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Foehn index
The foehn index is an objectively calculated index used to determine whether foehn is present at a number of weather stations on the northern and southern sides of the Alps.
The foehn index is an objectively calculated index used to determine whether foehn is present at a number of weather stations on the northern and southern sides of the Alps.
The foehn is a mild, dry wind that blows through the inner Alpine valleys on the northern and southern sides of the Alps. In order to determine the presence of foehn in real time, an index was developed for a number of stations in the MeteoSwiss automatic monitoring network. Most of these are located in the inner Alpine valleys, and there are also some on the Swiss plateau in the east of the country, where the foehn can blow in intense phases.
The main prerequisite for the occurrence of foehn on the northern slopes of the Alps is a southerly wind on the main Alpine ridge, which is measured at the Gütsch station, Andermatt, (GUE). Conversely, on the southern slope of the Alps, the wind on the main Alpine ridge must come from the north for foehn to occur.
The foehn index is calculated individually for each station using different measurements. There are fixed threshold values for each parameter. These enable the index to be objectively calculated for each station, depending on where it is located and how prone it is to foehn. The parameters used (measured every ten minutes) are average speed, wind gusts, wind direction, relative humidity and potential temperature. The potential temperature is a hypothetical value that the air would have if it were measured at sea level. It is calculated from the temperature and the air pressure.
There are three levels in the foehn index. In addition to the absence (Level 0) and presence (Level 2) of foehn, there is an intermediate level at which the criteria are partially met (tending towards foehn, Level 1).
| Level | Definition | Foehn duration: |
| 0 | No foehn | 0 minutes |
| 1 | Mixed foehn air (or tendency towards foehn): Foehn criteria are partially met. | 10 minutes |
| 2 | Foehn | 10 minutes |
The foehn index is updated every ten minutes and can be displayed under measurements at the weather stations. The values shown are only partially monitored. Values may also be missing at times.