Agriculture Reference
In-Depth Information
Sun
5800 K
50%
Far IR
1
3
50%
Far IR
300 K
2
Fig. 3.1. The greenhouse effect. It is complemented by the shelter or confinement effect of the air inside
the enclosure (see text). 1. The plastic lets a large part of the solar radiation pass through it (it transmits).
2. The greenhouse surfaces and the plants absorb the solar energy and re-emit (far IR) energy. 3. The green-
house cover absorbs the energy (far IR) and re-emits it from its two sides, inwards and outwards.
greenhouse and is absorbed by the plants
and soil. The plants and the soil are heated
and re-emit energy, mostly with wavelengths
of 10 mm but ranging from 2.5 to 25 mm (far
IR range), according to Wien's law (because
the temperature is about 300 K). This energy
re-emitted by the plants and the soil is inter-
cepted by the covering material (as the mate-
rials used are usually opaque to IR radiation),
which is reheated and re-emits energy in
turn outwards and inwards in similar pro-
portions (Fig. 3.1). The greenhouse air is
then heated, as it is confined and it is not
renewed with outside fresh air.
These phenomena generate a tempera-
ture increase that is very evident during the
daytime, in relation to the outside. This effect
will vary depending on the specific condi-
tions of transmission and absorption of the
cover to radiation and depending on the ven-
tilation and airtightness of the greenhouse.
At night, the temperature gradient with
the outside is the result of a complex bal-
ance influenced, mostly, by the sky temper-
ature and the temperature under the cover
and the air exchanges between the green-
house and the outside.
In an unheated greenhouse 'thermal
inversion' may occur under certain condi-
tions, depending on the cover type. For
example, on nights when the sky is clear the
energy losses in IR radiation to the atmos-
phere are very high (see Fig. 2.14). If the cov-
ering material is permeable to such radiation
(as is the case with standard polyethylene
(PE) films), on nights with no wind it may
happen that the immobility of the air inside
the greenhouse can cause a higher decrease
in temperature than outside, resulting in
thermal inversion. In other words, although
a similar cooling process may occur outside,
the free air movement and mixing with
 
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