Influence of cloudiness on sunshine duration

摘要

This paper analyzes the influence of cloudiness and cloud genera on sunshine duration based on a very long (1884-2007) homogeneous daily nephological and sunshine duration data series for the City of Krakow. Quadratic regression was used to describe the relationship between sunshine duration and cloudiness. It has been shown that cloudiness affects sunshine duration the most in June and July, and the least in December, January and February. High clouds (Cirrus, Cirrostratus, Cirrocumulus) do not interrupt the recording of sunshine duration even when they completely cover the sky. Layered clouds such as Stratus and Nimbostratus, on the other hand, do not transmit solar radiation at all. The degree of influence of different cloud genera on sunshine duration changes very little from season to season and with respect to the position of the Sun over the horizon. When the Sun is positioned higher over the horizon, clouds are less able to weaken solar radiation, resulting in larger sunshine duration values. This is especially true with respect to the following clouds: Cirrus, Cirrostratus and Cumulus. It has been shown that daytime sunshine duration patterns can be a very good indicator of cloud type and cloudiness variability.