How and why does the snowpack change over time?

Snow is a highly unstable natural material, always being relatively close to its melting point. Snow crystals that form in the atmosphere land on the ground or the snowpack and begin to change their form rapidly. This process of change is called metamorphism. When patterns of metamorphism take effect and continue over time, snow grains within layers exhibit characteristic physical traits. The form and size of snow grains are evidence of their history from their origins in clouds to subsequent metamorphic processes. Professional observers should understand the basics of the processes of faceting, rounding, and melt-freeze metamorphism, the conditions that promote these processes, and the structural changes that they cause within the snowpack. This understanding helps put field observations into a context which leads to improved craftsmanship, relevancy, and interpretation.

The Phases of Water

Water can exist in 3 different phases of matter: solid (ice), liquid (water, liquid water), and gas (water vapor). The transition from ice to liquid water is called melting, and the reverse is called freezing. The transition from liquid water to water vapor is called evaporation, and the reverse is condensation. The transition from ice to water vapor (without going through a liquid water phase) is called sublimation, and the reverse is called deposition.

Vapor pressure refers to the tendency for a substance to change state from liquid (or solid) to vapor. Ice has relatively high vapor pressure, which makes it volatile and relatively unstable. Water molecules actively transition between ice, liquid, and/or vapor and as they do, snow grains change their size, shape, and bonding strength to one another. Bonding changes affect the strength of snow layers. Where stronger layers overlie weaker layers across a slope, we may see instability and potential for slab avalanche.

Phase Change Graphic. AIARE Files.

Dry Metamorphism

In dry metamorphism (rounding and faceting), there is no liquid water present. Snow changes through sublimation and deposition at subfreezing temperatures. The primary driver of these processes is snow temperature change over height, or temperature gradient, in the snowpack. This is because temperature is directly correlated to vapor pressure, and vapor pressure affects the rate and pattern of sublimation and deposition of water molecules. As snow observers, we can’t measure vapor pressure directly, so we instead measure snow temperatures as a proxy. When the temperature difference is less than approximately 1°C per 10 cm of height the temperature gradient can be described as “low,” and when the difference is more than approximately 1°C per 10 cm of height then it is “high.” This distinction is important for anticipating dry metamorphic processes within the snowpack, but there are other factors as well that affect how snow changes, such as: the load of snow (slab thickness), density of snow layers, pore space size and shape, grain size and shape, presence of crusts, and the interface characteristics between grains.

Rounding

When dry snow layers are subjected to a low temperature gradient (< 1°C / 10 cm) sublimation of water molecules occurs from grain scale areas of higher vapor pressure, through pore spaces, and are deposited onto nearby areas of lower vapor pressure. Convexities in the ice have higher vapor pressure than concavities (due to an increased surface area to volume ratio). The net result over time is that pointy, convex areas become eroded, and concave hollows fill in. This creates a rounding effect on snow grains. Once we can see the evidence of this as the predominant grain form, we classify them as: rounded grains, or rounds for short.

As grains become rounded they contact each other, and these contact points also create concavities. As ice continues to sublimate from convexities and deposits in concavities, it forms ice between the grains, forming visible ice to ice bonds through a related process called sintering. Sintering creates stronger snow layers. Settlement is the vertical deformation of the snowpack and in dry snow is associated with rounding and sintering.

Is rounding good or bad for snow stability? Well, that depends. Rounding can strengthen the snowpack and weak layers decreasing instability. It can also form slabs over weak layers and increase instability. Observers and practitioners should note what grain types they observe within the snowpack and use that along with broader evidence to form opinions about stability.

Advanced rounding is referred to as sintering. Sintering is a process in which grains form bonds between each other to create cohesion amongst a layer. Sintering is generally associated with settlement within the snowpack and happens over longer periods of time.

Faceting

When dry snow layers are subjected to a high temperature gradient (> 1°C / 10 cm), sublimation and deposition occur, however, in a different pattern from the process of rounding. With a high temperature gradient, there is a larger difference in temperature over height. Physical laws of thermodynamics dictate that air moves from high to low pressure, and for our discussion, from warmer to colder. In the snowpack, the ground is typically warmer, near 0°C and the snow acts as an insulating blanket for this warmth. Alpine animals and plants have adapted to live near the ground, under the snow, for this reason. The heat flux through the snowpack is typically from the ground up in the winter months.

