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.
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. However, to be perfectly accurate one must account for the fact that snow, which is ice, does not have a density of 1.0g/cm3 like water, but instead is 0.92g/cm3. Therefore to properly account for the fact that water expands as it becomes a solid means that a snow sample with a density of 50kg/m3 is actually (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).
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 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 firstname.lastname@example.org, or the editor.