Agriculture Reference
In-Depth Information
3
3.1
Introduction
effect' (convective effect), derived from the
decrease in the air exchanges with the out-
side environment, and which is perceptible
even in greenhouses that are very permeable
to the air; and (ii) an effect caused by the
existence of a cover with low transparency
to far IR radiation emitted by the soil, the
plants and all the inner elements of the
greenhouse exposed to sunlight (visible and
short IR radiation, to which this cover is
very transparent; Fig. 3.1). This second
effect is sometimes referred to as the 'radia-
tive greenhouse effect' or 'heat trap'.
According to Wien's law, the product of
the temperature of a radiant surface (in K)
by the dominant wavelength of the emitted
rays (in microns) is constant and equal to
2897 (K mm).
The average temperature of the Sun's
surface, assimilated to a black body, is
5800 K. The Sun emits radiation (ranging
from 0.3 to 2.5 mm), with a dominant wave-
length of 2897/5800 = 0.5 mm (i.e. in the
margin between green-blue and yellow of
the visible range).
The materials that cover greenhouses
are transparent to solar radiation, transmit-
ting most of it but the plants and the soil
absorb
Inside the greenhouse, the radiation, temper-
ature and composition of the atmosphere are
modified, and this results in a different micro-
climate from the one outside. The modifica-
tions depend essentially on the nature and
properties of the cladding material, the air
renewal conditions, the shape, dimensions
and orientation of the greenhouse, but also on
the plant canopy and the possibilities for eva-
potranspiration (Berninger, 1989).
This microclimate is not uniform and
varies from the centre to the borders of the
greenhouse, from the ground to the roof and
from the limits of the canopy to its interior.
The so called 'spontaneous climate' is
the one generated without important human
or energetic intervention, especially without
heating, forced ventilation or water spraying.
A greenhouse, normally, has a crop
which is irrigated and its soil is wet. An
empty and dry greenhouse is only of theo-
retical interest, as it is not representative of
real conditions.
a
large
amount
of
it,
of
all
wavelengths.
During the daytime, the majority of the
solar radiation passes through the cover of a
The 'greenhouse effect' is the result of two
different effects: (i) a 'shelter or confinement