Colorado’s dynamic snow: Comparing winter and spring snowpack

When I think of snow, my mind reflects on my first official “powder day” as a brand-new skier in Vail. I remember peering down the run and scanning the blanket of fresh snow that awaited me as I mentally prepared myself for the challenge.

As I skied my way down, I remember thinking how it wouldn’t be long until the very layer of snow I was carving through would melt. I began to wonder about the intricacies of that process — how does the snowpack itself change as we transition from winter to spring? How will this powdery January snow compare to April snow?

Walking in a winter wonderland … atop a weak snowpack

Our snowpack begins accumulating in late fall and usually reaches its peak in April. The snowfall we receive tends to be a lot drier than in states closer to the coast or other large bodies of water. When moist air is forced up and over the Rocky Mountains, it cools rapidly due to decreasing atmospheric pressure. This process, coupled with cold temperatures, produces those light, fluffy snowflakes that we know and love.

From the moment a snow crystal forms, it is being changed by multiple forces, such as weather. In the winter, we often experience periods of snowfall followed by periods of dryness. During these dry periods, there are typically significant temperature differences between the snowpack and the air outside.

Snow maintains a relatively constant temperature of 32 degrees Fahrenheit, whereas the outside temperature can dip well below freezing. This causes water vapor to move upward through the snowpack, changing the snow crystals closest to the ground into angular, jagged shapes called facets.

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This snow is called sugar snow and, as you can imagine, looks and feels much different than its powdery counterpart. Since those facets are so angular, they cannot bond to each other well, and create a loose, weak layer in our snowpack. These weak layers are often the failure point when avalanches occur.

Sweet spring stabilization

Generally, we can expect to find these layers of faceted snow intermingled with layers of powdery snow throughout the winter. Come spring, our snowpack continues to change and stabilize. During the day, temperatures rise and warm the snowpack, driving snowmelt. Temperatures drop during the night, causing the snow to refreeze. This melt-freeze cycle rounds out the snow crystals, allowing them to bond together more securely than the faceted snow we see in the winter, thus allowing for a firmer, more stable layer in our snowpack.

This can also cause the top layer of snow to appear crusty. As the snow melts during the day, it loses its structure and becomes mushy and slushy by day’s end. It will freeze throughout the night, transforming into a nice layer of crunchy, “crust snow” that will await you the next morning. However, give the sun a little time to rewarm the top inch or so of this crusty layer, and you will be rewarded with yet another kind of snow — corn snow. This snow has a granular or corn-like consistency and sits atop layers of that harder-packed snow.

Snow long, farewell …

Snowmelt typically begins in April and lasts throughout two to three months. As heat is absorbed from the sun and the ground, our ever-cherished snow crystals continue to melt into liquid water.

Although snow lovers are sad to see our snow leave, this water is much-needed. About 60-80% of mountain streamflow originates as snowmelt, and about 83% of our usable water relies on surface water supply from streams and rivers. Snow is our largest reservoir for water — from the moment it accumulates in late fall right up until it begins to melt in the spring.

Callie Pehl is a naturalist at Walking Mountains, a self-proclaimed winter enthusiast, and an avid snow enjoyer — in any form that may take (she’s not picky).