CLIMATOLOGY OF SOLAR RADIATION IN EUROPE

SEASONAL VARIATIONS

The daily irradiation exhibits seasonal variations. At regional scale, they are mostly due to the changes of the solar declination. The declination is the angle between the equatorial plane and the plane containing the movement of the Earth around the Sun. This angle varies from +23°27' (21 June) to -23°27' (21 December) with a period of one year. These variations induce changes of the sun elevation for a same geographical location. In June, the Northern hemisphere is the closest to the Sun. The sun is the highest above horizon for a same hour and the duration of the day is the largest. The daily irradiation is increased by this mechanical effect compared to the winter solstice in December.

The changes of the distance between the centers of the Earth and the Sun have much less influence on the daily irradiation. This distance varies by ± 1,7 % only. It is minimum on January 2 and maximum on July 2.

Irradiation is minimum in December (map for December)

Irradiation is maximum in June - July (map for July)

Observe the changes in radiation throughout the year (animation over the 12 months)

Map of daily irradiation for December (about maps)

The daily irradiation is varying strongly with latitude. Irradiation is maximum in the South, where values of 4,5 kWh/m² can be observed and decreases towards northwards. North of the latitude 64° N, the daily irradiation is quasi-null. Actually, the sun is almost always below horizon: at zenith, elevation is at most equal to 4°. This demonstrates that the main cause of the changes in radiation is the change in sun elevation that creates these latitudinal (or zonal) variations. On the average, the influence of the cloud cover is of lesser importance: it exhibits local effects that disturb the latitudinal distribution.
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Map of daily irradiation for July (about maps)

Like the other months, the irradiation varies strongly with latitude. It decreases northwards. Irradiation is large in South Algeria, where values of 8 kWh/m² can be found. Compared to winter, the sun is high above horizon at very high latitudes. Irradiation reaches 4 to 5 kWh/m² and these areas are sundrenched.

Zonal variations are less marked compared to winter. Because of the greater range of values found in Europe, the contrast between areas is larger and this enhances the disturbances of the zonal distribution. The influence of the cloud cover is clearly visible, especially along the Spanish Atlantic coast, in the Southeast of France, in the plain of the Po river, in Scotland and Northwest of Eire.

The Scandinavian mountains have a strong influence on the spatial distribution: the structures of the irradiation are parallel to these mountains. In general, one may note that all mountains influence the irradiation: the high plateaus of the Sahara and the Atlas, the mounts Zagros and Elburz in Iran, the mountains along the Red Sea, the Taurus mountains in Turkey, the Spanish mountains, the Pyrenees, the Appennini and the Alps in Damatia (Dinara Planin). This influence is not the same throughout the year. For example, the influence of the Alps is not noticeable on this map, while it is on the map of December.
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Changes in radiation throughout the year

The daily irradiation increases generally from January to June, then decreases from July to December for most locations seen in this map. It follows the cycle of the orbit of the Earth around the Sun. Some local effects can be observed that depart from this general rule.