Verhalten abseits der Piste Avalanche accidents Wissenswertes über Lawinen Literatur Kernteam Lawinenausbildung Exceptional avalanche situations
Guidance on the use of measured data
Data recorded by the automatic measuring stations in the avalanche warning networks (IMIS) are made available every half-hour. The SLF is unable to check or verify their accuracy. The measured data are therefore unchecked raw data.
Errors can arise from the following factors:
- The sites for the IMIS stations are selected according to avalanche-related technical deliberations. The stations are situated in the vicinity of avalanche starting zones and are generally not connected to the electricity network. In view of the solar energy supply, neither heated nor cooled sensors can be used. This impairs the data quality in a number of ways. In some cases, the quality does not comply with the customary meteorological standards. Anemometers can become iced-up, for example, and indicate excessively low values (cf. Fig. 1) and, especially when there is no wind, excessively high air temperatures can be measured.
- In the event of a sensor defect, an immediate repair
may be impossible in some circumstances, depending on the weather, because many
IMIS stations are accessible only by helicopter. In such cases, an error
message is not recorded.
Abb. 1: IMIS wind station Pizzo Tremorgio (TI) covered with rime. The rime or ice does not always disappear due to meteorological conditions (sun, heat) and it can take days or weeks until someone can free the station from ice or rime (photo: E. Salinetti, 08.12.2008).
The main purpose of
the automatic measuring stations is to collect data quickly from avalanche
starting zones for those who are responsible for public safety in settlements
and on the roads. These individuals are trained to interpret the data and are
expected to be able to cope with any erroneous values. Anyone using these data,
when planning backcountry tours for instance, must take into account the issue
of data quality. The SLF cannot accept any liability for problems arising
during backcountry touring or off-piste activities that are attributable to
flawed measured data.
Snow depth (HS) and new fallen snow (HN)
The chart tracking snow depths over time allows conclusions to be drawn about snowfall, settling and melting. In case of snowfall, the modelled new fallen snow depth (6h) can serve as a useful aid in interpreting the data. In the event of a major snowfall, for example, the snow cover can settle significantly, its depth either remaining constant or decreasing, even while snow is still falling (example). In other words, the amount of fresh snow cannot be determined simply by measuring snow depth. Such measure of fresh snow depth is modelled by the snowpack simulation program and may not accurately reflect real conditions.
Problems can arise from the following factors in connection with snow depth measurement:
Wind speed measurements are a key aid when estimating the formation of snowdrift accumulations. The wind charts recorded by the automated stations plot mean wind speed (vectorial average over 30 minutes), peak gusts (highest wind speed over 30 minutes) and wind direction (averaged wind direction over 30 minutes).
Problems can arise from the following factors in connection with wind measurement:
Air and snow temperatures are key factors in assessing avalanche danger, particularly in springtime when wet snow avalanches are most likely. The pattern traced by the two temperature curves also permits conclusions to be drawn about the extent of cloud cover. When the air temperature and snow surface temperature curves trace a similar pattern, the weather is overcast; when they diverge, the skies are clear (example).
Problems can arise from the following factors in connection with air temperature measurement: