Snow, Glaciers and Avalanches

SNOW, ICE, AVALANCHES AND GLACIERS

The instant a [snow]flake has sunk to earth, changes in its structure begin to take place. As we gaze at the whitened woods stilled to silence or look through the tiny window of an alpine hut upon the dazzling fields, conveying to us the false message of an inert nature standing still, we are really looking upon a supremely busy labor in which, in sum total, vast energy is at work inducing all kinds of physical changes, so that in a short time nothing is left of the original flake of yesterday’s blizzard save its whiteness.

G. Seligman,

Snow Structure and Ski Fields

(1936)

The presence of frozen water in several forms is fundamental at high altitudes and provides the essential ingredient for the development of avalanches and glaciers. These interrelated phenomena, which contribute much to the distinctiveness of high mountain landscapes, offer a considerable challenge to the inhabitants, both plant and animal, of these regions.

SNOW AND ICE

Snowfall and New Snow

Snow is precipitation in the solid form that originates from freezing of water in the atmosphere. This leads to one of the great mysteries of nature, why should snow fall in the form of delicate and varying lacy crystals rather than as frozen raindrops? The commonly held assumption that water must freeze at 0oC (32oF) is incorrect. The freezing temperature can range as low as –40oC (–40oF) which, coincidentally, is the crossover point of the two temperature scales. Water that remains liquid when cooled below 0oC is referred to as supercooled water. The actual freezing point of water in the atmosphere depends not only on ambient temperature but also on water droplet size, droplet purity, and mechanical agitation. Smaller droplets are more resistant to freezing. Very small droplets may resist freezing to the –40oC value mentioned above. Dissolved salts will retard freezing but certain particulates will enhance freezing (i.e. promote freezing at temperatures closer to 0oC) (Knight 1967).

Clouds form most readily around certain contaminants in the atmosphere. These contaminants can be divided into two classes depending on their ability to promote either condensation or freezing. Condensation nuclei are hygroscopic materials that attract water, such as salt and smoke. Freezing (more properly called deposition) nuclei generally are particles that mimic the hexagonal crystal structure of ice, although dry ice is also an effective freezing nucleator based on its low temperature. Effective freezing nuclei include clays, certain bacteria, and silver iodide. In nature, clouds contain a mixture of water droplets formed around condensation nuclei and small ice crystals formed around freezing nuclei. At typical cloud temperatures of –10oC (14oF), the freezing nuclei are effective in overcoming the “activation energy” and hence allow the surrounding water to freeze. Droplets formed around the condensation nuclei are too small or too salty to freeze directly at this temperature. Most storm clouds, therefore, are a three-phase mixture of water vapor, supercooled liquid droplets, and small ice crystals. The affinity of ice surfaces for attracting water vapor is slightly greater than that of the supercooled liquid surface (stated another way, saturation vapor pressure is lower over ice than over liquid water at the same temperature). Therefore, water vapor molecules have a tendency to deposit more rapidly on small ice seed crystals (hence drying the air); while water vapor tends to evaporate from supercooled droplets (thus moistening the air). The net result is a vapor flow from the supercooled droplets to the ice crystals causing shrinkage of the former and growth of the latter (Figure 1)(Knight, 1967). Thus, it can be seen that snow crystals grow molecule-by-molecule (analogous to bricks placed one-by-one in a complex building project) and helps explain why snowflakes can be so delicate and varied. This mechanism is referred to as the Wegener-Bergeron-Findeisen process named after persons involved in the development of the theory.

Snow and ice crystals grow in some variation of the hexagonal (six-sided) crystal system (Figure 2). This was one of the early scientific observations of snow and was made by the famous astronomer Johannes Kepler. Once formed, ice crystals and snowflakes are subject to continual change. They may grow through deposition and accretion or diminish through sublimation and melting, and they may be fragmented and recombined in numerous ways. The variations on the basic hexagonal pattern display almost infinite variety. We are taught from childhood on that every snowflake is different! In absolute terms this is true but most often snow crystals falling from homogeneous cloud conditions resemble one another closely in basic shape. Snow crystals are generally small and simple when first formed in the cold dry air of high altitudes. As they fall, snow crystals can become larger and more complex when they encounter warmer or more moisture laden atmospheric layers often becoming large enough to earn the name snowflakes. Thus, snow received at the summits of mountains is often quite different from that received on middle slopes of ranges and, in fact, may melt to rain by the time it reaches lower elevations. Most rainfall outside the tropics begins as snowfall at high altitudes.

