Assessment of Sunshine Duration for Various Time Resolutions Based on Pyranometric Data (An Example from Temperate Transition Climate of Central Europe)

Sunshine duration (SD) is an essential meteorological variable. It represents the sum of time for which direct solar radiation with an intensity above 120 W∙m−2 reaches the Earth’s surface. In the contemporary observational routine, automatic electronic devices are in use. The pyranometric method based on global solar radiation measurements (Kglob) is also proposed by the WMO to assess SD. The aim of the paper is to study the accuracy of the Slob–Monna method (SD-WMO), recommended by the WMO to calculate sunshine duration. Alternatively, the author’s method, which is based on the Ångström clearness index (SD-ACI), was used to approximate SD. For this purpose, a two-year series of SD and Kglob observations at four locations in Poland (well representing the Central European transitional climate zone) was analyzed. The result shows that, for SD-WMO, sunshine duration values are on average 16% higher than observed ones. For the SD-ACI method, they are only 5% higher. When verifying the accuracy of SD-WMO and SD-ACI approximations, we have found that, both for daily and monthly periods, the calculated SD sums are closer to the observed ones in the case of SD-ACI than for the SD-WMO method. The correlation coefficients are, respectively, 0.98 and 0.82 for daily sums and 0.99 and 0.88 for monthly sums.