Nothing can lure the fun-loving child out of a grown adult more quickly than a snowstorm. Particularly in places where it doesn't fall very often, snow is a holiday-maker, an enforced interruption to our routine. Both kids and kids at heart fling snowballs; the more meditative among us stroll through a panorama of softened sound and muted colour. There's shovelling, to be sure, but on the whole it seems a fair exchange for a day or two of magnificent transformation.
For those who live on the northern flanks of North America, Europe and Asia, snow is integral to everyday life. Transportation, housing, food – any, or all, of these may depend on the quality and timing of the winter snowpack. Airplanes and snowmobiles have introduced some flexibility to life in the Arctic, but they haven't altered its essence. Every winter, trucks rattle along a Canadian ice highway that runs over 560km/350 miles from the town of Yellowknife to diamond mines on the other side of the Arctic Circle. When the snow and ice melt, the highway is gone – until the next winter.
Shapes in the sky
The first step towards understanding snow is to ditch the stereotypical image of a monotonous field of white. There are endless variations on snow, starting with its hue. Specialized algae that live within snow can tint it blue, green or pink. Pollen falling from cypress and pine trees can even produce yellow snow. Snow's variety starts in the air. Every schoolchild learns that no two snowflakes are alike, but that assumption was proven incorrect in 1988. Two Colorado researchers examining snowflakes preserved from a research flight found conjoined twins: two perfectly matched columns attached to each other. Given the uncounted numbers of flakes that form each year across the world, it's highly likely that there are other duplications out there. Whether or not each one is original, the shapes snow can take are stunning.
Every snowflake begins as a single, microscopic ice crystal that usually forms on a mote of dust in the middle of a cloud. Among all the particles that float through the air – including salt, soot and other debris – only a select few have the right molecular structure to allow ice to form on them. Windblown dust is the most common of these.
Raindrops grow mainly by bumping into each other, while ice crystals grow into snowflakes by attracting water vapour. In a process called deposition, the vapour goes directly onto the ice without ever passing through a liquid phase. As the crystal grows, temperature is the main factor that controls what shape it will take. If the air in its cloudy womb is hovering close to freezing, the snowflake is likely to turn into an unadorned needle or plate. The classically pretty, six-branched flakes known as dendrites tend to develop at a cooler temperature, ranging as low as about -18°C/0°F. Hexagonal plates (less frilly than dendrites) form when it's as cold as -28°C/-18°F. Any colder and the result is usually a six-sided column.
As they descend, some ice crystals bump into supercooled water droplets. These have remained liquid below freezing because they lack nuclei that would help ice form. The result of these collisions, called riming, is less flaky, more rounded snow. If it grows large enough, rimed crystals fall as pellets called graupel, which resemble tiny, lightweight hailstones. Large, wet snow-flakes – especially dendrites – glom together as they fall, sometimes with dramatic results. The biggest flakes can span more than 5cm/2in in diameter.
As long as the temperatures are right for snow production within a cloud, it's not all that relevant how cold it is at ground level. In fact, it's usually snowing in the topmost layers of a summertime thunderstorm, even as temperatures sizzle near the ground. Snow can accumulate on grass and other surfaces that are well above freezing, as long as it falls quickly enough to compensate for melting on the warm ground By the same token, it's never really too cold to snow. The peak amount of moisture in the air does go down with air temperature, so if it's extremely cold at all levels it's unlikely to snow very hard. However, if a batch of near-freezing air moves on top of a shallow layer of much colder air, then heavy snow can fall into the frigid surface layer, even if temperatures near the ground are below -18°C/0°F. Indeed, some of the worst snowstorms on record have occurred in temperatures far below freezing.
The geography of snow
Low-pressure centres that sweep across mid-latitudes in the winter bring most of the northern continents their snow. Many such storms have a zone where the temperature and moisture profiles are just right to yield the best ice-crystal production. This zone can range from about 80-160km/50-100 miles wide, and is typically located about that far to the left of the surface low as it moves along. The classic result is a swath of heavier snow bracketed by lighter amounts on either side. Radar often shows a bright band marking the transition aloft from rain to snow production. Forecasting where the rain/snow line will occur at the surface is a difficult task, however. A span of less than 50km/30 miles can separate drenching rain from paralysing snow.
The Southern Hemisphere isn't renowned for its snow, although Antarctica certainly has plenty of it, and hefty amounts can fall each year on various low-latitude mountains and on the highlands of Chile, Argentina, South Africa, Australia and New Zealand. The Northern Hemisphere hogs most of the world's snowfall, mainly because it has more land area on which to generate cold air masses. By January, the majority of Canada, the extreme northern US, northeast Europe, Russia, Mongolia and northeast China – along with many Northern Hemisphere mountain ranges – are snow-covered and remain so until springs More sporadic snow-cover comes and goes across the bulk of the US, Europe and northern Asia. Further south, snowfalls are infrequent, though not necessarily minor. Close to the sub-tropics, a freak storm in February 1895 gave Houston 51cm/20in of snowfall and New Orleans 20cm/8in. At roughly the same latitude, Jerusalem was blanketed by some 60cm/23in of snow in January 2000. Even Miami Beach saw snowflakes in January 1977 – a chilling affair that hasn't been duplicated since.
Mountains are practically synonymous with snow. Their high altitude means they experience cold air more consistently, and the peaks serve as a barrier to wring the moisture out of ascending winds. Any side of a mountain can get substantial snows. Some parts of the Tibetan plateau see snow more often in the monsoon-dampened summer than in the winter. The west-facing slopes of mountains at mid-latitudes encounter the prevailing westerlies, so they see the most prodigious amounts of snow on earth (as long as they're cold enough). Japan's Mount Ibuki holds the world record for snow on the ground with a staggering 1182cm/465in. In the US state of Washington, Mount Baker broke the seasonal world record held by neighbouring Mount Rainier when it notched 2898cm/1140in in the winter of 1998-99. Cities next to mountain ranges can also rack up huge snowfalls if cold, moist winds blow up-slope for long periods. The Denver, Colorado, area has seen over 100cm/39in fall in a single storm.
Even the tropics can experience snowfall with the help of altitude. When a mid-latitude winter storm drifts toward the equator, it can bring once-in-a-lifetime snows to higher terrain. In December 1997, Guadalajara, Mexico, suffered its first snowfall since 1881. Nor was it a mere dusting: parts of town saw 41cm/16in.
When snowfall exceeds melting in one spot for many years, a glacier forms. Antarctica's glaciers are legendary with good reason: they hold some 70 percent of all the fresh water in the world Permanent glaciers dot many other parts of the world, including several Andean peaks as well as the African volcanoes Kilimanjaro and Kirinyaga. Low-latitude glaciers are sensitive barometers of global warming. The famed snow fields of Kilimanjaro shrank 80 percent during the twentieth century, and at least one study predicts them to vanish by 2020. However, this widely publicized decline may be the result of sparse snowfall more than a local temperature increase. A better example of warming at work would be the extensive glaciers of the Peruvian Andes, which are receding dramatically. Warmer global temperatures don't necessarily mean that all glaciers will shrink right away. In parts of Antarctica, warmer temperatures could bring increased snowfall; as temperatures there inch closer to freezing, the air tends to carry more snow-producing water vapour.