Snow

• 1. Introduction

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  Snow is capable of astonishing extremes. It fell hard enough at Mount Baker Ski Area in Washington state to leave 2,896 centimetres on the ground in a single season, 1998 to 1999, a world record for total seasonal snowfall. That number hints at how much complexity hides inside something that looks, from a distance, like the same soft white blanket every winter. Snow begins as individual ice crystals forming inside clouds, growing to millimeter size, falling, accumulating on the ground, and then metamorphosing, melting, sliding, or sublimating away. It shapes transportation, agriculture, sport, and warfare, and it insulates entire ecosystems through the coldest months. What turns a microscopic ice crystal into a blizzard, and what happens to snow long after it lands?

• 2. Born Inside A Cloud

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  A single snowflake is built from roughly 1019 water molecules, added to its core at different rates depending on the shifting temperature and humidity it passes through on the way to the ground. That process starts with a supercooled cloud droplet, about 10 micrometres across, which can stay liquid even below -18 degrees Celsius until a handful of its own molecules happen to arrange themselves like an ice lattice and give the droplet a nucleus to freeze around. In warmer clouds, the droplet instead needs an outside aerosol particle to act as that nucleus. Clays, desert dust, and biological particles can all do the job, and so, artificially, can silver iodide or dry ice, which is how cloud seeding stimulates precipitation. Once frozen, the crystal grows by pulling water vapor out of the surrounding, far more numerous liquid droplets, a process called the Wegener-Bergeron-Findeisen mechanism, until it reaches hundreds of micrometres or millimeters across and heavy enough to fall. Crystals collide and stick together into aggregates on the way down, and those aggregates are what actually reach the ground as snowflakes; their apparent whiteness comes from light scattering off countless tiny facets and imperfections rather than any color in the ice itself. Falling snowflakes settle into a handful of basic shapes, among them platelets, needles, columns, and rime, and physicist Ukichiro Nakaya's morphology diagram showed that shape depends on both temperature and how saturated the surrounding air is: crystals forming below the saturation line grow solid and compact, while those forming in supersaturated air grow lacy, delicate, and ornate. A crystal that starts forming in a column-growth regime around -5 degrees Celsius can sprout plate or dendritic shapes at its tips if it later falls into a warmer, plate-favoring layer of air, a combination known as a capped column. Starting in 1885, photographer Wilson Alwyn Bentley's micrographs of thousands of snowflakes first revealed how classifiable their variety really was, and researchers Magono and Lee eventually catalogued 80 distinct snow crystal shapes, documenting each with its own micrograph.

• 3. How Storms Make Snow

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  In the Northern Hemisphere, the northern side of a low-pressure system typically produces the most snow, since most snow clouds form as part of these larger weather systems built around warm and cold fronts. A cold front sliding through near-freezing surface temperatures can trigger a frontal snowsquall, an intense convective line similar to a rainband that produces whiteout conditions along its path; any single point usually experiences it for less than 30 minutes, even though the line itself can travel a long distance. A warm front works differently, sending moist air up and over a layer of below-freezing air to create snow that often turns to rain once the warm sector behind the front arrives. Lake-effect snow forms when a cold air mass crosses a long stretch of warmer lake water, picks up moisture, rises through colder air above, and dumps snow on the leeward shore; the same process over salt water is called ocean-effect or bay-effect snow, and it can deposit many inches an hour in narrow, intense bands. The resulting snowbelts include the areas east of the Great Lakes, the west coasts of northern Japan, Russia's Kamchatka Peninsula, and the shores near the Great Salt Lake, the Black Sea, the Caspian Sea, and the Baltic Sea. A third mechanism, orographic or relief snowfall, happens when moist air is forced up the windward side of a mountain range, cooling and condensing as it climbs and leaving drier air to descend the leeward side; mountain waves downwind of ranges can further boost precipitation by adding extra lift. Storm intensity has its own formal vocabulary, too: snow flurries, snow showers, snowstorms, and blizzards describe events of progressively greater duration and force. In the United States, a blizzard requires sustained winds or frequent gusts of 35 miles per hour and visibility cut below 0.4 kilometres for at least three hours, criteria Canada and the United Kingdom mirror closely; because the definition hinges on blowing snow rather than snow actually falling, wind alone can create what meteorologists call a ground blizzard. Snowfall itself is graded by how far a person can see through it: light snow allows visibility beyond 1 kilometre, moderate snow cuts that to between 0.5 and 1 kilometre, and heavy snow drops visibility below 0.5 kilometre.

