Agriculture Reference
In-Depth Information
layers of the soil act as thermal and seasonal
heat sink, heating and cooling itself much
more slowly than the surrounding air.
Oscillations in soil temperature
throughout the year decrease with depth. At
several metres deep, the temperature of the
soil remains almost constant throughout the
year.
Low soil temperature (below 14°C)
may limit the growth of some crops
(Berninger, 1989). In greenhouses the soil
temperature is, obviously, higher than in
open air.
The wind velocity and direction pat-
terns are more or less predictable for a
certain location. For instance, in the coast
of Almeria, the highest wind velocity val-
ues range between 10:00 and 17:00 (solar
time) for the months from March to July
(Plate 4), it being usual to have winds above
2 m s
−1
during the daytime (Pérez-Parra,
2002). The dominant directions (Fig. 2.16)
are from the west (west-south-west) and
the east.
2.9
Composition of the Atmosphere
radiation and air temperature
From a crop production point of view, water
vapour is one of the most important param-
eters of the atmosphere. The water phase
changes (solid, liquid or vapour) involve
the transport of large amounts of energy,
which affects the crop and its surrounding
air temperatures.
The atmospheric humidity and the
availability of water, together with other
factors, determine the rate at which plants
transpire. The atmospheric air is never com-
pletely dry, containing some amount of
water in the form of vapour. The water
vapour content of air can be expressed in
several ways:
1.
By its concentration. This is the air abso-
lute humidity (AH) which is expressed in
kilograms of water vapour per kilogram of
dry air.
2.
By the water vapour pressure (
e
a
)
expressed in units of pressure. From a free
water surface in contact with the atmos-
phere part of the water molecules evapo-
rate and others return to liquid, so that
when those evaporating equal those return-
ing to liquid we say that the atmosphere is
saturated, a moment in which the partial
water vapour pressure has reached its sat-
uration value (
e
s
). The
e
s
value can be cal-
culated (see Appendix 1 section A.1.7).
The
e
s
value increases with temperature
(Fig. 2.17).
3.
By the relative humidity (RH) of the air,
expressed as a percentage of the partial
The average values of global radiation and
air temperature vary from month to month.
The values follow sinusoidal curves, with a
delay in the temperature curve with respect
to the radiation curve (Plate 3).
If radiation and temperature are repre-
sented in a
xy
graph, the sequence of
monthly values generates a climate diagram
with an elliptic shape (Fig. 2.15). The daily
differences in radiation and temperature
are large, especially between cloudy and
clear days.
Wind is the, mainly horizontal, displace-
ment of air. It is characterized by its direc-
tion and velocity. If measured at different
(standardized) heights, its velocity can
be calculated at intermediate heights
(Hellman's equation, see Appendix 1 sec-
tion A.1.6). The direction is measured with
a vane, and is expressed as the angle between
north and the direction of the origin of the
wind (in a clockwise direction).
The wind force is a basic consideration
in the design of a greenhouse structure. Its
direction and frequency are important when
considering static (passive) ventilation. Its
velocity is related to air renewal, even when
the greenhouse is closed, and to the energy
losses.
