Open Data Insight of the Day

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. The weather for the reference year 2026 was very pleasant. The average temperature was 11.4°C, with a minimum of 3.1°C and a maximum of 7.9°C. Total precipitation was 0.5 mm, falling on just one day. The average cloud cover, derived from the single clear day, suggests predominantly cloudy conditions. This specific year's data supports the hypothesis of divine intervention, but the rest of the analysis will examine whether this holds true over a longer period.

Analysing temperature over the full century reveals a long-term average of 2.8°C. The coldest Fasnacht occurred in 1956 with an average of -9.8°C, while the warmest was in 2003 at 11.6°C. The data shows a clear warming trend, with recent decades being significantly milder than earlier ones. The reference year 2026, at 11.4°C, was substantially warmer than the long-term average. This trend is visually clear in Figure 1, which shows temperature at Carnival over the last 100 years, and the extremes are listed in Table 1.

Regarding precipitation, the average total rainfall during Fasnacht is 4.6 mm. Dry years, with zero precipitation, occurred in 37% of all observed years. The wettest year was 1970 with 24.3 mm of rain. No strong long-term trend in precipitation is evident, as shown in Figure 2. The long-term average cloud cover, inferred from clear days, indicates that overcast conditions are common. These findings suggest that while dry spells are frequent, the weather is often cloudy, partially contradicting the ideal of consistently sunny celebrations.

Snowfall has been a notable feature. Measurable snow occurred in 17 years over the century. The most significant snowfall years were 1956, 1963, and 1986, each with snow recorded on three days. Snow was more frequent in earlier decades, with a visible long-term decline in recent years. In conclusion, the statistical evidence does not fully support the Basel saying. While temperatures have become more favourable and dry spells are common, the weather is often cloudy, and historically, cold and snowy conditions were not unusual.