No long-term climate signal
In earlier normal periods before 1991–2020, there is no discernible pattern showing a drop in relative sunshine duration at the beginning of July, either for the Swiss plateau, the north side of the Alps (Fig. 1), or the south side of the Alps (Fig. 2). We can conclude from this that chance, i.e. the selection of the analysis period, plays a significant role in the determination of a regularly occurring weather singularity. In other words, something that might feature very prominently in one normal period may no longer be apparent in the next period.
An object lesson for country lore
A parallel can be drawn here with country lore, which describes typical weather patterns at certain times of the year, based on historical experience. Because of the variability and changes in the climate, however, what may have been true in a previous era may not be applicable to today’s weather. Country lore can therefore become completely invalidated as time goes on.