Very mild September

Preliminary monthly summary from GeoSphere Austria: One of the 20 warmest Septembers in recorded history. Precipitation (+4 per cent) and sunshine duration (-6 per cent) were around average in the Austria-wide evaluation, but with some significant regional differences.

September 2025 was mostly warmer than average. Especially around 20 and 21 September, temperatures in some regions of Austria still exceeded 30 °C. The highest temperature of the month was recorded by the Innsbruck University weather station on 20 September at 32.4 °C.

Erst gegen Ende des Monats lagen die Temperaturen in etwa im Bereich eines durchschnittlichen Septembers der jüngeren Vergangenheit (Klimaperiode 1991-2020) oder leicht darunter. Zum Beispiel liegen in den Landeshauptstädten die durchschnittlichen Frühtemperaturen des letzten Septemberdrittels um 10 °C und die Höchstwerte um 19 °C.

‘Looking at the month as a whole, September 2025 was significantly too warm, with temperatures in the Austrian lowlands 1.4 degrees above the average for the 1991 to 2020 climate period and 1.8 degrees above average in the mountains,’ says climatologist Alexander Orlik from GeoSphere Austria. ‘This puts it in 18th place in the series of warmest Septembers in Austria's 259-year measurement history in the lowlands and 19th place in the 175-year mountain measurement series in the mountains. This also means that around 90 percent of all Septembers in the measurement history were cooler than September 2025.’

Compared to the climate period from 1961 to 1990, which was not yet significantly affected by global warming, September 2025 was 1.8 degrees above average in the lowlands and 1.4 degrees above average in the mountains.

The number of summer days (days with a minimum temperature of 25 °C) in September was well above average and well below record levels:

The Vienna Hohe Warte weather station recorded 12 summer days last September, compared to an average of 6 summer days in September during the 1991-2020 climate period. The September record here is 21 summer days in 2023.

Eisenstadt: Last September there were 11 summer days, compared to an average of 6 summer days, with the September record being 22 summer days in 2023.

St. Pölten: Last September there were 7 summer days, the average is 5 summer days, and the September record is 20 summer days in 1961 and 2023.

Linz City: Last September there were 7 summer days, the average is 4 summer days, and the September record is 17 summer days in 2023.

Salzburg Airport: Last September 8 summer days, average 4 summer days, September record 15 summer days in 2023.

Innsbruck University: Last September 9 summer days, average 6 summer days, September record 19 summer days in 1961.

Bregenz: Last September 5 summer days, average 3 summer days, September record 14 summer days in 2023.

Klagenfurt Airport: Last September 10 summer days, average 5 summer days, September record 15 summer days in 1961.

Graz University: Last September 11 summer days, average 6 summer days, September record 20 summer days in 1947.

In the Austria-wide evaluation, precipitation in September 2025 was within the long-term average, with a deviation of +4 percent. There were significant regional variations:

For example, parts of the Waldviertel and Weinviertel regions were significantly wetter than average, with 50 to 100 percent more precipitation than in an average September. It was significantly too dry, especially in the area from the districts of Kufstein and Kitzbühel across most of Upper Austria to the western Mostviertel, with 35 to 55 percent less precipitation than in an average September.

In the warm weather of last September, the fruits of the chestnut trees ripened very early. The average start of horse chestnut fruit ripening in Austria was on September 2, 2025, almost two weeks earlier than the average for September in the period 1991-2020 and almost three weeks earlier than the average for September in the period 1961-1990. This ranks second in the observation series, which has been in existence since 1926. The only year in which horse chestnut fruits ripened slightly earlier was 1931.

Translated with DeepL.com (free version)