For forty years around the turn of the twentieth century, a dedicated photographer named Wilson Bentley took thousands of photographs of newly fallen snowflakes while braving the outdoors conditions of New England winters (Figure 3 and 4). Bentley cataloged his snowflake photographs into different types based on similar form characteristics (Bentley and Johnson 1931). During the 1930s-50s, a patient scientist from Japan spent a great deal of time studying the seemingly infinite varieties trying to make some physical sense of snow crystal form. Ukichiro Nakaya (1954) grew snow crystals indoors in a cold chamber where temperature and humidity could be carefully controlled. He grew snow crystals from small “ice seeds” frozen onto a strand of rabbit hair and noted the form results for varying temperatures and amounts of supersaturation. Nakaya’s original results are shown in Figure 5 and are summarized follows:

Temperature oCIce Crystal Habit

0-3Thin hexagonal plates

-3-5Needles

-5-8Hollow prismatic columns

-8-12Hexagonal plates

-12-16Dendritic, fern-like crystals

-16-25Hexagonal plates

-25-50Hollow prisms

We note that the crystal form changes in a consistent manner depending on cloud temperature and degree of supersaturation. It is most typical for one type of crystal to fall from a given cloud rather than having a mix of types all falling at once. The bottom line is if you can identify the basic form of the snow crystal at the ground you can tell what the conditions are in the clouds above. Nakaya referred to this connection between crystal form and cloud conditions as “letters from the sky”.

The principal forms of snow crystals falling from the atmosphere are generally grouped into eight to ten main types. The newer International Commission on Snow and Ice (ICSI) classification scheme shown in Figure 6 has eight types. The older scheme has ten classes including a spatial dendrite and capped column class both of which have been removed from the newer system. These classification schemes are applicable only to falling snow or snow that has been on the ground a short period of time (a few hours to days depending on temperature), which is referred to as new snow.

The Seasonal Snowcover and Old Snow

Upon reaching the ground, snowflakes quickly lose their original shapes as they become packed together and undergo metamorphism (Seligman 1936, Bader et. al. 1939, Alford 1974). Snow, then, displays continual change during formation, falling, and accumulation on the ground, until it eventually melts and returns to the sea. Snow may form in the atmosphere at any latitude, but in order to maintain its identity it must fall to the earth in an area with sufficiently low temperatures to prevent it from melting. Most snow melts within a few days or months from the time it falls (referred to as the seasonal snowcover), but snow can remain year-round depending upon the amount received and climatic conditions (Dickson and Posey 1967; McKay and Thompson 1972). Polar areas receive very little snow, owing to the extremely low temperatures there, but what does fall is preserved with great efficiency. On the other hand, snow may persist even in areas where temperatures are above freezing if sufficient amounts fall there. The snowline in the Himalayas extends much lower on the southern side than on the northern side because the greater precipitation received on the south side more than compensates for the effects of higher temperature. A similar situation exists in the tropics, where snow often reaches lower elevations in tropical mountains during summer (the period of high sun) than in winter. The increased precipitation and cloudiness in summer overrule the effect of the higher sun angle. Heavy snowpacks are found most commonly in middle-latitude and subpolar mountains, regions of relatively high precipitation and low temperatures. Even after the snow has disappeared from the surrounding lowlands in these areas, vast amounts may continue to remain in the higher elevations.

The build-up of a snowcover (also called old snow) is in many ways analogous to the formation of a sedimentary rock from geology. Snow accumulates as a sediment, with each layer reflecting the nature of its origin. Newly fallen snow has very low density, somewhat like fluffed goose down, with large amounts of air between the crystals. But with more accumulation, snow becomes compressed and settling takes place. Also a related series of changes take place over time at the crystal level referred to as snow metamorphism (just as in geology where the metamorphic rock class represents a changed form coming from other pre-existing rock types by increased heat and pressure). The exact behavior and characteristic of old snow depends upon its temperature structure, moisture content, internal pressures, and age of each layer in the snowpack. (Bader and Kuroiwa 1962; de Quervain 1963; Sommerfeld and LaChappelle 1970, LaChapelle and Armstrong 1977, Colbeck et. al. 1990). Snowpack metamorphism can take place by three fundamental processes, two that are largely two-phase, vapor driven processes (i.e. without significant melting) and one that is a three-phase, liquid driven process (i.e. melting is now significant and liquid water is in the pore space to some degree).

Equilibrium Metamorphism

The first process discussed here is equilibrium metamorphism (referred to in older literature as equi-temperature, ET, or destructive metamorphism)(Figure 7). This process occurs when the snowpack is subfreezing (i.e. is not melting) and free of large vapor pressure and temperature variations. When these conditions are met, grain geometry (crystal shape) and pressure contact between adjacent grains controls the metamorphism. Points of grains are locations of higher vapor pressure while grain declivities are locations of lower vapor pressure. A vapor flow is set up that transfers mass, molecule-by-molecule, from the tips of the grains to the branch junctions leading, in time, to a spherical form often referred to by workers in snow as rounded grains or rounds. Where grains are in contact in these conditions, sintering (i.e. bonding) can take place forming continuous ice “necks” connecting adjacent grains and hence a producing a mechanically strong snowpack (Colbeck 1983).

