Horizontal measurement of global radiation on the rooftop of a weather station is generally hindered by the presence of obstructions surrounding the pyranometer. To investigate the sheltering effect, measured data from two weather stations in Taiwan, namely the Taitung (TWS) and Penghu (PWS) weather stations, were compared with corresponding in situ data measured under zero-shelter environments at nearby locations: the Taitung Center of National Open University (TCNOU) and the Penghu University of Science and Technology (PUST). The shelter view factor around the installed pyranometer was determined using a fisheye-lens image together with a calculation method based on a polar grid representation with sufficiently fine annuli. The shelter view factors for TWS and PWS were 11.8% and 5.0%, respectively. Comparisons of the monthly global radiation data measured at TWS and TCNOU and at PWS and PUST showed that underestimations of global radiation ranged from 1.8 to 9.1% (2016–2017) at TWS and from 1.3 to 4.2% (May 2015–December 2017) at PWS. These underestimations were primarily attributed to the magnitude of the shelter view factor for all obstructions around the pyranometer but were also dependent on the local pattern of global radiation (that is, beam and diffuse radiation), which is a climatological factor.
Chung et al. (Thu,) studied this question.