The White Death

There is no sight in nature quite so peaceful as the white blanket of newly fallen snow which covers fields, woods and mountains alike. All living sounds are hushed and even human footfalls become muffled. Yet beneath this canopy of almost death-like calm, in reality there is no rest.

- Gerald Seligman, Snow
Structure and Ski Fields

From the earliest reaches of history, avalanches have had a way of humbling even the most heroic human endeavors. In 218 B.C., the Carthaginian general Hannibal left southern Spain with some 90,000 soldiers, 12,000 horsemen, and three dozen elephants, crossed France, and climbed into the Alps on his way to attack Rome during the Second Punic War. In the mountains, the elephants were placed at the front of the column; the belligerent Celts and Gauls who lived in the mountains reportedly "beheld these beasts with superstitious awe." After defeating its foes in a number of terrible skirmishes, Hannibal's army reached a pass-perhaps the Little St. Bernard Pass, perhaps the Col de la Traversette, perhaps the Col du Clapier- on October 26; to cheer his depleted, exhausted troops, Hannibal exclaimed that they had "climbed the ramparts of Italy, nay, of Rome. What lies still for us to accomplish is not difficult." The trouble was that the Alps on the way down toward Italy proved far steeper than on the way up from France. Worse, November storms had covered the glaciers with snow, concealing deadly crevasses and loading the steeper slopes with heavy blankets of snow. Although they encountered no enemies during their descent, thousands of soldiers and horses were lost to avalanches; at one stretch, an avalanche halted their progress for four days, and only the use of explosives was able to open the pass for the men and their elephants.

By the time the army reached the plains on the eastern slope of the mountains, some 18,000 men, 2,000 horses, and several elephants were lost, as many as half of them to cold and avalanches. An epic poem about the journey by Silius Italicus (A.D. 25-101) reports that "There where the path is intercepted by glistening slope, [Hannibal] pierces the resistant ice with his lance. Detached snow drags the men into the abyss and snow falling rapidly from the high summits engulfs the living squadrons."

In 1499, Kaiser Maximilian ordered 10,000 soldiers to invade the Engadine Valley; as the army crossed a high mountain pass, 400 men were carried away by an avalanche. "The panic created by this occurrence soon changed to laughter," a Swiss account reported. However, when the men buried under the snow emerged from it one by one as if they rose from the grave, and although many of them were injured, none were lost." The survival of so many men caught in a slide is, historically speaking, miraculous; it is rare, once caught, for even single victims of large slides to survive. The same year, 100 mercenaries were killed as they crossed Great St. Bernard Pass.

As people settled in Europe's mountain regions, death tolls began to rise proportionally. In 1569 an avalanche smashed through the ice of a lake near Davos, Switzerland, with such force that it killed a large number of fish by concussion and threw them out onto the land. In 1808 in Trun, Switzerland, it snowed more than 15 feet in three days. The sun finally emerged, precipitating a large avalanche. It came down in a westward direction, destroying a number of houses, then careened up the opposite side of the valley and destroyed a large forest. It then recoiled down, destroying more forest. Like a scythe slashing back and forth through the valley, the avalanche swooped again to the west, and returned again to the east, flattening a dozen cowsheds. Then it was back to the west again, this time burying a barn full of cattle. One more time it recoiled to the east, flowing over some hills with enough force to mass again; heading west, it buried houses to their rooftops.

Wide-eyed historical accounts of avalanches abound. An 1843 book entitled Travels in the Alps of the Savoy called avalanches the "greatest and most resistless of catastrophes which can overtake the Alpine pedestrian." A document known as the "Montafon Letter" reported that, after an avalanche buried 300 people in 1689 in the Montafon Valley, a priest carrying the sacrament to the dying was buried by one avalanche and then unburied by another. The lucky priest was hardly the first man of the cloth to find himself endangered by snow. In the tenth century, on the eve of his marriage, the wealthy Bernard of Menthon renounced all worldly temptations and devoted his life to saving others. After joining the Augustine order, and determined to help pilgrims crossing the mountains, he founded a monastery in A.D. 962 at an altitude of 8,000 feet on what was then known as Mount Jove. Every morning, monks would set out from the hospice in both directions, looking for pilgrims to guide back to the monastery. Between 1436 and 1885 (the year telephones allowed pilgrims to notify the hospice in advance), "not a single winter's morning passed, regardless of how vicious the weather, on which men did not set out from the St.. Bernard's Hospice to guide travelers, or to rescue those in trouble," Colin Fraser writes. The monks were kept busy; by the eighteenth century some 15,000 people per year crossed St. Gotthard Pass, and the St. Bernard Hospice found itself serving some 400 meals a day. The monks, of course, were also at risk; between 1810 and 1845 alone, a dozen were killed in high mountain avalanches.

