The Ice Saints — Mamertus, Pancras, Servatius, Boniface and Cold Sophie, observed each year from May 11 to May 15 — are rooted in the folklore that this five-day window brings the last cold snap of spring, with the risk of ground frost damaging crops and fruit blossoms. The Basel-Stadt temperature record, which stretches back to 1865, offers one of Switzerland's longest continuous climate datasets, making it possible to test this centuries-old belief against 160 years of measured data.

In 2026, the Ice Saints period in Basel-Stadt brought a minimum temperature of 4.5°C and an average minimum temperature of 5.8°C. No frost nights were recorded during this year's Ice Saints, meaning the mercury stayed well above freezing throughout the five days.

Looking back over the full record since 1865, frost during the Ice Saints has been a rare event. Only four years in the entire dataset recorded a minimum temperature below 0°C during the May 11–15 window: 1953, 1949, 1941 and 1935. The coldest Ice Saints on record was 1953, when the temperature dropped to -2.7°C, while the warmest was the year 2000, with a minimum temperature of 12.7°C. The historic average minimum temperature across the Ice Saints period stands at 8.3°C. The decade-by-decade trend shows that the Ice Saints have become progressively milder over time, with average temperatures rising steadily from the pre-1950 era to the post-2000 period.

Does the statistics confirm the folklore? The data suggests that mid-May has indeed historically been the period when the last spring frosts occur in Basel-Stadt. Before 1950, the peak frost risk date fell on May 3, while the last recorded frost during the Ice Saints occurred on May 14, 1935. Frost nights during the Ice Saints have historically been possible but not frequent — only 0.5% of all Ice Saints periods since 1865 have recorded a frost. The verdict is clear: the data confirms that mid-May is indeed the window when the last spring frosts historically occurred in Basel-Stadt, but frost during the Ice Saints has always been the exception rather than the rule.

Climate change has further weakened the signal. Before the year 2000, frost during the Ice Saints occurred in a small fraction of years; after 2000, no frost nights have been recorded at all during the period. The warming trend has accelerated in recent decades: the average temperature increase per decade for the Ice Saints period has been steeper since 2000 than over the full 160-year record. There has also been a notable calendar shift: the peak frost risk date has moved from May 3 before 1950 to May 6 after 2000, suggesting that the riskiest day for frost has shifted slightly later in the month.

In summary, mid-May remains the historical window for the last spring frosts in Basel-Stadt, but frost during the Ice Saints has always been a rare rather than regular event, and the signal has weakened considerably with rising temperatures. It is important to note that this analysis covers Basel-Stadt only — patterns may differ significantly in other regions of Switzerland, particularly at higher altitudes where frost risks persist later into the spring.