Agriculture Reference
In-Depth Information
All the energy contained in the electro-
magnetic spectrum travels at the speed of
light and it is called radiation. It includes
the cosmic rays, gamma rays, X-rays, ultra-
violet (UV), visible light (blue, green, yel-
low and red), infrared, radar and radio and
television waves (Plate 1).
The solar energy, outside the Earth's
atmosphere, changes very little and it is
called the solar constant, but when crossing
the atmosphere the radiation is partially
reflected, absorbed or dispersed and suffers
quantitative and qualitative modifications.
The energy losses depend on the thickness
of the atmosphere crossed and its character-
istics (moisture content and gases, turbidity,
cloudiness) (Fig. 2.6). The higher the Sun is
over the horizon, the more energy reaches
the Earth's surface. The maximum is
received at noon when the Sun is in the
point of maximum elevation and the sky is
clear (Fig. 2.7).
With clear sky, the amount of solar
energy which reaches a point of the Earth's
surface depends on the Sun position, which
varies with latitude, season and time of the
day, besides the cloudiness and turbidity.
The Sun's position at any moment is given
by its coordinates: Sun elevation (
h
) and
geodesic azimuth (g). The geodesic azimuth
is the angle (from 0° to 360°) of a certain
direction in a horizontal plane, measured
from the south direction (0°), following the
movement of the clock hands (south-west-
north-east) (Fig. 2.7). The topographical azi-
muth, used in topographical operations, is
the angle measured from the north direc-
tion, following the movement of the clock
hands. The zenith is the point of the sky
located in the vertical that passes through
the observer's head located at a certain
point. The zenith angle (
q
) is formed by the
vertical at the zenith and the line formed by
the solar rays (Fig. 2.8), so that: (
h
+
q
= 90°)
(Dufie and Beckman, 1980).
In mid-latitudes the solar rays impinge
more vertically in summer than in winter,
implying that the intensity of radiation is
higher in summer (Figs 2.8 and 2.9). Besides,
the days are longer in summer, thus the total
amount of solar radiation received each
day is higher than in winter (Fig. 2.8) as the
Sun covers a larger apparent trajectory
(Fig. 2.10).
Solar
rays
AB
Solar
rays
H
37°N
C
D
37°N
H´
Winter
Summer
BAH Measures the maximum solar elevation for
DCH´ the observer located at A and C, respectively
Angles
AH: Defines the horizontal (for the observer at A)
CH´: Defines the horizontal (for the observer at C)
Fig. 2.6.
In winter, the solar rays must cross a greater thickness of the earth's atmosphere (doing it more
obliquely, stretch AB) than in summer (stretch CD). The figure represents conditions at noon (the moment
at which the thickness of the atmosphere to be crossed is lower).
