Snow is made of clear ice, so why does it look white?
Each snow crystal is transparent ice, yet a snowfield glows white. The reason is scattering: countless facets, hollows and flaws bounce every colour of light around until it all comes back mixed. Inside each flake, roughly 10 to the 19th power water molecules have stacked up in a pattern set by the air it fell through.
A crystal begins with a supercooled cloud droplet about 10 micrometres across. Droplets that small can stay liquid below minus 18 degrees Celsius, because freezing needs a few molecules to line up by chance in an ice-like arrangement. In warmer clouds a speck of clay, desert dust or biological material must act as the seed, and cloud seeding supplies artificial ones such as silver iodide or dry ice. Once frozen, the crystal feeds on vapour and outgrows the far more numerous droplets around it, a mechanism named after Wegener, Bergeron and Findeisen, then collides with others to form the clusters we call snowflakes.
Shape depends on conditions. Ukichiro Nakaya mapped crystal forms against temperature and moisture, finding that drier air yields compact shapes and supersaturated air produces lacy, ornate ones. A crystal that starts as a column near minus 5 degrees and then drops into warmer air can sprout plates at each end, a capped column. Magono and Lee catalogued 80 distinct forms, and Wilson Bentley began photographing thousands of flakes in 1885, revealing endless variety within recognisable patterns, though closely matching crystals have been observed.
Most snow falls from low-pressure systems; in the Northern Hemisphere their northern side is the snowiest. Cold fronts can unleash squalls that bring whiteouts for less than half an hour at any spot. Lake-effect snow forms when frigid air crosses warmer water, picks up moisture and dumps it on the downwind shore, sometimes several inches an hour, creating snowbelts east of the Great Lakes, along northern Japan's west coast and on Russia's Kamchatka Peninsula. Mountains squeeze out extra snow as rising air cools.
On the ground, snowpack changes through sintering, sublimation and freeze-thaw cycles. Where it survives from year to year it can build a glacier; elsewhere its melt feeds rivers and groundwater, and in winter it insulates the plants and animals sheltering beneath it.
Source: Snow
