Aarau in September: September is the golden Indian summer month: stable high-pressure systems bring sunny, dry days around 20 °C while nights cool noticeably. Aarau is the capital of the canton of Aargau and lies at the foot of the Jura mountains on the banks of the Aare river. Climate change is leaving visible traces here too: deviations from the long-term mean reveal how weather extremes in Aarau have shifted over the past decades.
September in Aarau typically shows golden early autumn with warm days and cooler nights. Deviations from the 10-year average reveal how the local climate is changing.
Planning around September in Aarau? This page shows the average weather in Aarau during September 2025, compared against the 10-year climatological mean. The comparison covers daily maximum and minimum temperature, sunshine hours, precipitation, and peak wind speed. Those five variables most reliably describe whether a month felt warmer, sunnier, wetter or windier than usual.
Historical weather data for Aarau comes from the ERA5 reanalysis product, a high-resolution dataset maintained by the European Centre for Medium-Range Weather Forecasts (ECMWF) and delivered through Open-Meteo. ERA5 is one of the most accurate publicly available sources for historical Aarau climate values, updated daily and covering every month from 1940 to today. That means typical Aarau September conditions are grounded in measured values, not a statistical model.
The KPI cards on this page each show two numbers: the current September 2025 value for Aarau, and its deviation from the 10-year reference mean. A positive deviation means the month ran warmer, sunnier, wetter or windier than the recent climate normal for Aarau in September. Weather in Aarau Septemberhas shifted measurably over recent decades; to explore Aarau September climate year by year, open the climate trend page. To compare Aarau weather against another city for the same month, use the weather comparison tool.
This page is a retrospective, not a forecast. If you are planning travel, a garden project, a photo shoot, or an event in Aarau in September, use the measured historical values as a realistic baseline and treat them as a distribution, not a guarantee. The 10-year mean smooths over individual extreme years; the current-year values show where this particular September sat against that baseline. For typical Aarau September conditions across a longer window, the climate trend page covers every year back to 1940. For day-level detail rather than monthly aggregates, compare arbitrary date ranges for Aarau using the period comparison tool on the homepage.
This page compares the daily measured values of the current year against the average of the last ten years, the so-called 10-year mean. The deviation values in the KPI cards show directly whether the current month is warmer, sunnier, wetter, or windier than usual. The reference data comes from Open-Meteo's ERA5 weather archive and includes daily measured maximum and minimum temperatures, sunshine hours, precipitation totals, and wind speeds.
A ten-year average is short enough to reflect current climate trends, yet long enough to statistically balance out individual extreme years. The World Meteorological Organization (WMO) recommends 30-year normals as the official climate reference, our 10-year mean is deliberately more present-focused and therefore shows the climate change of the most recent decade more clearly.
The daily data for the current year comes directly from Open-Meteo in real time. The reference mean is calculated from actual measured values of the last 10 years, no statistical model, but real historical archive data. The basis is ECMWF's ERA5 reanalysis dataset.
The deviation shows the difference between the current monthly average and the 10-year reference value for the same month. A value of +2.3 °C means: the current month is on average 2.3 degrees warmer than the average of the last 10 years.
Weather review pages are available for over 100 cities in Germany, Austria, and Switzerland. Choose your city from the overview page or use the weather comparison tool on the homepage for custom periods.