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Clear day

Days on which the relative sunshine duration exceeds 80% are called clear days. If less than 20% of the total possible sunshine duration is achieved on a particular day, it is called a cloudy day.


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Swiss federal authoritiesSwiss federal authorities

The relative sunshine duration is the proportion of the measured sunshine duration in relation to the maximum possible sunshine duration at a given location. The relative sunshine duration depends on the weather (cloud cover). The maximum possible sunshine duration depends on one hand on the length of the day, i.e. on the season (greatest in the summer), and on the other hand, it is influenced by the amount and duration of shadow (e.g. from surrounding mountains) at the location in question.

The trend for the number of bright days saw an overall decrease between 1960 and 1980, while the number of cloudy days increased. Since 1980, there has been an increase in the number of clear days. This trend is probably strongly related to air pollution, which increased worldwide until the 1980s and has since decreased thanks to stricter laws.