2.2.6 Density
Although geography sets the stage for snowpack development, each type of precipitation event has its own impact on snowpack depth and density.
Before examining the precipitation types, we'll take a minute to discuss density. Density is the most important aspect of snowpack since it determines the amount of runoff and the stability of a snowpack. Stability is critical for avalanche considerations.
When discussing density, we tend to think in terms of weight per volume, with a typical snow density being 0.1 gram per cubic cm (g/cm3). That makes snow one tenth as dense as water because it is composed of both water and air.
When talking about snowpack, we express density as the depth of the pack vs. the depth of the water that would be produced if the snow were melted. This is known as the snow water equivalent or SWE. For example, a snowpack that is 100 cm (39.4 in) deep will melt to produce a volume of water 10 cm (3.9 in) deep.
Hydrologists like the term SWE because it tells them how much water will run off when the snowpack melts. Meteorologists typically use another term to discuss the density of snow: snow-to-liquid ratio or SLR. SLR is a unitless ratio of snow depth to liquid depth (SWE).
The SLR is the inverse of density, meaning that the higher the SLR, the lower the density of a snowpack. Using our previous example, snowpack with a density of 0.1 has an SLR of 10:1.
- SLR is high for light, powdery snowpack (up to about 40:1)
- SLR is low for snowpack with older or drifted snow (as low as 6:1 or 7:1)
- SLR is even lower for very wet snowpack (as low as 2:1)