## Precipitation

Since most avalanches are associated with recent or newly fallen precipitation, you should collect the following information for any recent and/or ongoing precipitation events:

• Amount of new snowfall
• Rate of accumulation
• Snow water equivalent
• New snow density

New snow amount: New snowfall is usually recorded to the nearest inch (cm) at 24-hour intervals or sometimes more often during storms. Measurements are taken manually from a storm board (snow board) at the site or automatically, using a remote sensing device, such as an acoustic snow height sensing instrument.

Snow accumulation rate: We measure the water content or SWE of new snow to help us determine its density and learn about the weight of new snow being added to the pack. Record SWE to the nearest one-hundredth of an inch (or tenth of a millimeter). Measure it using a melting precipitation gauge or by bringing a snow sample from a standard precipitation gauge indoors, melting it, and measuring the height of the water. It can also be measured by weighing a core sample of new snow using a calibrated scale designed for weighing snow collected with a snow tube. For more information, see COMET’s Snowmelt Processes module at http://www.meted.ucar.edu/hydro/basic/Snowmelt/.

New snow density: Snow density is another way of expressing how much weight is being added to the snowpack. For a given snow depth, high-density snow adds more weight (water) than low-density snow. Snow density is usually expressed as a dimensionless ratio of snow water content to snow depth (such as 1:10) or as a percentage of snow water content to snow depth (such as 10%).

To calculate density, divide the amount of water contained in the snow by the depth of the new snow. The higher the value, the more water is in the snow.

Here are a few exercises.

What’s the density of 20 inches (50 cm) of snow that contains 2 inches (5 cm) of water? (Choose the best answer.)

If you divide the snow water content (2 in or 5 cm) by the snow depth (20 in or 50 cm), you get 1:10 or 10%.

Which snowpack weighs more? (Choose the best answer.)

The first pack has a density of 10% (2 inches / 20 = 1:10 or 10%), whereas the second has a density of 5% (1 / 20 = 1:20 or 5%). The snow depths are the same but the denser snow contains twice as much weight as the less dense snow.

For more exercises like these, see the COMET module Snowpack and Its Assessment at http://www.meted.ucar.edu/afwa/snowpack/.

Storm trend: During the course of a storm, temperature fluctuations will produce snowfalls with varying new snow densities.

• Storms that begin with warmer temperatures and higher-density snowfall and end with colder temperatures and lower-density snowfall produce a more stable layer. These are known as right-side-up storms because lighter snow overlays heavier snow.
• Storms that begin with colder temperatures and lower-density snow followed by warmer temperatures and higher-density snow produce a more unstable layer. These are referred to as upside-down storms since heavier snow overlays lighter snow.

When tracking incoming storms, be sure to consider the nature of the underlying or old snow surface. If new snow falls on surface hoar or near-surface facets, an unstable situation may develop regardless of the new snow’s density.

Rainfall: Rainfall adds weight to a pack. Especially in maritime climates, you should closely monitor the snow/rain level during storms since rain on snow almost always causes avalanches.

Summary table:

You’ve finished the background information on precipitation. Scroll up and access the Operational Information tab.

Ideally, precipitation data should come from sites near the elevations of historical avalanche starting zones but below ridge-top level, in locations somewhat protected from the wind.

New snow amount: Gather all new snow depth measurements for the past few days. Determine if it’s snowing anywhere and if the reported snowfall exceeds the critical threshold.

Snow accumulation rate: Depending on the frequency of your new snow depth reports, determine snow accumulation rates for 6-, 12-, and 24-hr periods. Use the table to interpret the impact of your rates on avalanche potential.

Snow water equivalent: Collect SWE data and see if the amount of water in the new snow exceeds the threshold.

New snow density: Determine the density of newly fallen snow from the SWE and snow depth using this formula: New snow density = snow water content / new snow depth.

Storm trend: If it’s snowed, when you’re ready to start looking at current temperatures, determine if they warmed or cooled during the snowfall event. Rising temperatures during a snowfall destabilize the storm snow layer (an upside-down situation) whereas falling temperatures stabilize it (a right-side up situation).

Rainfall: Determine if the area has received any rain because it will have added significant weight to the pack. Rain has a particularly destabilizing impact.