World of Science | Snow climate classification

In snow and avalanche research, new techniques, models and verification options are constantly being researched in order to improve warning systems and forecasts and make them more accurate. In order to compare experience and expertise and to be able to generalise results, a rough classification of the snow characteristics typically occurring in the region can be used. These can indicate certain avalanche problems. Put simply: are there regions in which certain snow characteristics typically occur? If so, how can they be delineated and labelled, what does "typically" mean and what data is used?

Taken with a grain of salt: There are indeed regions in which certain avalanche problems occur more frequently than others. This can be verified by classifying meteorological data. Analysing this data enables a systematic classification and a deeper understanding of regional snow conditions and their dynamics.

Terms such as "typically", "generally" or "normally" imply a statistical basis that is based on long-term observations. In meteorology and climatology, an observation period of 30 years has become established in order to enable robust statements to be made about climate conditions and to identify typical patterns. This long-term database is essential in order to recognise which meteorological conditions in a particular region lead to the formation of specific avalanche problems and how these behave in comparison to other regions. How exactly the occurrence of certain avalanche problems caused by snow can be determined will be explained in more detail below.

These are primarily intended to provide users with an overview and assessment of the avalanche danger, but they only relate to a short-term and limited period, usually one day. A short-term and limited period, as snow is constantly changing under the influence of weather changes or day/night cycles, taking on different characteristics and thus changing the avalanche danger. In order to make general statements (the more typical, normal) or to get an overall impression of which regions are potentially more frequently affected by certain avalanche problems than others, it makes sense to look at several years. In meteorology, periods of 30 years or more are observed for such analyses (e.g. whether it tends to rain more or be warmer in one region than in another), the results of which are then referred to as climate. The same applies to analysing trends in snow and avalanche conditions. As the climate can be defined as a statistical average of weather series, it serves as a basis for characterising snow conditions. A classification of snow climates was developed in the USA and Canada in the 1990s and early 2000s respectively. According to this, snow climates are snow conditions averaged over a period of around 30 years for a specific location or region and are intended to provide an overall view of the conditions. Similar efforts have also been made in Europe, but compared to the USA, there is still no comprehensive snow climate classification. However, in the last three to four years, a scheme established in the USA has also been applied to the French Alps.

But why is it helpful to have a snow climate classification?

As already mentioned, meteorological conditions have an impact on the composition of the snowpack, which determines avalanche activity and its character. For example, when it snows a lot in early winter, the temperature gradient between the ground and the snow surface is very high, which leads to the formation of angular crystals within the snowpack, which represent a critical weak layer that can sometimes remain present until mid-winter or spring. Certain snow conditions are therefore linked to certain weak layers or the probability of a weak layer occurring. An example: In a region with frequently very cold temperatures and low precipitation, which leads to a thin snowpack, the probability of old snow problems occurring is higher, as these conditions favour a greater temperature gradient within the snowpack or individual layers. This in turn favours the formation of angular crystals, which reduce the stability of the snowpack and can represent a weak layer that can be triggered. In contrast, the risk of old snow problems is significantly lower in regions where warmer temperatures occur regularly, a lot of snow falls and rain occurs occasionally. Caution here: in regions with more precipitation (than rain and/or snow), other avalanche problems will dominate.

With this knowledge, a general overview of snow conditions and the associated avalanche problems within a region can be determined. This can serve as a basis for the awareness of an avalanche problem within a region. In addition, existing classifications can be used to determine changes in relation to climate change.

Outlook:

As mentioned at the beginning, the classification pattern described above has already been used for the French Alps, and avalanche climates were also determined using a different method (2). A numerical model was used to forecast the possible avalanche problems for each day and also to estimate how critical the problem is. A distinction was made between the avalanche problems (fresh snow, drifting snow, old snow and wet snow) and the type of triggering (spontaneous or caused by skiers). The typical combination of avalanche problems is then determined for each region. This results in a specific combination of fresh snow, drifting snow and other avalanche problems for a particular region.

These combinations can be used to identify regions with similar patterns, which can be divided into four classes (Pre-Alps, Northern and Southern Alps, and Inner Alps) and geographically localised. This classification extends the previous snow classification, as it relates directly to avalanche problems and not only to meteorological data associated with the occurrence of certain avalanche problems.

This approach supplements the snow climate classification with the prevailing avalanche problems, which would also be interesting for the Swiss Alps.

In summary, it can be said that there are regions that are characterised by similar snow conditions and consequently have similar avalanche problems. The categorisation of these regions according to their typical snow climates is particularly important in scientific analyses and in international comparisons, for example with countries such as the USA or Canada.

However, it must be emphasised that the current situation is decisive and that the immediate weather and snow conditions are paramount in the avalanche forecast, because even in a generally maritime region with plenty of snow, an old snow problem can occur.

Nevertheless, knowledge of snow climates provides valuable insights and promotes understanding of how snowpacks behave in different regions. This knowledge can also support planning, as it enables a more targeted response to specific conditions. For example, it can help to roughly estimate which regions have similar snow conditions when travelling to a new, as yet unknown region.

These opportunities for comparison expand knowledge in dealing with avalanche hazards and contribute to international co-operation and the exchange of experience.

More references:

The classification was created during an internship at the WSL Institute for Snow and Avalanche Research SLF in Davos under the supervision of Prof Dr Jürg Schweizer. The data used was also provided by the SLF.

For the snow climate classification:

1: Mock, C. J., Birkeland, K. W. (2000). Snow Avalanche Climatology of the Western United States Mountain Ranges. Bulletin of the American Meteorological Society, 81(10), 2367–2392. doi: 10.1175/15200477(2000)081¡2367: SACOTW¿2.3.CO;2.

For the avalanche classification:

2: Reuter B, Hagenmuller P, Eckert N (2023). Snow and avalanche climates in the French Alps using avalanche problem frequencies. Journal of Glaciology 69(277), 1292–1304. https://doi.org/10.1017/jog.2023.23.