Sion (German: Sitten) is the capital of the canton of Valais in the inner-alpine Rhône valley. Sheltered by the Alps, it enjoys a relatively sunny, dry climate with only around 600 mm of annual precipitation – the lowest of any major Swiss city. Summers are warm, winters cool with occasional Föhn. Compare the inner-alpine climate of Sion.
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The period comparison for Sion sits at the top of this page. Pick two time windows that interest you, for example the same month across two different years, and the tool puts maximum and minimum temperature, sunshine hours, precipitation, and peak wind side by side. The “Climate Trend Sion” link above offers a different lens: up to ten years of the same month in a single chart.
The monthly weather review gives you a third perspective. Instead of comparing two periods, you land on a single month page with context from previous years and a link back to the Valais region. That is particularly useful when you want to know whether a specific month in Sion stood out or tracked a familiar pattern.
All values on this page come from the ECMWF ERA5 reanalysis dataset, a global archive that merges satellite and surface measurements into a gap-free time series. For Sion that means: data from 1 January 1940, day by day, without gaps. The series is calibrated and extended daily with new observations.
Because ERA5 reconciles measurements retroactively against the full observation network, values stay comparable whether you set two adjacent years or 1970 against 2026 side by side. The comparison is free and ad-free. Maximum temperature, minimum temperature, precipitation, sunshine hours, and peak wind each follow the same rule: one value per day, one point on the map, one source.