Influence of atmospheric conditions on the ice crystals
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.
Snow cover measurements
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).