What is SWE?
SWE stands for Snow Water Equivalent.
What it measures is the amount of liquid water in the snow. If you took a height of snow and melted it, the height of the water created is SWE. For example, if 10″ of snow falls at 10% density, then there would be 1″ of SWE.
Snowfall is measured this way for a couple of reasons:
- The first is that the height and weight of water are constant, while snow’s height and weight vary considerably depending on the density.
- SNOTEL sites, which are remote weather stations, measure snowfall by measuring the displacement or weight in antifreeze, which is the liquid weight. The data from SNOTEL sites is primarily used to decide how dams and irrigation should be managed, and SWE is the most convenient measurement for this.
SWE is a convenient measure for estimating how a new snow load will impact avalanche conditions because it provides us with a constant value.
For example, a mid-winter storm that deposits 10″ of snow at 10% water content would have 1″ of SWE, a spring storm that deposits 4″ of snow at 40% water content would have 1.6″ of SWE and 60% more weight than the lighter, deeper snow.
Generally, mid-winter snow will be around 10% liquid water, and the rest will be air. This is a convenient number; it is easy to multiply the SWE by ten and estimate how much snow fell. Percentages can vary though, very light and dry powder, “cold smoke” can be 3% water content. Heavy spring or coastal snow, “cement,” or “mash potatoes” can be 20% or more water content.
The amount of water in snow can also be expressed as a ratio, Snow to Liquid Ratio, or SLR. You can see this on the backcountry forecast issued by the National Weather Center.
SLR estimates snow density. A 10:1 ratio would be 10% liquid content snow, higher ratios are lighter snow such as the 14:1 ratio above, and smaller ratios such as 1:8 are heavier snow.
You can calculate more accurate snow heights using the SLR from the forecast. For example, when snow is forecast to fall 1:14 ration and a SNOTEL in the area site reports 1″ of SWE multiple 1 by 14 to get 14″ of snow.
Some forecasts will also use percentages to estimate snow density. In this case, divide the SWE by the percentage to get height of snow. For example. 1″ of SWE at 7% density would be 1 X 0.07=14″ of snow.