Fig. 2 shows the annual mean values of global irradiation recorded during 2006 and 2007. The zones with the highest values of irradiation (greater than 14 MJ m-2 day-1) are located along the south-western border of the Ourense province. This zone is characterized by low precipitations and relative humidity (Fig. 3); however, lowest average annual temperatures (Fig. 4a) compared to the rest of the region are due to the lowest cold temperatures in wintertime, that are not balanced by its highest hot temperatures in summertime; this is typical from this clearer sky zone than the rest of the region.
Fig. 4: Annual mean values of (a) temperature (°С) and (b) daily sunshine hours (hr) over the region.
On the other side, the lowest values of irradiation (less than 11 MJ m-2 day-1) are found in the northern edge of the region, in the southern edge (around Monte Aloia station) and near the stations of Ourense and Sergude. Typical foggy conditions over valleys close to rivers (Ourense and Sergude stations) or along coastal line (northern edge) reduce the sunshine hours, as it is shown on Fig. 4b), so irradiation is lower than in the rest of the region. At Monte Aloia mountain, high precipitation values are achieved (Fig. 3a), due to typical wintertime wet air masses from the Atlantic Ocean carrying rain clouds over this coastal mountain (Foehn effect) .
Annual clearness index, KT, was estimated in order to evaluate both the influence of local conditions and geographical coordinates over the solar irradiation. As it is observed on Fig. 5 compared to Fig. 2, KT distribution is quite similar to global irradiation map, with extreme values located at the same zones. Therefore, local conditions are the most important affecting the solar radiation that achieved the ground level.
Fig. 5: Annual mean values of clearness index (Xt) over the region.
This comparison shows that either high resolution solar irradiation maps or long term solar irradiation measurements are required to evaluate the solar resource in any location of this region.