Science of Snow
By Lisa Jancarik
What is Snow?
Snow is an accumulation of packed ice crystals. A snow crystal is a single ice crystal with precise hexagonal symmetry, and nearly every snow crystal has a dust mote at its center. The term “snowflake” refers to one or more snow crystals. Snow crystals can form fluffy snowflakes when they collide and stick together, but the terms are not technically interchangeable. Observe, too, that snow crystals are not frozen raindrops, which are called “sleet.” A snow crystal forms directly from frozen water vapor without passing through the liquid state.
Strictly speaking, snow can be classified as a mineral. A mineral is a naturally occurring, homogeneous chemical substance with a specific composition and ordered atomic arrangement.
Snow Crystal Symmetry
The hexagonal symmetry of snow crystals arises from the ice crystal lattice in which water molecules arrange themselves. The six arms of a snow crystal grow independently. However, they all pass through the same conditions governing their formation between cloud and Earth’s surface, resulting in a certain symmetry. The symmetry isn’t perfect, though: irregular shapes are more common because the arms don’t grow at exactly the same rate. At the beginning of their formation, growing snow crystals actually can be identical. Despite any possible initial similarity, however, no two snow crystals follow exactly the same path to the ground, so no two are exactly alike upon arrival.
In the 1930s, Japanese physicist Ukichiro Nakaya studied the ways snow crystal morphology varies with temperature and humidity. He discovered that the stellar crystals we all think of grow at colder temperatures than the plates, prisms and columns he also described. Branching occurs with relatively high humidity.
Characteristics of Snow
The high percentage of air in snow makes the often repeated snow-to-water ratio of 10:1 not entirely true. In fact, melting 25cm of fresh snow might yield as little as 0.25cm of water or as much as 10cm of water, depending on the weather conditions associated with the snowfall.
The traits associated with snow can vary significantly with conditions. For example, fresh, fluffy snow effectively absorbs sound. If the surface melts and refreezes to become smooth and hard, it will reflect sounds, which will then seem clearer.
Snow as Insulator
Ironically, snow is an excellent insulator, so burrowing into a snow cave can be a good strategy for a hibernating animal to keep warm relative to the outside air. This effect is created by the high percentage of air trapped in freshly fallen, uncompacted snow. Because this trapped air can barely move, hibernating animals benefit from minimal heat transfer.
Igloos similarly take advantage of the insulating properties of compacted snow blocks, which are used in their construction ― distinguished from blocks of ice, which insulate poorly because ice traps much less air. Once the snow blocks have been placed, the bottom inside the shelter is dug out. Occupants of an igloo can have a fire and maintain an average inside temperature of 20° Fahrenheit (-6° Celsius), making an igloo much more comfortable than surrounding air, which can be 70° Fahrenheit (21° Celsius) cooler.
How Snow Forms Naturally
When atmospheric temperatures reach or dip below freezing, snow can form if there is a minimum amount of moisture in the air. Once snow crystals form in the atmosphere, they absorb nearby water droplets to form the snowflakes we see. Snowflakes typically reach less than 1.3cm diameter, but they can actually grow much bigger, depending on conditions.
Generally, snow will not reach the ground unless the temperature there is less than 5° Celsius (41° Fahrenheit). While it can never be too cold to snow, it is true that heavy snowfalls more typically occur when the air on the ground is relatively warm.
How Snow Forms at Ski Resorts
Manmade snow extends the ski season at most resorts, and many skiers claim they can tell the difference between natural and manmade snow. Still, most of them remain content enough with manmade snow to justify resorts' expenditures on snowmaking equipment, which can run to thousands of dollars per machine to purchase and operate.
The ability of these machines to make snow depends on the “wet bulb temperature,” the temperature adjusted for humidity. Low temperatures and low humidity are ideal snowmaking conditions. That said, if the humidity is sufficiently low, then an above-freezing temperature may still result in successful snowmaking.
Resorts typically have two kinds of snowmaking machines. The less expensive models feature air guns that break up the stream of water pumped through them. This kind of machine requires an air compressor or pump. The other type, fan guns, don’t require the air pump and can use up to 10,000 gallons of water per hour. Modern machines are controlled wirelessly from a computer running software that adjusts for changing weather conditions.
With either type of machine, droplets shoot into the outside air, freeze within seconds and fall as snow. The snow crystals start with a particle or single water droplet to which other droplets can freeze (or not, if the air is too humid). Along with factors like low temperature and low humidity, a long throw from the gun to the ground makes for bigger flakes.
Where it Snows
Worldwide, snow is most common in the high altitude and high latitude regions of the planet. Snow commonly reaches 46 million square kilometers of territory in the Northern Hemisphere. Interestingly, parts of Antarctica receive very little snow because these regions have too little moisture.
In the United States, lake effect snow is most closely associated with the Great Lakes region, although the phenomenon also occurs in other parts of the world. Lake effect snow is generated when cold, dry air passes over a relatively warm body of water. The moisture picked up forms clouds as the rising, warm air cools. Clouds of significant size will produce snow when they contact land. Valdez, Alaska averages 326 inches of snowfall annually, making it the “whitest place in the U.S.”