• 4. Snow On The Move

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  The International Association of Cryospheric Sciences defines snow metamorphism as the transformation that snow undergoes from the moment it is deposited until it either melts or turns into glacial ice. A settling snowpack typically compacts to about 30 percent of the density of water under its own weight, then reaches roughly 50 percent by late spring through repeated melting and refreezing. Water vapor also reshapes a snowpack from within, depositing ice crystals known as hoar frost during cold, still conditions and turning snow into what scientists describe as a highly porous, sintered material with a continuous ice structure and a connected network of pore space. Snow that survives into summer becomes neve, granular and partly melted and refrozen snow with a minimum density of 500 kilograms per cubic metre, about half the density of liquid water. Snow that persists for multiple years recrystallizes further into firn, denser than neve but not yet true glacial ice; it resembles caked sugar, strongly resists a shovel, and typically runs 550 to 830 kilograms per cubic metre. Deposited snow moves in four main ways: drifting when the wind carries unsintered powder, avalanches on steep slopes, seasonal snowmelt, and the slow flow of glaciers once compacted snow has turned to ice. Slab avalanches, which fracture cleanly out of the surrounding snowpack like a cut block, account for most back-country avalanche fatalities, while a powder snow avalanche, formed when snow mixes with air, can exceed 300 kilometres per hour and carry 10,000,000 tonnes of snow; a slower wet snow avalanche, moving at only 10 to 40 kilometres per hour, still generates powerful destructive force from its sheer mass. Where accumulation keeps outpacing melt, snow compacted inside a bowl-shaped basin called a cirque can eventually grow thick enough, sometimes with as little as 15 metres of snow-ice on a steep slope, for gravity and pressure to set it flowing as a glacier.

• 5. Measuring A World Of White

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  Glaciers and their permanent snowpacks cover about 10 percent of Earth's surface, while seasonal snow adds roughly another 9 percent, most of it in the Northern Hemisphere, where a 1987 estimate put the seasonal coverage at about 40 million square kilometres. A 2007 estimate found Northern Hemisphere snow cover ranging from a minimum of about 2 million square kilometres each August to a maximum of 45 million square kilometres each January, nearly half the hemisphere's land surface. A study of the period from 1972 to 2006 found that extent had shrunk by about 0.5 million square kilometres, part of a longer decline satellite records have tracked since the 1960s at roughly 1.3 percent per decade, with China standing out as an exception, its own snow cover expanding between 1978 and 2006. The National Snow and Ice Data Center tracks these changes with a normalized difference snow index calculated from visible and infrared reflectance, while passive microwave sensors let scientists map snow beneath clouds and in total darkness, filling gaps optical satellites miss over patchy or forested terrain. On the ground, observers still measure height of new snow the old way, with a ruler against a snowboard cleared every 24 hours, since melting, compacting, and drifting all complicate an accurate reading. The seasonal average snowfall record belongs to Sukayu Onsen, Japan, which measured 1,764 centimetres of snow a year on average between 1981 and 2010. The largest snowflake on record, according to Guinness World Records, fell in January 1887 near present-day Miles City, Montana, and measured 38 centimetres across. Among cities with more than 100,000 people, Japan claims the top annual snowfall totals: Aomori at 792 centimetres, Sapporo at 485, and Toyama at 363, followed by St. John's and Quebec City in Canada at 332 and 315 centimetres, and Syracuse, New York, at 325.

