Two different zero-degree isotherms
There are two different zero-degree isotherms. One is determined by means of weather balloons in the free atmosphere, and the other is derived from ground-level measurements at weather stations. The altitude of the zero-degree isotherm in free atmosphere is reported as a matter of course in Swiss weather forecasts. The zero-degree isotherm near ground level is used to analyse long-term changes, with the help of high-quality historic weather-station datasets dating back many years.
The temperature distribution near ground level, along with the zero-degree isotherm can fluctuate strongly over a very small area, especially in the six winter months. Fog or low stratus layers are an example of this: under the stratus the air is noticeably cold, while above the stratus in the sunshine, the temperatures are pleasant (see inversion). During such weather conditions, the zero-degree isotherm can actually be higher in the mountains than on the fog-covered Swiss Plateau.