The meteorological and climatic features of Galicia are monitored by a network made up by 93 weather stations, located all over the region covering the usual meteorological parameters. Ten — minute averaged data were collected for both monitoring and climatic studies. The network is managed by MeteoGalicia, the Galician weather service (www. meteogalicia. es). Currently, 25 stations provide the solar radiation measurements with Ph. Schenk first class pyranometers (Philipp Schenk GmbH Wien & Co KG, Wien, Austria) and Kipp & Zonen sunshine duration sensors (Kipp & Zonen, Delft, The Netherlands) located in the most representative sites of Galicia. These sensors are periodically cleaned and calibrated by specialized crew, following the manufacturer’s recommendations. Data are collected by dataloggers, sent to a central data server and stored in a database specifically designed for environmental data sets. Here, several filters are applied to ensure the data quality [6, 7].

Since the solar radiation network was gradually set up, eight stations count on a data set longer than five years. The main features of these stations are listed on Table 1, while geographical location is illustrated in Fig. 1.

In this work, the analysis has been focused on the trend of the values of global radiation, sunshine duration and clearness index KT. Other meteorological parameters that are relevant for the solar resource characterization (temperature, relative humidity and precipitation) have also been considered. Finally, monthly and yearly values of global solar irradiation measured in the meteorological stations have been compared with the results obtained by Vazquez et al. [4].

The clearness index has been evaluated by the ratio G/G0, where G is the ground measured irradiation and G0 is the extraterrestrial irradiation. For this purpose, a solar constant value of 1376 Wm-2 was adopted, as recommended by Davies [8]. Astronomical relationships were obtained following Iqbal [9].

3. Results