There is a saying in Basel: “God must be a Basel citizen.” It reflects the playful belief that the weather during Fasnacht is usually pleasant, as if higher powers were looking after the celebration. But is that really true? We analysed 100 years of weather data to find out.

During the past 100 years, Fasnacht was not observed 12 times: from 1914-1918 and 1940-45 during the world wars, and in 2020 and 2021 during the COVID-19 pandemic. For the reference year 2025, the weather was warm. The average temperature was 8.3°C, with a minimum of 3.0°C and a maximum of 5.7°C. Total precipitation was 14.4 mm, falling over three days, and there were no clear days, indicating overcast conditions. This wet and cloudy weather contradicts the hypothesis of divine intervention for pleasant conditions, prompting a longer-term analysis.

Over the 100-year period, the long-term average temperature for Fasnacht is 2.6°C. The coldest year was 1956 with an average of -9.8°C, while the warmest was 2003 at 11.6°C. A clear warming trend is evident, with recent decades being significantly milder than earlier ones. The reference year 2025, at 8.3°C, was well above the long-term average. Figure 1 illustrates this temperature trend, and Table 1 lists years with the most extreme weather.

The average total rainfall during Fasnacht over the century is 4.5 mm. Dry years, with zero precipitation, occurred 37% of the time. The wettest year was 1970 with 24.3 mm of rain. No strong long-term trend in precipitation is visible, as shown in Figure 2. The data on cloudiness, inferred from clear days, shows considerable variability. These mixed results only partially support the local belief in consistently fair weather.

Snowfall during Fasnacht was measurable in 17 years. The most significant snowfall occurred in 1956, 1963, and 1986, each with three snow days. Snow events were more frequent in earlier decades, showing a visible long-term decline in recent years. Statistically, the saying about consistently good weather does not fully hold up, as the data reveals a climate of significant variability, though with a clear trend towards warmer and less snowy conditions.