Snow is an especially important form of precipitation for Switzerland. Snow falls less frequently in low-lying areas than in the mountains, and the impact varies depending on the region, ranging from slippery roads in the lowlands to avalanches in the mountains.
Snow is a solid form of precipitation that is made up of hexagonal ice crystals, the shape of which varies depending on the weather conditions. How the different ice crystals form depends mainly on the humidity and the air temperature. Depending on the combinations of these two parameters, ice crystals take the shape of rods, platelets, flakes with six arms resembling small roof tiles, or dendritic shapes/stars.
Once an ice crystal has formed and is sufficiently large and heavy, it slowly falls towards the Earth’s surface, initially at a speed of a few centimetres per second. On its long journey towards the ground, the crystal encounters different temperature and humidity conditions to the ones it was formed in. This causes the crystal to undergo changes.
Snowfall occurs when temperatures on the ground are below or slightly above zero. The temperature of both the air and the surface of the ground, as well as the intensity of precipitation, influence how long snow stays on the ground and how high the snowpack becomes. The lower the temperature, the lighter the snow and the higher the air content of the snowpack. Conversely, the higher the temperature, the more compact the snow and the higher the water content of the snowpack.
If the air that the snow falls through is above zero, the snow begins to melt. Since snowflakes do not all melt at the same rate, there is a transition zone between snow and rain. The elevation at which precipitation is 50 percent rain and 50 percent snow is called the snowfall limit in weather forecasts.
At some weather stations in the MeteoSwiss monitoring network, measurements of snow depth are carried out manually. These take place in the morning at 6 a.m. UTC (7 a.m. local time in winter, and 8 a.m. local time in summer). The overall height of the snowpack is measured, as well as the depth of new snow that has fallen in the past 24 hours (from 6 a.m. to 6 a.m. UTC).
The Alps often act as a barrier to precipitation. For example, when the upper-level flow is coming from the north, snowfall can be heavy in the mountains on the northern side of the Alps, with record snow amounts measured in winter or early spring. The deepest snow cover is usually seen between March and May, depending on the year and elevation.
Even though periods of heavy snowfall still occur, the increase in Switzerland’s average temperature is having a noticeable impact on snowfall, especially at low elevations. This is demonstrated by the climate indices for days with fresh snow and the number of days with snow cover.
If snow is forecast, hazard warnings of between Level 2 and Level 5 are issued, depending on the amount of snow expected. Higher warning thresholds apply for the mountains (above 800 m on the northern side of the Alps and above 1,600 m on the southern side), as snowfall there is more frequent and heavier than in the lowlands. As far as lower elevations are concerned, the threshold values for German-speaking Switzerland and the Fribourg region are somewhat higher, as snowfall down to the lowlands occurs more frequently in those areas than in the rest of the country. If the expected snow amounts are below the Level 2 warning threshold for snowfall, Level 2 warnings of slippery roads are issued. When warnings are issued for heavy snowfall or slippery roads, it is important to follow the recommendations for action.