Figures 2 and 3 show the annual variation of global irradiation and sunshine hours for the stations considered in this work. Generally, they show a regular annual variation with a maximum between June and July and a minimum in December. The only exception is represented by the station of Pedro Murias, which has a maximum in May and a relative minimum in June. As expected, the highest value of global insolation is found at Ancares, due to its altitude and rural location. The evolution of the sunshine hours shows a similar trend, with maxima occurring in summertime and minima in winter.

Table 2 summarizes these results. Having a look at the mean values of insolation and sunshine hours, Corrubedo has the highest values. It should be noticed that this site, which is located at only 38 km west from Lourizan, has mean sunshine hours and insolation significantly higher than Lourizan, 1 hour and 2 MJ m-2, respectively. The two stations located in an urban environment (Lourizan and Ferrol) are characterized by similar levels of insolation and sunshine hours.

Fig.3: Monthly average daily sunshine hours for the stations analysed

On table 3 are represented the main features of the yearly and monthly mean values of temperature, precipitations and relative humidity. The trend of the temperature is similar in all the locations, with maxima in August and minima in February.

Table 2: Monthly and yearly averaged daily values of Insolation and Sunshine Hours in the stations. Mean Max Min Mean Max Min Station Insolation Insolation Insolation Sunshine Sunshine Sunshine (kJ m-2) (kJ m-2) (kJ m-2) Hours Hours Hours Ferrol 12446 20307 Jun 4069 Dec 5.6 8.5 Jul 2.7 Dec Lourizan 11898 20383 Jun 4194 Dec 5.7 9.1 Jul 2.8 Dec P. Murias 10511 16628 May 3754 Dec 4.7 6.5 May 2.3 Jan Alto Rodicio 12687 20962 Jun 4444 Dec 5.9 8.8 Jul 3.1 Dec Monte Aloia 12545 20697 Jun 5013 Dec 6.0 8.5 Jul 3.6 Jan Corrubedo 13842 22807 Jun 5185 Dec 6.6 9.6 Jun 3.5 Dec Ancares 13805 23748 Jul 5264 Dec 6.0 9.7 Jul 3.3 Dec Sergude 10629 18562Jul 3809 Dec 5.3 8.1 Jul 2.3 Dec

As it was already mentioned, mean precipitations are higher than the rest of Spain [10]. The highest values are found in Monte Aloia, due to the westerly air masses with high water content that

channel into the outfall of Mino river and ascend to the slope of the local mountain range, Sierra do Galineiro. In all the stations, maxima occur in October and minima in summertime.

Ferrol	Lourizan	Murias	Rodicio	Aloia	Corrubedo	Ancares	Sergude
14.2	14.8	13.8	10.2	12.7	14.8	8.4	13.5
23.7 Aug	26.3 Aug	22.5 Aug	22.7 Jul	24.1 Aug	23.3 Aug	20.1 Aug	25.9 Aug
7.2 Feb	5.8 Feb	6.4 Feb	2.3 Feb	5.4 Feb	8.6 Feb	0 Feb	4.3 Feb
82	77	88	84	79	82	79	84
85 Oct	83 Dec	92 Jun	91 Nov	87 Oct	88 Oct	87 Oct	90 Jan
79 Apr	73 Jun	84 Apr	77 Aug	75 Aug	72 Jun	72 Jun	78 Aug
1225	1574	918	1108	2097	1032	1406	1409
211Oct	331 Oct	131Oct	220 Oct	465 Oct	150 Oct	230 Oct	261 Oct
45 Aug	46 Jul	39 Jun	41 Jul	62 Jul	30 Jun	44 Aug	24 Aug

Table 3: Monthly and yearly averaged daily values of Temperature, Relative Humidity and accumulated Precipitations in the stations analysed.

Clearness Index: averaged values and frequency distribution

The monthly averaged daily data of KT are depicted in Fig. 4 for five stations. It can be noticed that the station of Ancares is the one with the highest values of the index all over the year; it can be explained because of its altitude, rural environment and relatively cloud-free atmosphere. These values have a maximum in summertime.

The lowest clearness indexes are found in Pedro Murias and Lourizan. The first station, located in a rural environment, is characterized by high values of relative humidity that produce, together with low values of temperature (Table 3) and persistent fogs, especially in summertime. The station of Lourizan, located in an urban environment, is characterized by the lowest values of relative

humidity (77% yearly average) and the highest value of temperature (table 3). These features suggest that low values of KT are produced by anthropogenic aerosols, generated by local factories and urban pollution.

The station of Ferrol, despite of its suburban location, close to a sea port, has values of KT higher than Lourizan, due to the presence of winds that clean up the atmosphere from aerosols and fogs.

3.1 Ground data vs. satellite observations

Yearly and monthly averaged daily values of global irradiation collected in the meteorological stations were compared with the averaged values derived from satellite images and collected in the Solar Atlas of Galicia [4].

Figure 5 shows, for the stations analyzed, the distribution of the yearly averaged daily values measured by pyranometers at five stations versus the estimated values by Vazquez et al. [4]. Global irradiances estimated from satellite images overestimates the data collected by the meteorological stations in every location, except in Ancares. In three cases (Ancares, Ferrol and Corrubedo) the agreement between ground and satellite is quite good, with relative differences less than 3% (Table 4). In the other sites, differences are higher, ranging from 10.7% (Alto do Rodicio) to 25.3% (Sergude). Relative differences in the monthly averaged daily values are greater, up till 76%. Mainly, overestimations by ground measurements occur during winter.

