Snow measurements are taken on flat, wind-protected sites of the SLF and MeteoSwiss.
Snow depth is measured either manually or automatically. The chart tracking snow depth over time allows conclusions to be drawn about snowfall, settling and melting. The amount of fresh snow is measured manually or modelled by the automated IMIS snow stations of the SLF. Fresh snow plays a key role in the formation of avalanches.
The map shows the most recent available value for each data point. If the most recent measurement was taken more than 210 minutes earlier, the station is greyed out and depicted without a value. Some of the depicted values originate from raw data that have not been checked or validated.
Snow depth and fresh snow are measured once a day, in the mornings between 7 and 7.30 am. Snow depth is measured by the gauge installed at the measuring site. The fresh snow is measured with a white board placed on the snowpack, flush with the snow surface. Between 7 and 7.30 am the next day, the amount of snow that fell during the previous 24 hours is measured by placing a yardstick on the board.
The values measured by hand are very accurate. Nevertheless, most of the measuring sites are situated in low and intermediate altitude zones, that is to say, below approximately 2000 m, so that if the fresh snow at the site is moist or if rain is falling, the amounts of fresh snow lying at higher altitudes will be greater. In such cases it makes good sense to consult the modelled fresh snow values recorded by the automated IMIS stations at higher altitudes as well.
The data collected by the automated IMIS stations are neither checked nor corrected. They are updated every half hour. To determine snow depth, an ultrasound sensor mounted above the snowpack measures the distance to the snow surface.
The amount of fresh snow cannot be measured directly by automated stations, rather it is modelled from the data collected by the automated IMIS stations using the snowpack model SNOWPACK. Given that the snowpack is constantly settling, the amount of fresh snow does not equal simply the increase in snow depth, but is determined by subtracting from the overall snow depth the amount of settling of the underlying old snow. Above all else, in the case of prolonged snowfall, snow can settle to such an extent that snow depth remains constant or even decreases, despite continuing precipitation.
Errors can occur both in automated snow depth measurements and in modelling the amount of fresh snow. Since measured snow depths are also used to calculate the amount of fresh snow, errors made when recording these values often give rise to inaccurate fresh snow values. Errors can sometimes be detected if the time chart is consulted alongside the current value.
Fresh or old snow transported by wind: Sharp upturn or downturn in snow depth curve. Even if this event actually occurred at the measuring station and was measured accurately, the indicated snow depth does not correctly reflect conditions prevailing in the surrounding area. Drifting snow deposited at the measuring station can trigger overstating of fresh snow quantities on the days that follow as well (because of inaccurate settling calculations).
- Temperature dependence of snow depth measurement: On sunny days the snow depth curve regularly dips marginally around midday. This response is caused due to temperature correction performed by the ultrasound sensor, which is too high because the temperature sensor is not cooled.
- Avalanches: In isolated cases, automated snow stations can be struck by avalanches. The snow depth curve responds by rising abruptly. In such instances the measured snow depth does not accurately reflect conditions prevailing in the surrounding area, and modelled fresh snow values are incorrect.
- Interruptions in snow depth measurement: Heavy snowfall can interfere with the measuring signal, or the sensor can become blocked by snow. This can cause failure of individual measurements.
- Measuring of grass in the summer: Grass grows around numerous IMIS stations in the summertime and is measured by the ultrasound sensor which indicates an increase in ‘snow’ depth. If subsequently snow falls, it compresses the grass and triggers a sudden decrease in measured snow depth. For this reason, calculated fresh snow values recorded by the automated stations have to be treated with particular caution in summertime and at the start of the winter season.