The greatest snow abundance occurs for cold / moist winters in the both observed sites of Chateau d'Oex ad Arosa, while the least abundance is associated with warm winter in Arosa.

Warm and moist winter at medium elevation is associated with the fall of precipitation in the form of rain rather than snow. Hence the extent of the winter snow pack is rapidly reduced. At higher elevation, warm/moist winters lead to greater snow accumulaton.

At higher elevations, on the other hand, the warm-moist winter mode leads to greater total snow accumulation than the warm-dry mode, but does not offset the negative influence of the higher temperatures compared to cold winters. It can also be seen that the duration of the snow season is strongly modulated by the various combinations of temperature and precipitation, the longest being related to the cold-moist winter mode, the shortest to the warm modes (dry at high elevations and moist at low elevations).

The longest duration of the snow season is related to the cold/moist winter mode, the shortest to the warm modes (dry at high elevation and moist at low elevation).

There is a non-linear increase in seasonal snow accumulation for a linear increase in snow duration. For example, in the case of wetter-than-average winters, an increase in snow duration by 60 days, from 30 to 90 days is accompanied on average by a rise in snow accumulation of 225 cm, from 50 to 275 cm. The same increase of 60 days, but from 240 to 300 days, results in an average additional accumulation of 315 cm, from 960 to 1275 cm.

Under changing climatic conditions, the separation into different winters modes can be used as an indication of the type of snow cover that could be expected in the future, by analogy with current conditions.

Examples are given in the study: A rise in temperature of over 5°C at Arosa and a 40% reduction in precipitation leads to a switch from the cold/moist to the warm/dry modes, leading to a reduction of 54 days in snow cover and almost a 50% reduction in the seasonal snow accumulation. For an equivalent rise in temperature and reduction in precipitation at the lower site of Chateau d’Oex, there is a reduction of over 100 days of snow cover, from 125 days to 22 days, and an 80% reduction in snow cover.

The sensitivity of snow cover to warming is geater for dryer-than-average winters than for wetter-than-average winters.

The volume of snow cover remains maximum at 2000 m for both + 2°C and + 4°C scenario ; the reduction in volume corresponds to 10% per degree of warming. The snow cover duration is reduced of 15-20 days per degree of warming. The shortening of the snow season concerns more the end (spring), rather than the beginning (autumn), so that it should be expected that snow melt would intervene much earlier in the season than under current conditions.

An average warming of 4°C for the period 2071-2100 (simulation from the PRUDENCE project) suggests that snow volume in the Swiss Alps may respond by reduction of at least 90% at altitude close to 1000m asl, by 45-60% at 2000m asl, and 30-40% at 3000m asl.
            
The duration of snow cover may be sharply reduced in warmer climate, with a termination of the season 50-60 days earlier at high elevation (above 2000-2500m asl) and 110-130 days earlier at medium elevation -close to 1000m asl). Reduction in snow-cover duration of 15-20 days for each degree of wintertime warming.

(4) - Remarques générales