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.
