The average altitude of the zero degree line over the Payerne weather station in the period of 1991-2020 was 2,570 m.a.s.l. As is the case with temperature, the zero degree line over Switzerland has a distinct annual pattern, with its minimum in winter and its maximum in summer. In winter, the average altitude of the zero degree line in a free atmosphere is between 1,000 and 2,000 m. However, in strong inversion conditions or when milder air is flowing in from the south or south-west, values of 3,000 m or more are possible. On ice days the notional zero degree line has a negative value. In summer, the average zero degree line fluctuates between 3,000 m (in June) and almost 4,000 m (in July). On cooler summer days, values under 2,500 m are possible, while on extremely hot days, the zero degree line can rise up to 5,000 m.
The climate report provides an in-depth description of the previous year’s annual cycle of the zero degree line.
Changes in the near-ground zero degree line over the long-term
As is the case with temperature, the altitude of the zero degree line over Switzerland is increasing markedly due to climate change. This is most apparent from the near-surface zero degree line derived from weather-station measurements. Since records began in 1864, the near-surface zero degree line has risen by around 200 to 700 metres.