like or wedge-shaped, stepped, striated
crystals--sometimes mistaken for facets or stellar snow that
falls from the sky.
Also
called:
hoar frost, frost, feathers
Distribution Pattern:
Open areas without trees or sparse trees exposed to a clear
sky, lower elevations as opposed to upper elevations, the bottoms
of mountain basins, beneath thin fog layer, the top of a thick
fog layer or stratus cloud layer, shady, calm areas, near streams
Persistence:
Extremely persistent weak-layer--one week to months depending
on temperature. Especially persistent and dangerous when on
top of a firm ice crust.
Best snowpit detection method:
Shovel shear test or compression test. Look at the bottom of
the block to see the crystals.
Forecasting considerations:
Carefully map the distribution of surface hoar BEFORE it is
buried by subsequent snow. Be suspicious of it with each loading
event. Surface hoar is guilty until proven innocent.
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