Backcountry: Explained | Snow Water Equivalent and Its Role in Avalanche Risk

Clay Malott | | AvalancheAvalancheBrainsBrains
backcountry, swe,
Snow water equivalent. Photo credit: UCAR

Backcountry skiing is a wonderful activity that wouldn’t be possible without snow. To put it simply, snow is very complex. It can do all sorts of things once it has hit the ground: turn into different shapes, gain and lose stability, and more.

However, something that comes up a lot when talking about snow before and while it’s falling is snow water equivalent (SWE). Snow water equivalent is essentially a proxy for snow density. If you have 20 inches of snow and melted it down and ended up with 1 inch of water, the ratio would be 20:1. SWE has many different applications in the snow sports world. Understanding and contextualizing snow water equivalent is hugely important to backcountry safety.

The first and most obvious implication of snow water equivalent is the density of fresh snowfall. Due to lower elevations and higher moisture content, maritime areas (like the Cascades, Sierra Nevada, and Coastal Ranges) often get denser snow. This is characterized by a higher snow liquid ratio (SLR), typically hovering around 12:1. High and dry continental areas, like the Rockies or Wasatch, often receive ratios upwards of 16:1. In general, lighter snow with a lower SLR is more fun to ski. It takes less effort to turn through than heavier snow experienced in areas closer to the ocean.

Snow with a low SLR is fun to ski. Photo credit: RootsRated

Another, more nuanced implication for SWE, and more important for backcountry safety, is avalanche risk. I like to think of avalanche risk like a see-saw. Say you have a see-saw where one end is hanging off a cliff. If the see-saw is perfectly balanced, the avalanche risk is high; just a bit of material placed on the cliffside of the see-saw could tip the scale and send someone careening down the cliff. If there is significantly more weight on the see-saw’s non-cliff side, then the balance can tolerate much more weight before ending in a similar fate as the first instance. The cliff height is like the consequence of the avalanche, a small cliff will hurt and could be deadly if you land wrong, and a big cliff means almost certain death.

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Seesaws are like avalanche risk. Photo credit: Wayfair

In this see-saw example, to an avalanche forecaster, giving how much snow fell is like telling them how much volume of “stuff” you put on the see-saw. But this is irrelevant – on the see-saw, placing 1 cubic foot of cotton isn’t at all comparable to placing 1 cubic foot of steel. This means that the likelihood of it tipping over the cliff (or the likelihood of an avalanche occurring) is unknown. However, snow water equivalent is knowing exactly how much weight was placed on the see-saw, regardless of the material density.

Hence, snow water equivalent is much more useful to forecast avalanche risk than total snowfall. 10″ of heavy snowfall and 10″ of light snowfall do not load the snowpack equally, but 1″ of SWE in heavy snow and 1″ of SWE in light snow do load a snowpack equally. Knowing the snow water equivalent is hugely useful to avalanche forecasters, but it can also be incorporated into your avalanche terrain decision-making, as well. SWE can be used to assess how much the load on the snowpack increased over a storm cycle. Hence, by checking the snow water equivalent totals from weather stations in the area, you can get a better idea of stability than just by looking at how much snow fell.

Previous articles in this series:

Backcountry: Explained | What Are Wind Slabs and How Do They Form?

Backcountry: Explained | Why Does the Snowpack Gain Strength in the Spring?

Backcountry: Explained | Snow Water Equivalent and Its Role in Avalanche Risk

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