## 1 Introduction
This occurs when the solar disk suddenly becomes obscured by rapidly moving clouds with no sunshine recorded by the automatic instrument, but the burning of a trace on the CS sun card continues. It was also found that this 'overburning effect' is at its highest intensity in the presence of Cumulus Congestus clouds [12; 23]. On the other hand, SD recorded by CSD3 sensor can be significantly reduced by high clouds such as Altostratus or Cirrostratus. [PERSON] et al. [11] found also that the larger differences between CS and automatic sensors in Austria were attributed to the higher sensitivity threshold of the CS instrument. [PERSON] [15], based on parallel measurements of CS and CSD3 at 16 sites in England, has proposed a quadratic relationship between the two instruments. The Angstrom-Prescott model was checked by [PERSON] et al. [14] at 10 stations in Spain using CS and CSD3 Kipp & Zonen sensors. The results of this study showed that the Kipp & Zonen sensors can be considered to be suitable for measuring sunshine duration and can provide a significant reduction in the statistical errors when solar radiation is estimated. [PERSON] and [PERSON] [10] found strong relationships between CS and CSD1 automatic sensor at five locations in south-western Poland. The estimated mean differences in daily totals of sunshine duration were \(\pm\)0.3 h. The differences were both positive and negative with no pattern of occurrences. Comparing CS and CSD1 in England, [PERSON] and [PERSON] [24] found that the CS recorder overestimates the sunshine due to the spreading of the burn on the card. This occurs in cases when cloud cover is broken, and the sun is high in the sky. They showed that the overestimation of the sunshine was about 20% in summer and 7% in winter. This seasonal variation was attributed mainly to the influence of solar elevation. [PERSON] et al. [25] have investigated the relationship between the burn width of CS and direct solar irradiance using digital image machine for the Girona station, a Mediterranean site in Spain. This semiautomatic method showed very good results as far as SD measurements. [PERSON] et al. [18] analyzed the daily SD data of 13 stations in Germany using CS sunshine recorders and automatic measurements using SONIe instrument. They found that the manual observations are larger during the summer, especially in cases of frequent alterations between sunny and cloudy conditions. [PERSON] and [PERSON] [16] have developed algorithms for the estimation of sunshine duration using pyranometers and pyheliometers. The uncertainty in the yearly sums was estimated to be around 0.5%. [PERSON] et al. [21] have developed a model to estimate the turbidity of the atmosphere based on SD from CS instruments. Similarly, [PERSON] et al. [26] have developed a method to estimate aerosol optical depth from SD measurements. Therefore, by means of the CS SD records it is possible to create a time series of atmospheric aerosol going back to the starting point of sunshine duration measurements at the given location. [PERSON] et al. [17] have demonstrated that the Jordan sunshine recorder was compatible with those from the DFC2 photoelectric sunshine meter and the Kipp & Zonen CMP11 pyranometers under all sky conditions, but they were considerably different from those observed by CHP1 pyrheliometer. They showed that the potential influencing factors on the measurements are the solar zenith angle, the spectral range of the automatic sensors, the relative humidity, and the sky conditions. On clear days, the SD observed by the Jordan sunshine recorder was generally less than the DFC2 photoelectric sensor and the CHP1 and CMP11 thermoelectric radiation meters. However, with an increase in cloud cover and the appearance of low- or medium-level clouds (cumulus, cumulonimbus and altocumulus clouds), the Jordan sunshine recorder overestimated the SD due to the 'overburning effect'. Finally, the estimation of daily SD from satellite-based data is the subject of study for [PERSON] et al. [9] in Polland.