Fig. 5: Distribution of the yearly mean values of global irradiation for the stations analyzed

The reason for these disagreements may be found in the normalization of the satellite data adopted by Vazquez et al. [4]. To obtain monthly averaged daily values, the authors apply only one coefficient for the entire region, instead of dividing the territory in areas characterized by similar climatologic features and calculating different normalization coefficients for each area. In their work, Vazquez et al. [4], compare their results only with one station, obtaining a very good agreement. For that, they assume the goodness of the results achieved for the entire region that is, as previously stated, characterized by complex topography and high climatologic variability.

Table 4: Relative differences (satellite-pyranometer) between the yearly and monthly averaged daily values of global irradiation in the location analysed. Month Ferrol Lourizan Murias Rodicio Aloia Corrubedo Ancares Sergude Jan -3.9 8.7 16.2 -2.4 3.7 -5.2 -23.6 24.7 Feb -5.9 -7.7 9.2 -9.0 1.3 -17.7 -23.6 42.2 Mar 13.5 44.0 23.2 22.0 36.6 18.7 10.4 32.2 Apr -3.06 19.5 12.7 13.5 13.4 1.0 1.8 36.4 May 0.8 19.9 12.6 13.6 11.8 -0.2 -1.4 23.4 Jun 2.8 13.0 24.2 15.1 13.1 5.8 -0.2 29.7 Jul 6.9 34.8 21.2 18.4 18.9 9.5 -4.5 16.4 Ago -1.5 8.4 18.4 10.3 13.7 3.0 -8.5 12.2 Sep 2.9 -4.9 19.9 4.9 8.5 7.3 -1.8 21.9 Oct 6.4 3.6 75.9 -10.6 15.4 0.6 6.2 22.4 Nov 14.3 26.8 25.5 24.6 5.8 -16.1 1.1 19.3 Dec 6.2 11.6 15.1 1.3 0.5 -6.3 -17.9 18.1 Avg 2.7 15.0 18.2 10.7 13.4 2.7 -2.2 25.3

4. Conclusions

The evaluation of the solar resource in eight sites of Galicia has been carried out for the period 2001-2006. The analysis of global irradiation, sunshine hours, clearness index, together with other meteorological parameters — precipitations, relative humidity and temperature — allowed a characterization of the solar resource in this region.

Monthly averaged daily values of global radiation and sunshine hours point out the complex climatology of the Galician territory. In an area of less than 30.000 km2, distributed over 2° of latitude and longitude, differences in the yearly mean daily values of global irradiation of more than 3 MJm-2 per day were found between the locations considered. This range of variability in the values of global irradiation is comparable to that of Germany (Meteonorm; Bern, Switzerland).

In the same way, yearly averaged daily values of sunshine duration show differences of almost 2 hours per day between the sites.

The analysis of precipitations, temperature and relative humidity, combined with the study of solar radiation, evidence the presence of persistent fogs in particular zones of Galicia. The station of Pedro Murias represents the most remarkable example. In the same way, the analysis of the solar radiation combined with the mean values of precipitations points out that extremely rainy areas, as that represented by the station of Monte Aloia, are not necessarily associated with low insolation.

The complexity of the Galician climate comes also out analysing the mean values of the clearness index: while the highest values are found in high elevation sites, as a consequence of a clearer atmosphere, the lowest values occur in Lourizan and Pedro Murias due, respectively, to the urban pollution and to persistent fog episodes.

The comparison of solar irradiation ground measurements vs. satellite observations gives the opportunity for making several remarks. The solar irradiation maps obtained by Vazquez et al. [4] evidence two different trends: a) the incoming solar radiation increases with the decreasing latitude and b) coastal zones receive more radiation than the inner nearby areas.

The ground measurements of global irradiation recorded in eight sites of Galicia depict a much more complex pattern not so easy to generalize. The latitude effect seems to be inexistent or, at least, is masked by more important local climatologic factors. Thus, Corrubedo and Lourizan, being at the same latitude, show yearly averaged daily differences of 2 MJ m-2. The same

differences are found in Ferrol and Pedro Murias, that have similar geographical features. In these areas, Vazquez et al. found differences of 0.36 MJ m-2 per day.

The stations of Ferrol and Alto do Rodicio, located respectively in the coast and in the interior, have similar mean values of irradiation even they are separated by 1° in latitude. The highest levels of radiation are found in Corrubedo and Ancares, located at the same latitude, but respectively on the western and eastern edges of Galicia. All these facts drive to the conclusion that the distribution of the solar resource in a region such as Galicia cannot be explained without the support of other climatologic and topographic features.

The present work was intended for characterizing the solar resource in eight sites of Galicia over a period of 5 years. In the future, the data record will increase also due to the installation of 17 more first class pyranometers and 30 pyranometers with Photovoltaic sensor. This will allow a more detailed characterization of the solar resource in time and space.

Due to the complex topography and climatology of Galicia, this will not be enough to obtain solar maps of the region since, in these cases, interpolation techniques do not provide sufficiently reliable results [11]. However, data from this solar radiation network will represent a very important tool to validate and calibrate the methods to estimate the solar resource.

Acknowledgements

Meteorological dataset provided by MeteoGalicia (Xunta de Galicia) from its web page is acknowledged This work was partially funded by Galician R&D Programme under project 07REM02CT.

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