Know the Snow: Rain on snow

A photo of wet grains, some of these grains are held together by the surface tension of the water surrounding them.

Last weekend we were trying to figure out if we wanted to go skiing.


It was raining outside and a quick look at the Federal Aviation Administration’s vertical velocity sonar and weather stations across the area indicated that it was also raining up at Eaglecrest Ski Area.

This wasn’t an isolated event.

It has rained a lot this winter. In fact, at higher elevations, it has rained more than I remember in the previous three winters.

When I sat down to write this column, it was easy to decide what to write about this week.

No one likes rain on snow. It increases the avalanche danger, melts the snow, gets folks wet and cold and can ruin skiing, in general. In fact, the only people I know who don’t mind the rain and who actually enjoy skiing in such conditions are my 3 and 5-year-old kids. Probably because they don’t know any better. (This goes to show you that although low standards can manifest mediocrity, it also has it benefits.)

Depending on the intensity, the duration and the snowpack, rain on snow can trigger different types of avalanches in several ways. It can trigger wet slab and surface avalanches by weakening the snowpack, or it can trigger a dry slab avalanche by adding additional load and softening the slab above weak layers.

How much rain is needed to trigger a wet slab avalanche is still unknown. Several years ago, Erich Peitzsch and Karl Birkeland tried to apply colored water and see how much water is needed to penetrate and to different snowpack depths. Their conclusion was that it depends on so many variables that it is impossible to be reliably estimated.

When liquid water is added into the snowpack, a race begins. If the snowpack manages to let the water flow through and out of the layers, it remains stable. If the liquid water manages to accumulate in a thin layer (or interface) in the snowpack to a point where the snowpack becomes slushy, an avalanche occurrence becomes likely.

As liquid water flows through snowpack, it dissolves the bonds between the snow grains. With these bonds dissolved, the snow grains only remain attached to each other if air bubbles are present between the grains. The grains are held together by the surrounding water’s surface tension. To visualize how surface tension works, think about a cup full of water. It can be filled to where a barely-bulging bubble of water can be seen at the lip of the cup. What keeps this bubble intact and prevents it from spilling is the surface tension. As more water is introduced into layers in the snowpack, the air bubbles and the surface tension — what’s keeping the snow grains together— begin to disappear. The snow grains become suspended in the liquid and the snowpack loses it strength.

In their research, Peitzsch and Birkeland discovered that the most common snowpack locations where liquid water tends to accumulate, dissolve bonds and trigger avalanches, are in the interfaces between layers where small grains exist above a layer with large grains.

Over time, a process we call cannibalism occurs as larger wet snow grains grow even larger, at the expense of the small grains. The snowpack becomes porous, developing flow channels for the water to drain through and becomes less susceptible to additional rain on snow events.

On that weekend day, we decided to stay at home and not ski. We went skiing the next day. It still rained some, but the snow wasn’t as slushy as the day before and skiing with the kids was wet, but also a lot of fun.

Maybe my standards are slowly dropping to match the conditions this winter brings.


• Ron Simenhois is an avalanche forecaster who lives in North Douglas; contact him at


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