The welcome spate of good weather in autumn known as an Indian summer is one of the best-known regular weather patterns. In the lowlands, the sun can be obscured by mist or low stratus even in fine weather, which then attenuates the effects of an Indian summer.
A regularly occurring weather pattern or singularity is defined as a deviation from the average annual course of meteorological elements that occurs on certain days in the annual calendar with reasonable regularity. The Indian summer phenomenon is most evident in mountainous areas above the blanket of stratus. One ideal measuring station for climatologically recording this weather pattern is Davos, where records of daily sunshine hours stretch back more than a century. The metric used for identifying the phenomenon is the frequency of fair (fine) weather days. MeteoSwiss defines a fair weather day as one on which the relative sunshine duration is higher than 80 percent. In other words, on a fair weather day, the actual sunshine hours recorded correspond to more than 80 percent of the absolute possible sunshine hours from sunrise to sunset for that day.
When we examine the frequency of fair weather in the months of September and October in Davos, it becomes clear that the Indian summer is a phenomenon encountered in the middle of October. Looking at the entire period of 1901–2020, fine weather occurred most frequently on the days between 12th and 17th October. Indian summers were most prominent during the period of 1901–1960.
Even in today’s climate, Indian summers in October still occur, albeit not to the extent seen in the first few decades of the 20th century. In the climate period of 1991–2020, the 13th, 22nd, 25th, 26th and 27th October stand out as having a greater frequency of fair weather. On the other hand, what was once typically a fair weather period in the middle of October is no longer seen in the analysis of the 1991–2020 climate period.
The origin of the German term for an Indian summer – “Altweibersommer” (sometimes translated as old wives’ summer) – is not discussed in the literature. According to Lutz Röhrich’s dictionary of proverbial idioms, the term comes from the spider threads that are seen floating in the air in autumn. In Old German, “weiben” meant to weave, with reference to spider webs. According to popular belief, these cobwebs were also thought to be the webs of elves, dwarves and other mythological figures. According to the Duden dictionary, the term “old” is understood to mean “late”. The Altweibersommer is therefore late summer or post-summer.