The zero degree line in a free atmosphere is determined from the temperature profile that is measured in Payerne on a daily basis by means of weather balloons. This climate index has a distinct annual pattern, with its maximum value in summer and its minimum in the winter. Over recent decades, anthropogenic climate change has caused the altitude of the zero degree line to rise significantly in every season.
Temperature decreases with altitude, and at a certain point on the vertical axis the temperature falls below 0°C. This specific altitude is known as the zero degree line. The zero degree line can either be deduced from near-surface measurements at weather stations or determined via weather balloons in a free atmosphere.
In Switzerland, MeteoSwiss weather balloons are deployed to conduct soundings at the regional centre in Payerne. Weather balloons are launched twice a day (11:00 AM and 11:00 PM UTC). As they rise, they take a vertical profile of temperature, moisture and wind measurements. It is not always possible to determine the altitude of the zero degree line unequivocally. In an inversion situation multiple zero degree lines can be present in the same vertical profile. In such circumstances, the highest value is taken. If the soil temperature is below 0°C at the time of the radio sounding, a notional zero degree line is calculated (for every 100-m downward correction, the soil temperature is increased by 0.5°C). In very cold, winter conditions, this leads to a zero degree line that is below sea level (i.e. a negative value). The average of the values from both soundings is taken for the daily data.
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.
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.