CLIMATOLOGY OF SOLAR RADIATION IN EUROPE

LOCAL EFFECTS

The changes in radiation are mostly zonal (latitudinal) on large scale. Effects due to permanent cloud cover can be observed in these climatological means. Orographic effects can be also observed that are due to the presence of mountains. These local effects are illustrated by the case of an area in the South-West of Europe, comprising several chains of mountains, including the Alps and Pyrenees (orography map ).

The following examples illustrate the orographic effects. The mountains may induce local changes of the cloud cover or shelter some areas from oceanic influences or others.

Observe large irradiation values on mountains in March (map for March)

Reliefs shelter some areas from frequent cloud coverage (map for July)

Map of global daily irradiation for March (about maps)

There is no change of radiation with latitude for the month of March in this area. The spatial distribution has a strong local component that is linked to the relief. The radiation is maximum in the South of Spain, on the high plateaus where daily irradiations of 5 kWh/m² can be observed. A local minimum is found in the local depression caused by the Ebro river. Values in the Pyrenees are of order of 4 kWh/m², much greater than those found on the Atlantic coast that are approximately 3.3 kWh/m².

The most striking feature in this map is the large irradiation found in the Alps, from Austria to Provence. Values of 4 kWh/m² are observed while there is only 2.8 kWh/m² in the Po plain. Such high values are also found in the Apennini in Italy.
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Map of global daily irradiation for July (about maps)

In July, the radiation has a strong local component, not always directly related to relief. Irradiation is maximum in the South of Spain, with values of approximately 7.2 kWh/m². As in March, a local minimum is found in the local depression caused by the Ebro river. Irradiation in the Pyrénées is greater than 7 kWh/m², superior by far to that found on the Atlantic coast: approximately 6 kWh/m².

The Massif Central shelters the South-East of France, and especially the Rhone river, from oceanic influences. The cloud cover in the South-East is less frequent than in the South-West. Consequently, the daily irradiation is greater in the former area. Hence, one may observe 7 kWh/m² in the Provence, 5.8 kWh/m² in the Aquitaine and 6 kWh/m² in the Massif Central.

The Venetia and the whole area, north of the Adriatic Sea, exhibit a large contrast with more southern areas. While 5.7 kWh/m² are observed in Venetia, values greater than 7 kWh/m² are found in the south. The Alps and the Appennini limit the extension of the cloud cover and protect the southern areas from wet air. Finally, one may note the micro-climate with respect to solar radiation in Vendée - Poitou.

The map above displays the direct component of the daily irradiation, that is the irradiation that is due to the radiation when viewing directly the sun, without taking into account the diffuse radiation originating from the sky vault. The global irradiation is the sum of the direct and diffuse components.

The local component of the direct irradiation is strong. The features are the same than in the map of the global irradiation with more contrast. This is due to the fact that the diffuse irradiation exhibits a very smooth spatial distribution with low local changes.

The limits of the Mediterranean climate in France are highly visible in this map. In Spain, the importance of the relief on irradiation is demonstrated. The Atlantic influence induces a frequent cloud coverage of the Spanish northern coasts and a low direct irradiation. The relief plays also an important role in Italy, where the influences of the Alps and the Dolomitiche combine to create a mosaic of micro-climates with respect to solar radiation.