Kinetic Metamorphism

The second process discussed here is the kinetic metamorphic process (referred to in older literature as temperature gradient, TG, or constructive metamorphism)(Figure 8). In this process the snowpack is also subfreezing (i.e. is not melting) but, unlike equilibrium metamorphism, this process is dominated by large vapor pressure and temperature variations across sections of the snowpack, usually in a vertical direction (e.g. a shallow snowpack with a warm ground interface and a cold air interface displaying a temperature difference greater than approximately 10oC per m depending on the layer temperature, snow density, and other factors). When these conditions are met, large amounts of water vapor flowing through the pores between the individual grains controls the metamorphism. Grain bodies serve as areas of vapor deposition (i.e. the change of state from a gas directly to a solid) while the grain contacts receive little deposition. As a result, grains can become very large with angular and stepped edges growing into the direction of the vapor flow. These growth forms are often referred to as angles or facets and can become completely three-dimensional cup crystals if sufficient space is available. It is interesting to note that these kinetic crystals are relatively strong in compressive strength (top to bottom loading), but are very weak in shear strength (sideways loading). The rate of grain growth overpowers the sintering (bonding) effect, resulting in larger grains with fewer bonds per unit volume and a correspondingly weaker layer (Colbeck 1983). Several subtypes of this process occur depending on the location and source of the vapor and temperature gradients (i.e. rates of temperature change). Steep temperature gradients near the ground (a common condition in cold mountains with low snowfall) can lead to weak zones lower in the snowpack called depth hoar (McClung and Schaerer 1993 p. 49) while temperature gradients at or near the surface can lead to surface hoar formation (Figure 9)(McClung and Schaerer 1993, p. 44), and at least three types of near-surface faceting (Birkeland, 1998), including radiation re-crystallization (Armstrong and Ives, 1977). In all cases this type of metamorphism leads to weak layers of varying thickness and location within the snowpack, a key ingredient to many avalanches.

Melt-Freeze Metamorphism

The final type of metamorphic process discussed here is melt-freeze metamorphism (also referred to as MF metamorphism)(Figure 10). This process occurs where the melting point has been reached. This could be just a surface layer during a sunny period or could include the entire snowpack when the isothermal condition (melting throughout) is reached in the spring. This process is more complicated than the first two as it involves all three phases of water occurring at once! Here, liquid water fills the intergranular pore space to some degree. During the melt phase, large grains grow at the expense of smaller grains due to small but significant shape-related temperature differences (Colbeck, 1983). The result is that large poly-granular units form over time often referred to as corn snow. In the warm part of the day the snow may be mechanically weak due to the melting of intergranular bonds while in the cold part of the evening the snow may be very strong due to re-freezing of the liquid water especially near the surface where radiant energy exchange is pronounced. The process of repeated freezing and thawing causes increased densification and consolidation and is responsible for the formation of firn or neve, which is dense snow at least one year old. The snow may now be as much as fifteen times more dense than when it first fell, and it is well on its way toward becoming glacial ice (de Quervain 1963, p. 378).

The International Classification for Seasonal Snow on the Ground

A comprehensive snow classification system exists for all types of seasonal snow (including new snow described previously) that is known as the International Classification for Seasonal Snow on the Ground (ICSSG) (Colbeck et. al. 1990). The ICSSG is fairly involved but at the most coarse level it consists of nine fundamental snow and ice types based mainly on grain shape:

1Precipitation particles (identical to the eight ICSI classes)

2Decomposing and fragmented precipitation particles (blown new snow)

3Rounded grains (equilibrium metamorphisms)

4Faceted crystals (kinetic metamorphisim)

5Cup shaped and depth hoar crystals (advanced kinetic metamorphism)

6Wet grains (melt-freeze metamorphisms)

7Feathery crystals (surface and cavity hoar)

8Ice masses (horizontal ice layers and vertical columns from piping)

9Surface deposits and crusts (wind and rain stiffened layers)

This system is the standard that is used by most workers in snow related endeavors around the world.

The Mountain Snowpack as a Water Resource

The implications of mountain snow for human existence are discussed later in the book (see pp. 348-53 FIX), but the importance of meltwater cannot be stressed enough. Numerous estimates indicate that 66 to 75% of all water resources used in the western United States originate as snowfall. The Pacific Northwest of the United States is largely dependent upon hydroelectric power from streams that head in the Cascade and Rocky Mountains, and California's bountiful farm production is derived largely from meltwater from the Sierra Nevada. In fact, it is safe to say the economy of the entire western United States is dependent upon meltwater from mountains. The mountain snowpack is becoming increasingly valuable as a source of water worldwide. It has become fashionable to apply the term watertowers of the world to mountain watershed areas.