Agriculture Reference
In-Depth Information
Table 3.1.
Brief description of the behaviour of the main microclimate parameters of the greenhouse in
winter, depending on the weather conditions (adapted from Berninger, 1989).
Greenhouse
Type of sky
Outside
Day
Night
Clear sky
Large difference between day
and night temperatures. High
solar radiation (especially
direct radiation). Low RH,
especially if it is windy. At night,
cold air, 'cold' sky
High solar radiation (direct
and diffuse). High
ventilation to limit tempera-
ture rise and avoid CO
2
depletion. High thermal
storage. High evaporation
Possible heating to
maintain temperature.
High RH (without
heating)
Cloudy sky
Stable temperatures. Weak solar
radiation, diffuse. High RH.
'Warm' sky
Weak solar radiation, diffuse.
Ventilation to limit the
confinement (high RH, lack
of CO
2
). Scarce thermal
storage. Low evaporation
Limited heating or may
be unnecessary,
except where there is
a high plant disease
risk associated with
high RH
descending warmer air masses partially
compensates for this cooling, resulting in a
higher outside temperature at ground level.
In the past, the 'radiative greenhouse
effect' was considered responsible for the
greenhouse microclimate, but nowadays the
importance of the convective effect has gained
prominence, due to the air confinement, so
the use of the expression 'greenhouse effect'
must refer to both processes, radiative and
convective (Papadakis
et al
., 2000).
Table 3.1 summarizes the behaviour of
the main microclimate parameters of the
greenhouse in winter.
and in its photomorphogenic effects (Baille,
1999), but also the insects and microorgan-
isms in the greenhouse.
On single-span greenhouses and on the
greenhouse sidewalls of multi-span green-
houses, an important part of the penetrating
light is lost through the sidewalls. Therefore,
the use of reflecting surfaces on the north
sides of greenhouses (in the northern hemi-
sphere) contributes to an increase in the
available light (Day and Bailey, 1999).
Equally, the use of reflecting surfaces
over the soil, to reflect the light not inter-
cepted by the crop, allows for an increase in
the light available for the crop.
3.3
Solar Radiation in Greenhouses
3.3.1
Introduction
The fraction of global solar radiation trans-
mitted inside the greenhouse is designated as
'greenhouse global transmissivity' (Zabeltitz,
1999). The limitations to productivity caused
by low levels of radiation inside the green-
house in autumn and winter in Mediterranean
coastal areas on vegetable crops, which are
highly light demanding, have been well doc-
umented (Castilla
et al
., 1999; Gonzalez-Real
et al
., 2003). Maximizing the radiation inside
the greenhouse is in fact a desirable objective
in all latitudes, especially during the autumn
and winter seasons.
The solar radiation conditions in the green-
house are very important from the point of
view of production, not just quantitatively
but also qualitatively. The first alteration
which the greenhouse causes on the micro-
climate parameters is a decrease in available
solar radiation (Fig. 3.2). The radiometric
characteristics of the greenhouse cover may
also significantly modify the quality of the
radiation (distribution spectrum or propor-
tion of diffuse radiation) affecting the crops,
mainly in the efficiency of use of radiation
