The measurements used in this study were obtained from a
network of 12 stations equipped with Kipp & Zonen pyranometers
(types CM10, CM11 and CM21). The aforementioned pyranometers
comply with the specifications of the first class “High Quality” as
defined in the “Guide to Meteorological Instruments and Methods
of observation”, of the World Meteorological Organisation (WMO).
The installation of instruments started in 2011 in the frame of the
Hellenic Solar Network (HSNE) project [2] and was completed in
2012. The locations of the stations, shown in Fig. 1b and summarized
in Table 1, were selected to represent, as much as possible,
regions with different cloud coverage characteristics [20,58,59] to
account for the variability in solar irradiance due to regional
weather and cloud regimes. At all stations the field of view of the
pyranometers is unobscured by local obstacles, at least for solar
zenith angle (SZA) smaller than about 80. Before deployment in
the network stations, all instruments were first calibrated in the
laboratory by comparison to a standard instrument using a 150 W
Halogen radiation source. The measurements derived from the
network of pyranometers include one-minute data of GHI together
with the respective standard deviation of the values recorded
within each minute (usually between 50 and 60).