The higher the temperature gradient, the stronger this heat flux is, and the faster the air moves upward through the pore spaces. This increases the rate of sublimation and deposition and the crystal growth rate. Tiny sublimated water molecules in pore spaces within a high temperature gradient, are like helium balloons in a gale force wind. The wind supersedes their normal tendency to move as they would in calmer conditions. Sublimated water vapor molecules are essentially caught in the upward moving “wind” and the effects of the curvatures on deposition patterns is overridden. Vapor deposits onto the convexities and edges of ice above in the snowpack. The effect of this process is that ice mass migrates upwards and forms snow grains that are distinctly angular, with clean edges and faces (like the cut facets of a gemstone) and become poorly bonded to one another (like sugar). These grains are classified as faceted crystals, or facets for short.

Facets result from faster crystal growth rates, and develop at a faster rate than rounds. They can grow to be relatively large. When they grow large enough (generally >~3mm) and show evidence of cup shapes or growth layers (striations) they are classified as depth hoar, a type of advanced facet with its own separate distinction.

Due to the characteristic larger grain/pore size and angular shape of facets and depth hoar, these grain types generally do not round and strengthen quickly. They persist in the snowpack, and they create persistent weak layers that observers should take care to note and consider for avalanche operations. That said, is faceting, good or bad for stability? Well, again, it depends. Faceting can weaken slabs and decrease instability, but it can also form persistent weak layers that lie underneath slabs and increase instability.

Detph Hoar. Adapted from The International Classification for Seasonal Snow on the Ground. IHP-VII Technical Documents in Hydrology N°83, IACS Contribution N°1, UNESCO-IHP, Paris. By: Fierz, C., Armstrong, R.L., Durand, Y., Etchevers, P., Greene, E., McClung, D.M.,Nishimura, K., Satyawali, P.K. and Sokratov, S.A. 2009.

Melt-Freeze Metamorphism

When there is liquid water present in the snowpack, melting and freezing phase changes dominate over sublimation and deposition. Diurnal fluctuations in temperatures and solar radiation cause cycles and variations in intensity of melting and freezing within the mountain snowpack over time and across terrain. It is important for observers and practitioners to understand the fundamental changes that occur through these cycles. Snow very near its melting point (0°C) is inherently weak, and after it gets wetted and is refrozen it gains considerable strength.

When solar radiation and warmth add enough energy to the snow surface to begin to melt the snow, it is the smaller particles (with higher vapor pressure) that generally are first to melt. This water enters the pore spaces in the snow and the snow grains become rounder in shape (not to be confused with rounds, which are created through a dry metamorphic process). Liquid water in pore spaces lubricates and weakens snow layers. When the snow surface eventually cools, the liquid water refreezes which forms bonds of ice between the grains, increasing the size of the grains, and making the snow layer stronger. Rain is another condition that can add liquid water to the snowpack and has a similar effect, though it can occur anytime, day or night. When liquid moisture enters pore spaces between the ice grains and the ice grains become rounded, it creates melt forms. Melt forms can be found in their wet or refrozen state.

Clustered rounded grains. Adapted from The International Classification for Seasonal Snow on the Ground. IHP-VII Technical Documents in Hydrology N°83, IACS Contribution N°1, UNESCO-IHP, Paris. By: Fierz, C., Armstrong, R.L., Durand, Y., Etchevers, P., Greene, E., McClung, D.M.,Nishimura, K., Satyawali, P.K. and Sokratov, S.A. 2009.

“Corn snow” is a common term used by backcountry skiers and snowboarders to describe the large grained look and smooth feel of surface melt forms that have been through multiple melt-freeze cycles, found in their wetted state, with enough of a supporting layer of still frozen melt forms that ski penetration is shallow and riding conditions are optimal.

When pore spaces in the snow get fully saturated and melting continues, water will penetrate deeper into the snowpack. Water can percolate down through channels, and it can pool above a deeper, more impermeable layer such as a buried crust or the ground. When snow is water saturated with high liquid water content, it is considered a melt form. Once a saturated layer refreezes into a translucent layer of water ice, with pores that do not connect and no more recognizable individual grains, it becomes an ice formation.

Ice Layer. Adapted from The International Classification for Seasonal Snow on the Ground. IHP-VII Technical Documents in Hydrology N°83, IACS Contribution N°1, UNESCO-IHP, Paris. By: Fierz, C., Armstrong, R.L., Durand, Y., Etchevers, P., Greene, E., McClung, D.M.,Nishimura, K., Satyawali, P.K. and Sokratov, S.A. 2009.