Agriculture Reference
In-Depth Information
M
Sun's
trajectory
Noon
East
South
h
Observer
γ
A
North
West
Fig. 2.7. The Sun's position at any moment (M) of the day is given by its coordinates: geodesic azimuth
( g ) and solar elevation ( h ), for an observer located at A (adapted from Wacquant, 2000).
The relative differences of solar radi-
ation received between regions are high if the
latitudes are distant, especially in winter
(Plate 2).
For greenhouse crop production, the
most important factors are: (i) the total avail-
able solar energy; (ii) the duration of the day
and of the night; and (iii) the quality of the
radiation, because of its influence on the
photosynthesis and in photomorphogenesis
(Hanan, 1998).
Solar radiation acts on: (i) the plant,
affecting photosynthesis (intensity, quality
and amount of light), phototropism (rele-
vant role of the red and the blue light), pho-
tomorphogenesis (photoperiod, i.e. the
period of time per day that an organism is
exposed to daylight), transpiration (the
opening of the stomata), etc.; and (ii) the
energy balance of the greenhouse, affecting
the soil, water, air and plant temperatures,
and the temperature of other objects absorb-
ing radiation.
day of the year and time of the day. The cir-
cumsolar radiation represents the radiation
coming from the region close to the Sun. The
diffuse radiation comes from all directions of
the whole of the sky, due to reflections, devia-
tions and scattering caused by the clouds,
gases and aerosols present in the atmosphere
(Hanan, 1998). The sum of direct solar radi-
ation, circumsolar radiation and diffuse radi-
ation is called global solar radiation.
In practice, the circumsolar radiation is
considered together with the diffuse solar
radiation, because the measuring method
used integrates both of them (Day and
Bailey, 1999).
The cloudiness, turbidity and transpar-
ency of the atmosphere have a great influ-
ence on the proportions of direct and diffuse
radiation, as well as the solar elevation. On
sunny days, with a clear atmosphere the
percentage of direct solar radiation may
reach a maximum of around 90% of the glo-
bal daily radiation.
In urban or industrial areas, when the
Sun is clear the direct solar radiation pre-
dominates on many occasions only if the
Sun elevation is higher than 50°, due to the
influence of the air's turbidity, whereas in
coastal areas this predominance occurs at
30°, and at altitudes of 3000 m it occurs at
only 6°, due to the great transparency of the
air (Seeman, 1974).
2.4.2
Quality of solar radiation
The incident solar radiation is composed of
direct solar radiation, circumsolar radiation
and diffuse radiation. Direct radiation comes
directly from the solar disc, travels straight
and its direction is determined by the latitude,
 
 
 
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