Given the loveliness of the snow crystals that create them, it can be said that even monstrous avalanches are born serene. Snow begins simply enough, forming somewhere between the troposphere and the stratosphere, between 30,000 and 40,000 feet above the earth, where atmospheric temperatures dip down to 75 degrees below zero. Floating above the clouds are thousands of species of bacteria, fungi, and protozoa, along with the pollen of some 10,000 species of flowering plants and untold quantities of atmospheric dust-each a possible nucleus around which water molecules may freeze. These nuclei are exceedingly small: it takes as many as ten million of them to form a single raindrop, and perhaps a million frozen ice crystals to make one snowflake. The flakes they form are legion: a million billion may fall on a single acre of land during a ten-inch accumulation.

As these clusters float downward, they grow from invisible particles to fragile collections of crystals, depending on the amount of water vapor and the temperature of the air. For reasons peculiar to the bonds of hydrogen and oxygen-as true for ice cubes as it is for snow-snow crystals initially take the form of flat hexagons, but then gradually change as if an invisible hand were turning a kaleidoscope. Winds can sweep them thousands of feet up or down, and with each change in temperature the crystals change form. As they evolve, the six points of the hexagonal plates gradually grow or lose their spindly arms, or dendrites; high winds can break these arms, sending smaller fragments forth to begin evolving in their own right.

Like children, individual snowflakes are, during their growth period, as varied as the forces that touch them. The shape of a new snow crystal depends in part on the temperature of the air around it. Although scientists have over the years developed a number of different systems for categorizing basic snow crystals, the International Commission on Snow and Ice has come up with 10: plates, stellars, needles, columns, capped columns, spatial dendrites, graupel, sleet, hail, and a catch-all category called "irregulars." As even the music of their names suggests, there is a great aesthetic difference in the way these crystals look. Capped columns are structurally simple, at least compared to ornate stars.

They look like empty bobbins of thread, but their peculiar shape extends their reach: while still floating in the atmosphere, they can act as countless millions of prisms, refracting light in such a way that halos are perceived surrounding the sun or moon on a winter day. Graupel, the homeliest crystal, is formed when water droplets float through regions of fog or cloud and become bonded to hardened pellets; by the time it reaches the ground, graupel looks like frozen blobs of brain tissue. Stellars, with their ornate lattice of six delicate arms, are of such variety that their number borders on the infinite, and there is some poetry in this. It would be a shame if there were three kinds of stars and thousands of varieties of miniature floating brains.

Flakes descend at an average rate of a foot per second, although needles, more aerodynamic, fall faster than stars. In most cases, flakes that fall the greatest distances attain the most beautiful and ornate patterns; flakes that land atop high mountains, deprived of their chance to evolve, are generally simpler than the ornate structures that fall all the way to sea level. With so much evolution, it is often said that no two snowflakes are alike. Whether or not this is true, a Russian meteorologist claimed in 19 10 to have observed 246 different kinds of snow in a single year near St. Petersburg. Wilson Alwyn Bentley, a Vermont farmer, published a photographic portfolio with 2,453 microphotographs of individual flakes taken between 1884 and 193 1. He also knew that he had barely begun his list. Indeed, since a single cubic foot of snow may contain as many as ten million individual flakes, and since over time enough snow has fallen on the earth to cover it to a depth of 50 miles, the claim of eternal snowflake idiosyncracy seems mathematically unfathomable.

It should also be remembered that snow isn't white. Snowstorms are made up of billions of tiny, clear prisms, each of which breaks up all light that strikes it into the entire spectrum; snowflakes are, like the water that forms them, actually clear. Refracting all the light that passes through them, snowflakes flood the visual field not with one color but with all colors. The confused eye, unable to handle such a burst of sensory overload, turns the flood of colors back into whiteness, often with serious physical side effects. Blinded by so much whiteness-a so-called whiteout-a skier or hiker can quickly become completely disoriented and nauseated, much as he would feel on an utterly gray day spent rolling on a tumultuous sea. Losing any sense of a horizon, those caught in a whiteout can become overwhelmed by vertigo, a state in which the connection between the eye and the inner ear becomes so confused that one's very sense of footing becomes unreliable. In a whiteout, a skier may be so caught in the thrall of a storm that he is unable to see the gloved hand in front of his face, let alone the slope passing beneath his feet. Assuming he is standing still on a flat surface, he may discover that the slope is in fact nearing vertical, and that he is in fact flying down a hill at 30 miles per hour; tragically, he may discover this only after he has crashed into a tree, or gone sailing off a Cliff.

