VOL. 30, NO. 2, DECEMBER 2011
THE AVALANCHE REVIEW
10.2011 AAAP was tour:Layout 1 10/20/11
PAGE 7 t
what's new
Why Measuring Snow Density Matters
Story by Craig Dostie
Why does knowing the density of snow matter?
There are three common reasons. 1) To increase
understanding of the snowpack for predicting avalanche
hazard on a particular slope, 2) to estimate the amount of
water in the snowpack (which contributes to forecasting
knowledge), and 3) to know the density of snow as a percent,
something skiers and snowboarders enjoy bragging about.
Bragging Rights
Strictly speaking, percent snow is a measure of porosity – a ratio
of the volume occupied by snow crystals versus the total volume
of the sample. Snow density on the other hand is simply the mass
per unit volume. However, since snow is frozen water, the easiest
way to deduce the percentage of snow is to make a snow density
measurement, then convert that value to percent.
A simple way to do that is to simply take the snow density
value and move the decimal point to the left one digit. Thus
a snow sample with a density of 50kg/m3 is approximately
5% snow.
Snow Density Versus
However, to be perfectly
Snow Percentage
accurate one must account
for the fact that snow, which Snow Density Percent Snow
kg/m3
%
is ice, does not have a density
500
54.35%
of 1.0g/cm3 like water,
but instead is 0.92g/cm3.
200
21.74%
Therefore to properly account
100
10.87%
for the fact that water expands
50
5.43%
as it becomes a solid means
20
2.17%
that a snow sample with a
10
1.09%
density of 50kg/m3 is actually
5
0.54%
(5 x 1.09) percent, or 5.49%
snow (see table at right). In reality, few skiers bother to prove their
claims, spouting percentages merely to appear superior.
Estimating Water Content
For years hydrologists have been measuring snow density
and converting it to the snow water equivalent (SWE) value
to understand the amount of water held in a snow pack. This
has practical value for estimating spring runoff in rivers,
and water volume available for farming. It also has value in
estimating avalanche activity.
According to The Avalanche Handbook, “The total precipitation
(in mm of water equivalent) is more fundamental than new
snow depth for avalanche prediction.” Whether the snow falls
as light and deep, or heavy and dense, either condition could
be produced by an equal amount of water and thus create the
same avalanche potential, though triggered by different types of
events. Thus, knowing the SWE of a particular storm snowfall
helps with the overall understanding of the snowpack.
To convert snow density to its equivalent water volume,
or the snow water equivalent (SWE) you simply need to
weigh a known volume of snow and then convert that to the
equivalent volume of water for the same weight since the
weight of water present, in either liquid or solid state, is the
same. All that changes is the volume (and temperature).
WeightSnow = WeightWater (same # molecules)
DensityWater = 1.0g / 1.0cm3 = 1.0g / 1.0ml
Therefore:
WeightSnow / DensityWater = VolumeWater
Avalanche Forecasting
While there are many other properties of a snowpack that
will give a faster and more accurate indication of avalanche
potential than snow density – such as relative the hardness of
layers – measuring and keeping track of the snow density can
provide important information for avalanche prediction.
There are many factors that contribute to the stability of a
snowpack and the likelihood of an avalanche occurring. Super
dense snow may be an indication of a stable snowpack if this
condition exists throughout the snowpack. Changes in density
can indicate weaknesses too, or help to quantify differences
between layers in the snowpack such as a buried surface hoar
layer, a strong melt freeze crust, or changes in grain types.
These are structural factors that are difficult to interpret
because they don’t indicate instability by themselves.
Though there is no threshold value for snow density to
indicate the formation of a slab, the denser a snowfall is, the
more likely it will form a slab. Avalanche instructor and one
of the guiding forces in AIARE curriculum, Colin Zacharius,
says, “One condition of particular concern is when warmer,
denser snow falls over cooler, less dense snow.” This has
the signature of a heavy slab over a weak layer that could
shear. The question is, what sort of trigger could make that
happen? That’s where SWE comes in, as it helps to quantify
the load intuitively. With experience, you can anticipate the
level of trigger needed: a breath of wind, a snowboarder, a
snowmobile, or hopefully, nothing.
Measuring snow density can be done in a crude form by
documenting hardness changes in penetration, such as fist,
finger, pencil hardness. A snow-density gauge adds more
precision to such measurements which will certainly help
with an overall awareness of conditions over time, provided
density is measured in a consistent way throughout the
season, but may not necessarily help with prediction at a
specific time and location. Snow density is one more factor
that professionals use along with other snow-structure factors
in their evaluation of instability. Knowledge is power when
used in the right hands.
There are two things to keep in mind when performing
this measurement. The first is that it is difficult to obtain
an accurate sample of snow in a container without causing
a shift in the density of the sample being measured. The
volume of the container may not be completely filled, or the
snow will be packed tighter in the container as a result of
attempting to fill it.
In the case of very light density snow, the simple act of
transferring it to a container will cause settling and packing
of the snow which increases its density. So sampling errors
can be a problem with measuring snow density, especially
with low density snow. As the density of the snow increases,
the probability of sampling error is reduced though it is still
dependent on good technique.
Craig Dostie is the former publisher of Couloir magazine and
a freelance writer. While editing product copy for Brooks-Range
catalogs and Web site the desire to revise their snow-density gauges
arose. This article was written in conjunction with developing a
new set of snow density/SWE gauges for Brooks-Range. For more
detailed snow-density research figures, please contact Craig Dostie
at [email protected], or the editor.
R
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market and available as soon as DOTs have completed their paperwork and issue the formats.
R
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