Summer 2023 Warmer Than Long-Term Average, Fewer Heat Days Observed

September 1, 2025

This summer was warmer than the long-term average but consistent with recent decades, featuring notable high precipitation on its wettest day and fewer summer and heat days compared to recent years, while aligning with broader trends of increasing temperatures and sunshine.

The recent summer season has been characterized by a range of weather conditions that offer both typical and notable deviations from historical patterns. The average temperature for the season was 20.79°C, with temperatures ranging from a low of 8.8°C on June 9th to a high of 36.0°C on August 13th. The total precipitation was marked by a particularly wet day on August 21st, when 44.9 mm of rain fell. The average humidity was 67.2%, and cloud cover was relatively low at 50.7%. Radiation levels averaged 246.9 W/m², indicating a season with substantial sunshine, as evidenced by the average sunshine duration of 496 minutes per day. This summer saw 58 summer days and 22 heat days, but no tropical nights were recorded.

When we compare these figures to the past 20 years, we see that the average temperature of 19.86°C was slightly lower than this year's average. The historical range for the warmest day over the last two decades reached up to 37.4°C, which is slightly higher than this year's peak. The coldest day in the past 20 years was 3.7°C, significantly cooler than this year's low. The average humidity over the last 20 years was slightly higher at 68.8%, and cloud cover was also higher at 58.7%. The number of summer days and heat days over the last 20 years were 1051 and 338 respectively, indicating that this season had fewer such days compared to the recent past.

Looking at the long-term records since 1865, the average temperature for summer seasons has been 18.24°C, which is notably lower than both the recent season and the 20-year average. The historical maximum temperature reached 39.0°C, higher than this year's peak. The coldest recorded summer temperature was 1.1°C, much lower than this year's minimum. Historically, the average humidity has been higher at 72.1%, and cloud cover has been similar to the 20-year average at 57.7%. The long-term record shows 5619 summer days and 1507 heat days, indicating that this season's counts are relatively low.

In summary, this summer was warmer than the long-term average but consistent with recent decades. The season's wettest day was notable for its high precipitation, and while the number of summer and heat days was lower than in recent years, the overall weather patterns align with broader trends of increasing temperatures and sunshine.


Table 1: Comparison of Summer 2025 with Historic Seasons: Since 2000 and Before 2000
Parameter Current Season Since 2000 Before 2000
Max. Temp [°C] 36.0 38.6 39.0
Min. Temp [°C] 8.8 3.7 1.1
Avg. Temp [°C] 20.8 19.8 17.9
Avg Precipitation [mm] 331.6 263.1 263.1
Avg. Sunshine duration [h] 761.2 687.9 549.9
Avg. Summerdays (>=25°C) 58.0 49.0 31.0
Avg. Heatdays (>=30°C) 22.0 16.0 8.0
Max. consecutive days >25°C 5.0 12.0 17.0

Data source: Opendata Basel-Stadt

🤖 This text was generated with the assistance of AI. All quantitative statements are derived directly from the dataset listed under Data Source.