The boundary surface between two layers is called the tropopause. The tropopause is always characterised by a marked change in the temperature profile, and represents the lowest temperature between these two layers. Its height depends strongly on the geographical location, the season and the prevalent weather situation. When polar air masses flow into the upper troposphere above Switzerland, this leads to a lowering of the tropopause. Warm air from the subtropics, on the other hand, results in the increased altitude of the tropopause.
The height of the tropopause is significant in aviation. Moreover, processes of exchange between the troposphere and the stratosphere take place at the tropopause that have an impact on the radiation balance of the Earth.
Determining the height of the tropopause
According to the definition of the World Meterological Organisation (WMO), the tropopause height is the lowest level in the atmosphere at which the air cools at a maximum rate of 0.2°C per 100 m of altitude. The definition also specifies that this cooling rate of 0.2°C per 100 m must not be exceeded within a 2-km-thick layer above this level.
As is the case for the zero degree line, the height of the tropopause can also be determined from the temperature profile compiled with the help of weather balloons. In Switzerland, MeteoSwiss conducts weather-balloon soundings at the regional centre in Payerne. Weather balloons are launched twice a day (11:00 AM and 11:00 PM UTC). The average of the tropopause height from both soundings is taken for the daily data.
The average altitude of the tropopause over the Payerne weather station over the period of 1991-2020 was 11,210 m a.s.l. and the peak altitude of the tropopause at just over 12,000 m occurred in August. In summer, altitudes of over 15 km are possible, whereas an altitude of less than 10 km is rarely seen.
The height of the tropopause is significantly lower in winter and spring. On average, it fluctuates between 10,500 and 11,000 m a.s.l. in the colder half of the year. The tropopause can, however, sink below 10 km for several days at a time in these seasons.