Agriculture Reference
In-Depth Information
In a similar way, the components of the
atmosphere emit far IR radiation (in all
directions), depending on the state of the
atmosphere (i.e. humidity, turbidity, etc.).
The wavelength of these types of radi-
ation ranges between 5000 and 100,000 nm,
although the highest emission takes place
between 8000 and 13,000 nm, as the temper-
ature of the emitting bodies normally range
between 0 and 30°C (equal to 273 and 303 K),
an interval called the 'atmospheric window'
(Rose, 1979) (Fig. 2.14), because within this
interval, and if the sky is clear, the atmos-
phere is very permeable to radiation. The IR
band between 2500 and 25,000 nm is usually
designated as medium IR (MIR).
The balance of the Earth's radiation,
which some authors call also atmospheric
radiation, is always negative at night (i.e. the
ascendant radiation surpasses the descend-
ant radiation) and it is generally negative
also during the daytime, if only the far IR
radiation is considered (Berninger, 1989).
the same solar radiation, it is colder by
the end of March than by the end of
September.
The measurement of the average tem-
perature of the outdoor air is performed
inside a meteorological box under normal-
ized conditions, protected from solar radi-
ation and with circulating air, located
between 1.5 and 2 m above the ground. It
ranges between a minimum (normally at
dawn) and a maximum (usually 1 or 2 h
after noon).
The temperature is normally meas-
ured in degrees centigrade or Celsius (°C).
In the Kelvin scale, the temperature is
expressed in Kelvin (K). In the Kelvin scale
T
(°C) =
T
(K) −273°C. The Fahrenheit scale,
which is much less used, expresses the
temperature in Fahrenheit degrees (°F).
The conversion is:
9
(2.2)
T
( F)
°= °+
T
( C) 32
5
5
[
]
(2.3)
T
( C)
°= °−
T
( F) 32
9
The measurement of the outside air
temperature in greenhouses is sometimes
performed over the cover (at several metres
height) which alters how representative it is
compared with normalized measurements.
The altitude implies a decrease of the aver-
age temperatures of around 0.6°C per 100 m
of elevation.
The 'actinothermal index' indicates
the night temperature by means of a ther-
mometer exposed horizontally in open air
(Berninger, 1989). This situation permits
radiative exchange without any limita-
tions. With dry weather conditions and
clear sky, without wind, the actinothermal
index during the night reaches a lower
value than the ambient air temperature.
This index is more representative of the
plant temperature than the air temperature
during the night.
2.6
Net Radiation
The net radiation is the difference between
the radiation flux received over a surface
and the radiation flux emitted from the
Earth's surface.
It involves the energy available to all
physical and biological processes. The glo-
bal solar radiation flux is always positive
during the daytime and zero during the
night, whereas the net radiation flux is neg-
ative at night (Rosenberg
et al
., 1983).
2.7.1
Air temperature
For each location, the air temperature var-
ies with the evolution of solar radiation,
in 24 h cycles and with changes caused
by the seasons of the year. The average
temperature follows, with a certain delay,
the evolution of solar radiation (Plate 3).
For instance, during the equinoxes, with
2.7.2
Soil temperature
The soil temperature determines the tem-
perature of the subsoil organs. The surface
