The Formation of New Snow

When the environment is right, snow crystals form in the atmosphere. These crystals are created when water vapor condenses as ice on a crystalline nucleus (or dust molecule). Depending on the temperature and humidity in the regions where snow is forming, new snow crystals take a variety of shapes and fall to earth in a variety of sizes.

The classic six-sided “dendrite" or "stellar” crystal is what most people visualize when we talk about a new snow crystal. In reality, new snow comes in a variety of shapes and sizes. We recognize a number of sub-classes that reflect the main types of new snow. Large irregular snow grains including plates, needles, or columns may form weak layers if covered by storm snow.

Each of the sub-classes in turn has numerous variations. More than one sub-class and/or variation can form in a single storm as the temperature and humidity regimes change. It is not uncommon to see several different types of new snow during a single storm, sometimes changing back and forth over relatively short periods of time (a few minutes or hours).

Observers are interested in observing and recording precipitation type, rate, and intensity. Even when the snowpack is stable, avalanches can result when precipitation rates exceed 1 inch per hour and deposit 12 inches or more on the mountain slope. Varying types of snow falling during a storm cycle can result in a buried weak layer, for example, large, well- shaped dendrites forming a thin layer under 12 inches of small, more densely packed precipitation particles.

Riming, Graupel, and Hail

Under some conditions, tiny water droplets form in the atmosphere and remain in a liquid state at temperatures below 0 deg C due to a lack of a freezing nucleus. These water droplets are described as super-cooled. When a super-cooled water droplet comes into contact with any surface or object, it immediately adheres to the surface or object and freezes, forming a small spherical piece of ice. This process is called riming. The tiny ice spheres are referred to as rime.

The most visible form of rime is when super-cooled water is driven against a surface by wind. Under these conditions, rime accretes on the windward side of the surface and creates a kind of icy stalactite formation that grows larger as additional rime is added. These rime formations are often seen on rocks, trees, communication towers, etc., in wind-exposed areas, especially in maritime climates.

If a snow crystal comes into contact with super-cooled water droplets, riming occurs and the rime accretes on the crystal. When this happens, we refer to the new snow crystal as being rimed. When only a few of these ice spheres exist, they are almost invisible to the naked eye; however, they can usually be observed using a simple hand lens magnifier. As the amount of riming increases, rime becomes visible to the naked eye.

Under heavy riming, new snow crystals can accumulate so much rime that their original form becomes complete- ly obscured, eventually forming a roughly spherical (seldom a perfect ball) pellet. Sometimes referred to as “pellet snow,” this is what we call graupel. Graupel particles that ride atmospheric updrafts and accrete multiple layers of rime can eventually form hail.

Riming may occur to individual snow crystals, or it can be deposited directly onto the snowpack if conditions are right. For example, if super-cooled water is present and the wind blows directly onto a slope where rime can be de- posited and accumulate, a rime crust may form on the surface of the snowpack. While not a new snow crystal, rime added to the snowpack in this way becomes a layer that is part of the snowpack. Rime crusts are generally white and opaque, rough, and feel crunchy underfoot or to the touch.

Formation of Layers and Interfaces

Each time snow falls from the sky, the snowpack gains a new layer. In some cases, where the nature of the storm changes, a single storm may produce multiple layers. As snow changes over time, multiple adjacent snowpack layers may become quite similar in their grain size and form. In practice these layers may be considered a single layer. Other times a single buried layer may be exposed to different conditions at various heights within that layer, becoming multiple layers over time. In short, avalanche practitioners consider a snowpack layer to be a band of snow grains with similar characteristics.

The boundary between two adjacent layers is referred to as the interface. This distinction between layers and interfaces will play an important role for the snowpack observer. Since the layers are different from one another, the layers may not bond to each other. It is this layering that is the basis for the formation and release of avalanches.