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CLIMATOLOGY OF SOLAR RADIATION IN EUROPE

INTRODUCTION

The solar daily irradiation expresses the quantity of energy coming from the sun and received on a surface unit during a day. Usual units are J/m² or J/cm² or Wh/m² or kWh/m².

Daily irradiation depends upon many factors, and especially the cloud cover, day length and the angular height of the sun above horizon (sun elevation).

The day duration and the sun elevation depend on the sun-earth geometry. They are function of the latitude, day and hour only. More details on sun elevation.

The cloud cover is more variable and depends on the local meteorological conditions. It creates the local variations of the irradiation, except for shadow effects that can be observed in very steep relief. Other parameters intervene, such as the aerosols and relative humidity. The former scatter the radiation and may increase or decrease the radiation available at ground level. The water vapor absorbs the radiation. The greater the relative humidity, the smaller the irradiation.

The combination of these parameters produces the variability of the irradiation in space and time. The climatology of the daily irradiation depicts these variations at large scale.

The climatology of the solar radiation is illustrated by a series of maps for Europe and the Mediterranean Basin (how to read the maps). Each map shows the the monthly mean of the daily irradiation. The monthly mean has been computed over ten years (1981-1990), that is each map shows the mean of the daily irradiations observed during the same month for the ten years. These maps were obtained by combining the measurements made at ground level by the meteorological stations and those made by satellite.