Snow falls in soft crystals of infinite variety, but as the pioneering 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."
How ice crystals are formed
Air temperature decreases with height, and the amount of water vapour that air can hold is proportional to the temperature. In other words, the warmer the air, the more moisture it can hold. As air rises - by convection or to cross a mountain, for example - its temperature falls and if it falls to below the "dewpoint" temperature some of the water vapour will condense into droplets and a cloud will form. If the temperature falls below freezing, the water vapour changes directly to ice without passing through the liquid phase.
This sounds straightforward, but it is not quite so simple. Water vapour will not condense unless there are minute particles - cloud condensation nuclei - onto which the droplets can form. These are usually abundant over land, but even so it is common for air to be slightly supersaturated. Ice crystals will not form unless there are freezing nuclei present, such as fine clay particles, onto which water vapour can freeze. These are much less plentiful than condensation nuclei and most do not initiate freezing at temperatures above about 14°F (-10°C). Consequently, most clouds capable of releasing precipitation contain a mixture of ice crystals and supercooled water droplets - liquid droplets that are several degrees below freezing temperature. Once ice crystals begin to form, they do so at the expense of the supercooled droplets. Water evaporates from the droplets and is deposited onto the ice crystals.
Where the temperature is between about 14°F (-10°C) and -4°F (-20°C) a cloud contains approximately equal amounts of water droplets and ice crystals. Ice crystals predominate where the temperature is below -4°F (-20°C) and there is almost no ice in a cloud that is warmer than 14°F (-10°C). If the temperature is 75°F (24°C) at the surface, it will be 14°F (-10°C) at about 20 000 feet (6 km). That is about one-third of the way from the top of an average cumulonimbus cloud capable of delivering a heavy shower.
From ice crystals to snowflakes
Because the shape of its molecule determines the way it crystallises, water invariably freezes into a hexagonal crystal. This grows larger as more water freezes onto it, but it preserves its hexagonal shape. Ice crystals vary in shape, but whatever their shape they always have six sides. Crystals are classified according to their shape. The classification recognises seven crystal types, with a standard symbol to designate each of them, as well as three more symbols for graupel (soft hail), sleet (minute ice crystals, not the mixture of rain and snow that is called sleet in Britain), and hail.
Once ice crystals have formed they start to fall and as they drift this way and that, they collide. When large ice crystals collide they often shatter. Such collisions release tiny splinters of ice that act as freezing nuclei for the formation of more crystals.
When large crystals collide with smaller ones they tend to stick together. That is how ice crystals grow into snowflakes. The process works best if the temperature is above about 23°F (-5°C), so there is a plentiful supply of supercooled water droplets. Water droplets also collide with the ice crystals, forming a thin layer of water that freezes when another ice crystal arrives, so the water acts as an adhesive. Really big snowflakes form at this temperature, from smaller snowflakes that join by interlocking and thereby preserving the six-sided symmetry. Colder clouds produce smaller snowflakes. Powder snow - the consistency that is best for skiing - forms in very cold cloud.
The snow pack
If the snow falls through air that is warmer than freezing it will start to melt. If it falls for more than about 820 feet (250 m) through air warmer than about 35°F (2°C) the snow will melt and fall as rain. If it falls as snow it will not settle unless the temperature in the air below the cloud base and on the ground is below about 39°F (4°C).
Once snow has fallen, it packs together under its own weight and becomes denser by squeezing out the air between grains. The character of the pack depends on the amount of liquid water that is trapped between crystals. Wet snow contains 3-6 percent of water. This makes it dense and heavy. It is excellent for making snowballs, but not so good for skiing. Wet snow of this type falls on the windward side of coastal mountain ranges where it is moist air approaching from the sea that produces the climate, for example in western Norway and in the western Alps.
Powder snow, which is much drier and therefore lighter, falls farther inland, for example in central Scandinavia, the central Pyrenees, and the central and eastern Alps. Freshly fallen powder snow consists of small flakes with tiny pockets of air between them. It is very light and loosely textured, but the spikes on the snowflakes interlock, so powder snow has considerable cohesion. Skiing across it has the effect of packing it very hard in some places, but throwing it into much softer heaps in others. In this condition it is sometimes called "crud".
During the day the snow surface may warm to above freezing. The snow will start to melt, so a thin layer of water covers the surface, but as the temperature falls again this water will freeze to form a crust of ice coating the layer of soft powder snow. Freezing rain, comprising super-cooled raindrops that freeze on contact with a cold surface, falling on the snow pack will produce a similar crust. The crust may be so thin that it will not support the weight of a person or so thick that one can ski over the top of it. Between these conditions the crust may be thick in some places but thin in others, giving the snow an uneven consistency and making skiing difficult. Towards the end of a prolonged dry spell when the surface has been thawed and frozen several times, the old snow is called "sugar snow" because of its texture. Repeated melting and freezing of a snow crust compacts the snow, producing ice.
More prolonged melting increases the proportion of liquid water. Interlocking snow crystals then turn to larger, shapeless, ice grains. "Slush" is the resulting mixture of ice grains and water. The appearance of slush usually heralds the complete melting of the snow, but if slush freezes it forms a rough surface of solid ice that is useless for skiing.
Physical changes can also take place near the base of a snowpack. Even when it is densely packed, a layer of snow contains tiny pockets of air between the individual crystals. At the base of the pack crystals may sublime - change directly into water vapour - into these air pockets. The water vapour is then immediately deposited once more as ice crystals. These crystals are called "hoar" - not to be confused with hoarfrost. They are dense, but they pack loosely and can flow like a liquid. The process of sublimation and deposition may then spread upward until a substantial part of the base of the pack has been transformed. The snow surface has not changed, but the snow beneath the surface layer is now in a potentially dangerous condition because the pack is no longer securely bonded to the ground beneath it. It can move, triggering an avalanche.