Relationship between daily relative sunshine duration and relative sum of direct irradiance at Tartu-Tõravere meteorological station in 1967–2008

Abstract

Solar radiation effects at any site depend strongly on the presence of direct sunshine and on solar elevation. The daily sum of direct irradiance S' on horizontal surface is a more appropriate measure of sunshine energy than the daily sunshine duration Sundur. The latter, however, is more frequently recorded. A long-term data set of both quantities at a typical Estonian rural site, at the Tartu-Toravere Meteorological Station (58°16'N, 26°28'E, 70 m a.s.l.) is examined on the monthly level. The used daily values are expressed as the ratios relative to normal cloudless conditions S'/S'(subscript clear) and Sundur/Sundur(subscript clear) as well as their ratio R=(S'/S'(subscript clear))(Sundur/Sundur(subscript clear)). The study was performed on the level of monthly totals and also of daily values meeting the condition Sundur/Sundur(subscript clear)＞0.1 integrated within each month. The minimum values of R, 0.80 for monthly totals and 0.75 for the amount of selected days, were found in July and August due to frequent convective clouds around noon. In March to September the estimated monthly relative direct irradiances using Sundur/Sundur(subscript clear) and R agreed with the measured ones within ±10% in more than 70% of cases and within ±15% in about 90% of cases.