Sunshine duration is calculated from the cloud cover, the angle of the sun and the sunrise and sunset times.
The actual duration may change because of local variations or time differences.

A precise measurement of sunshine duration is the solar radiation. Radiation is, however, often different from perceived sunshine duration. Slight high cloud cover can reduce  radiation substantially, but still create a perception of sunshine. On the contrary, a slight intermittent cloud cover is perceived as less sunshine time if temperatures are low, because of the cooling effect. 

More information on the subject will be added - watch our NEWS.