Snow and Climate
Snow has an important role in regulating Earth’s surface temperature. Globally, snow cover regulates heat exchange between the planet’s surface and its atmosphere. The high reflectivity of snow – it reflects 80 to 90 percent of sunlight ― cools the Earth. The insulating property of snow impacts climate, too. Thirty centimeters of snow protect the soil and the organisms within it from extremes of air temperatures. Snow holds in the heat and prevents the soil from losing moisture.
Climate change impacts the amount of snowfall and the timing of the snow season. Current models suggest that snow will cover less of the Earth in coming years. Europe and Asia will experience this change in snow cover most dramatically. According to the U.S. Environmental Protection Agency, the United States has seen a decrease in snowfall over the decades since 1930, the first year for which widespread records are available. In recent years, nearly 80 percent of the various locations where snowfall has been recorded have seen more rain than snow as winter precipitation. By contrast, the Great Lakes region has actually seen slightly more snow than in the past. Over the past nearly 40 years, the average fraction of North America covered by snow has trended down significantly, although the records show wide variability year to year. Decreases in snow cover have largely occurred in spring and summer, times that can particularly impact water supplies.
Snow and Water Supplies
Globally, melted snow fills rivers and reservoirs. For example, mountain snowfields in the western U.S. provide as much as 75 percent of the water supply for these regions. At one time, the South Platte River flowing through Colorado and Nebraska effectively turned off in the late spring, when snow had entirely melted off (today, irrigation water seeps into the river to keep it flowing year round).
For states like California, a shrinking Sierra Nevada snowpack means continued drought, as this snow normally supplies 30 percent of the state’s water. Last April, Governor Jerry Brown announced statewide water restrictions in no small part because the snowpack in that state has radically decreased in the past few years.
This winter’s anticipated El Niño is expected to impact weather and snowfall across the country. While the northwest will have a much drier winter, California may see as much as three times the snowfall as last year’s total in its central and northern mountains. In the eastern U.S., less lake effect snow should leave snowfall totals in the Great Lakes region lower than normal.
An avalanche is defined as the rapid flow of snow down a slope, generally that of a hill or mountainside. Although sheets of powdery snow can slide down a hillside in sufficient quantity to be called an avalanche, another kind of avalanche occurs when a large slab of snow separates from its resting spot and slides down at a speed that can reach 130km/h. A large avalanche can release up to 230,000 cubic meters of snow, according to the National Snow and Ice Data Center in Colorado.
While an avalanche can occur on any slope under the right circumstances, they typically happen when the angle of the slope is 30° or greater. Other factors influencing avalanches include temperature and weather as well as slope orientation, wind direction, terrain and nearby vegetation. Some of these factors change hourly, like temperature or general snowpack conditions. Frequent changes in combinations of these factors can make the likelihood of avalanche low, moderate or high.
The Slab Avalanche
An avalanche has three main parts: the starting zone, the avalanche track and the runout zone. The starting zone is the volatile area of the slope where the snow slab separates. As you might expect, the starting zone is usually relatively high up the slope, but a snow slab can fracture from any point. The avalanche track is the channel the snow follows down the slope. Long, chute-like clearings vertically on a mountainside can indicate the track of one or more previous avalanches. The runout zone is where the snow and the debris it pushes ahead pile up.
Snow scientist Jordy Hendrykx of Montana State University describes snow as “one of the most variable materials we can imagine encountering at everyday temperatures.”
The variability one finds in snow means that some layers of it are weaker than others. Any additional load on a weak layer ― like a skier, fresh snowfall or wind-borne snow ― can trigger its collapse. Most people die in avalanches they triggered, according to Karl Birkeland, director of the U.S. Forest Service National Avalanche Center in Bozeman, Montana. While most avalanches tend to happen between December and April, avalanche fatalities have been documented for every month of the year.
Surviving an Avalanche
Precautions are the best way to avoid dying in an avalanche. Skiers and other winter sportsmen should consult the local avalanche center for mountains in the Western U.S. The U.S. Forest Service has 14 regional avalanche centers to help study avalanches and provide avalanche warnings. When traveling avalanche-prone areas, parties should spread out so that a trapped individual can be reported to rescuers by the survivors. Survival chances for full burials are as high as 91 percent if the victim is found within the first 18 minutes, and that means companions are doing the rescuing. Those odds fall off quickly as time passes, however, dipping to just 30 percent after 30 to 40 minutes.
At the first sign of an avalanche, trying to “swim” downhill ― thrash wildly, in other words ― can help one stay atop the sliding snow. A skier should ditch poles, skis and any large equipment. If caught, a person should try to keep one arm straight above his or her head. The idea in this case is to provide orientation of up from down, which is hard to keep track of while being tossed along with the other debris. When things come to a stop, this outstretched arm may extend above the snow as a gloved hand visible to rescuers.
Avalanche forecaster John Snook recommended that a trapped person create an air pocket with the buried hand before the snow freezes. Finding and rescuing an avalanche victim takes some time, so the air will be necessary.
Survivors who are not trapped should not leave trapped companions to go for help. Instead, using a phone or radio to call for outside help is crucial, because most rescues are performed by traveling companions.