Snow - Guidebook to Understanding, Testing and Interpreting

This practical guide is aimed at anyone who wants to acquire or improve their knowledge of snow and avalanche release. It describes the formation and evolution of snow, how to identify the many forms snow takes, and how to assess avalanche risk more precisely and reliably.

Understanding snow

How snow is formed

Water is always present in the atmosphere in the form of water vapour, an invisible and odourless gas. But the colder the air is, the less water vapour it can hold. A fall in atmospheric temperature (which occurs as air rises, for example) may therefore lead to the condensation of water vapour and so to the formation of clouds.

The process of the formation of snow begins in the cloud with the birth of a minute ice particles: this is called nucleation. In pure water, nucleation (referred to as ‘homogenous’ nucleation) does not take place until the water reaches about -40ºC; so water can remain liquid at temperatures below freezing in a state known as ‘supersaturation’. But in a cloud, in the presence of freezing nuclei (mineral or organic particles from 0.1 to 10 microns), nucleation (known as ‘heterogenous’ nucleation) usually takes place at higher temperatures (below -10ºC, but it can still occur up to about -3ºC).

Starting with a germ of ice, a snow crystal develops by sublimation and deposition, sometimes with the added freezing of surrounding water droplets. This development takes place through the growth of the faces, sides or points of the germ, and leads to the formation of crystals which are prism-, plate- or star-shaped. The snow crystals continue their growth at the expense of smaller crystals until they reach a mass at which they are too heavy to remain suspended in the cloud. When they have obtained this ‘critical’ mass (which is greater if the cloud contains strong turbulence), they fall, more or less joined into snowflakes. If the atmospheric temperature remains sufficiently cold all the way down to ground, then we have a snowfall (the snow–rain limit is normally about 300m below the level of the 0ºC isotherm.)

Note: technical terms in bold are explained in greater detail in the Glossary on pp. 60–61.

The variety of snow crystals
The form and size of snow crystals varies considerably depending on the atmospheric conditions (temperature, humidity and turbulence) in which they are created. Several hundred distinct forms, derived from the basic types, have been catalogued. The World Meteorological Organisation (WMO) has divided snow crystals into ten large families. In order to predict likely changes in newly fallen snow, it is important to take into account the snow’s initial crystallography, distinguishing especially between dendritic crystals, which generally have a branched form, and round crystals, which are much larger.

Dendritic crystals
The commonest dendritic crystals are star-shaped crystals, which form when the temperature within a cloud is low (below about -12ºC). It is easy to see them with the naked eye during snowfalls in calm weather, when they fall gently to the ground individually or assembled into light snowflakes. But there are many other less branched forms of dentritic crystals, such as needles and columns as well as all kinds of irregular particles. On the ground, snow made up of these kinds of crystals is subject to major changes.

Rounded crystals
These are mainly seen in the form of graupel, which is common in the mountains in winter. Graupel crystals are easily recognised by their quite rounded form and fairly large size: their diameter is often more than 5mm. They are formed in turbulent clouds with large vertical development (typically large cumulo-nimbus), and are created by water droplets freezing onto the crystals before they fall. This obscures their original form and gives them their characteristic appearance – like small polystyrene beads. They often fall during brief but violent thunder showers. They do not change much once within the snowpack, except by melting.

Surface Hoar
If the layer of air in contact with the snowpack is moist, and if the surface of the snowpack is subject to strong cooling, this leads to the formation of surface hoar crystals. These may become very large (several centimetres) and often resemble fern leaves in shape. They can often form quite thick layers when skies are completely clear. So this type of snow does not fall from the sky, but forms on the ground! As with rounded crystals, surface hoar crystals undergo little change when they are in the snowpack and will remain there until melting takes place.

Atmospheric riming
Atmospheric riming (generally known as ‘fog crystals’) appears when there is thick cloud along with windy and cold weather. It takes the form of deposits of compact white ice, made up of air and frozen droplets. In dense supersaturated cloud these deposits build up into the wind on crags and other objects. Atmospheric riming, which is common in regions of wet–cold climate and in tropical high mountain areas, also occurs in the Alps, where it can form a sheath of ice on rock faces. It can also cause serious damage to infrastructure, especially to aerials and electricity cables, which may collapse under the load.