• 6. When Snow Meets Civilization

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  Keeping roads open costs real money: a 1994 report by Kuemmel, surveying practices across 44 US states and nine Canadian provinces, estimated that North America spent about $2 billion a year on roadway winter maintenance. Snow tires improve traction by compacting snow into their tread, but the deeper fix is an anti-icing program that combines chemical treatment with plowing before ice can bond to the pavement. Airports face a stricter constraint: the chloride chemicals used on roads corrode aluminum aircraft, so airfields rely mainly on mechanical brushes and rotary snowplows, and clearing a single terminal apron can mean removing snow from 6 hectares or more. In the air, properly equipped aircraft can fly through snowstorms under instrument flight rules, but they still need deicing fluid beforehand to keep snow and ice from compromising their wings and fuselage. Railroads clear track with two kinds of plows, a wedge plow that casts snow to both sides and a rotary plow, invented around 1865, that throws heavy snowfall far to one side; in the Alps and the Rocky Mountains, railroads also build snow sheds, structures that cover the track outright to keep avalanches and heavy snow off the rails. On the ground, agriculture depends on snowmelt as a seasonal water source; the Ganges, fed largely by Himalayan tributaries, irrigates much of northern and northeastern India, the Indus River carries meltwater from Tibet and the western Himalayas into Pakistan and northwestern India, and the Colorado River supplies irrigation water to about 4 million acres from Rocky Mountain snowpack. Compacted snow can even carry aircraft: the Phoenix Airfield in Antarctica uses a snow-compacted runway engineered to withstand roughly 60 wheeled flights of heavy-lift military aircraft a year. Structures face their own snow math: engineers in Europe follow Eurocode 1 and those in North America follow the ASCE's Minimum Design Loads standard to translate regional ground snow loads into roof design loads, while ice dams, formed when meltwater refreezes at a roof's eaves under an insulating blanket of snow, can damage buildings or injure anyone who tries to remove them. Skiing remains the dominant snow sport: an estimated 65 to 75 million people skied worldwide as of 1994, about 55 million of them on Alpine skis, and by 1996 there were roughly 4,500 ski areas worldwide running 26,000 lifts for some 390 million skier visits a year. Ski wax reduces the friction between a ski and the snow beneath it, though a skier must first overcome that friction's static maximum before the ski will even begin to slide, and resorts increasingly rely on manufactured snow, made by forcing water and pressurized air through a snow gun, to extend their seasons whenever the wet-bulb temperature drops low enough to make the energy-intensive process worthwhile.

• 7. Snow And War

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  The French invasion of Russia offers the starkest example: poor traction for ill-shod horses left supply wagons unable to keep pace with advancing troops, and by the time the retreating army reached the Neman River in December 1812, only 10,000 of the 420,000 soldiers who had invaded that June remained. In the Winter War of late 1939, Finland's army used superior over-snow mobility, camouflage, and terrain knowledge to hold off a Soviet attempt to seize Finnish territory. The Battle of the Bulge, a German counteroffensive that began on the 16th of December 1944, unfolded under heavy snowstorms that grounded Allied air support even as the same weather hampered German supply lines. During Operation Barbarossa, the 1941 Nazi invasion of Russia, both Russian and German troops endured brutal winter conditions, and while the Red Army made common use of ski infantry, Germany fielded only a single division trained to move on skis. The Korean War, which ran from the 25th of June 1950 until an armistice on the 27th of July 1953, saw much of its fighting take place in winter conditions, most starkly during the Battle of Chosin Reservoir, where cold disabled vehicles and weapons as much as any enemy action.

• 8. Snow Beyond Earth

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  NASA's Phoenix Mars lander observed water-based snow crystals falling at high latitudes on Mars, evidence that snow is not unique to Earth. Carbon dioxide separately precipitates from clouds during Martian winters at the poles, becoming the main ingredient of that planet's polar ice caps. On Venus, the Magellan spacecraft detected a metallic substance precipitating onto the planet's highest mountain peaks, nicknamed Venus snow, and researchers suspect the culprits are lead sulfide or bismuth sulfide, minerals that can form a reflective coating even at Venus's extreme surface temperatures. Around Saturn, the Cassini-Huygens mission found evidence on the moon Titan of crystalline deposits made of methane or another hydrocarbon. Farther out still, NASA's New Horizons probe showed that on Pluto, methane condenses at high altitude and falls back down to the surface as frost.