As ephemeral as snow is in the air, it becomes even more mysterious once on the ground, and has long been a source of rhetorical rumination for poets and scientists alike. A swirl of ethereal, airborne snow flowing over a high mountain ridge mirrors in both its aspect and physics a plume of sand blowing over a dune. Snow "falls in soft crystals of infinite variety yet lies in a heavy sameness on the land;' writes Bernard Mergen, in his book Snow in America. "It obscures the familiar yet reveals new shapes, it comes in the season of darkness yet makes both day and night more brilliant with reflected light, it arrives with the killing frost and the disappearance of many plants and animals yet preserves seedlings and tiny creatures under its warming blanket, it is pure and beautiful yet volatile and transitory, it confines the body yet releases the imagination." Twentieth-century snow science has confirmed what superstitions used to suspect: snow, particularly as it masses and moves, turns into one of the strangest, most intriguing, and most potentially destructive substances on earth. "Upon one characteristic of snow, probably all observers can agree without reservation: it is the most recalcitrant substance on earth, particularly when one tries to study it," the pioneering American avalanche researcher Monty Atwater writes. "Snow seems averse to being studied. When it is poked or disturbed or manhandled in any way, it changes quicker than a chameleon, from one kind of snow to another, leaving the observer baffled."

A very dry snowpack may in fact comprise 95 percent air and only 5 percent water; very wet snow might hold as much as 50 percent water. An easy way to visualize this is to think of the amount of melted snow required to make an inch of water: 20 inches of snow at 5 percent water density, or just two inches of snow at 50 percent density. The weight of any pile of snow is thus the product of its depth multiplied by its water density. Since the average ten-inch snowpack can weigh as much as one hundred tons per acre, it is easy to see why shoveling snow has become such a regular cause of suburban heart attacks. If the average water density of snow is about 10 percent, in places like California's Sierra Nevada snow can contain 40 percent water and weigh two hundred pounds per square foot; since it snows up to four hundred inches a year there, it is also easy to understand what Californians mean by "Sierra Cement."

External factors, such as a strong wind, can also have an effect on the density and weight of a snowpack. A strong wind can jam crystals together and so fill empty spaces with ambient moisture that the pack reaches a density five to ten times that of the original snowfall. Once a windblown layer is buried under new snow, of course, things get more complicated. Cohesion is suddenly a matter not just between crystals, but between layers of snow as well. As the snowpack gets deeper, each layer is compressed by those above it, gradually becoming thinner and thus more dense. Denser layers below can be stronger than fluffier layers above, and become increasingly so as the weight above increases. Snow is also "visco-plastic"; it can flow like a liquid, and stretch or compress without losing its structure, like some solids. A snowpack on a slope can stretch or "creep" downhill for some time before it finally breaks along a fracture line. Analogies for this are hard to come by, since snow has such unique physical properties. It is as if bread dough, spread over a tilted cutting board and adhering to the wood, began to stretch downward until the gluten bonds snapped and sent the released half crashing to the floor. The variations in cohesive strength between one particle of snow and another, between one layer and another, or between snow and the ground are, according to Ed LaChapelle, another early American avalanche researcher, "among the widest found in nature."

This is particularly true given the wide variety in the shapes and sizes of individual snow crystals. Any given layer of snow can be 85 to 95 percent air, with the interlocking arms of individual crystals forming a kind of web around it, but the hardness of a wind-packed layer of old snow may be 50,000 times that of light, fluffy snow. Cohesion between layers depends primarily upon two factors: "interlocking" between the arms or necks of the different snow crystals and "sintering," or cementing, between layers. Snow crystals can last an instant and melt. At other times, they can pack so tightly that they almost achieve the consistency of stone; snow compressed over hundreds of years into glacier ice is hard enough to carve valleys through mountains and turn boulders into powder. Snow cover varies with virtually everything that influences its fall, from the temperature, moisture, and wind speed of the air to the geography, latitude, and altitude of the ground on which it falls. And since snow falls on roughly half of the North American continent, its characteristics and personality are as varied as the people it falls upon.

The places where snow falls most heavily are the world's mountain ranges, which make up about 20 percent of the earth's continental land mass, and which, by holding weather systems in place, become some of the wettest places on earth. Snow will also vary depending on what layer it occupies; snow deposited on the ground in December will have characteristics decidedly different from snow layered above it in February. Individual layers can be thick or thin, frozen solid or soft and damp. As a rule, thick layers of snow represent consistent snowfall within a single storm; thinner layers can be formed by wind or snowmelt between storms, or by "surface hoar," the wintertime equivalent of dew. Usually formed by nighttime moisture precipitating onto a cold snowpack, surface hoar is beautifully constructed of feathery crystals, makes for great ski conditions, and is one of winter's most delicate features. Under a microscope, surface hoar crystals are often revealed to take the shape of hexagons, each with sharp angles formed by two opposite sides longer than the other four. This geometry means that surface hoar is also a significant factor in avalanche formation, since hexagons, their angular sides scraping up against each other rather than locking together, have trouble bonding, especially on the horizontal plane. If a layer of surface hoar becomes covered by subsequent layers of snow, it may be able to withstand compression from above but will slide instantly once set in motion. One beautiful, utterly common layer of feathers, laid down in a single night, can mean a month of danger.