Agriculture Reference
In-Depth Information
2
2.1
Introduction
which determines the Earth's decline (see
Appendix 1 section A.1.1).
The local climatic conditions, to a large
extent, determine the microclimate inside a
greenhouse and its future management;
therefore knowledge of the prevailing cli-
matic conditions is necessary before design-
ing and building a greenhouse.
The climate of a certain locality is the
result of the radiative exchanges between
the Sun and the Earth. In relation to green-
houses, the most important elements of the
climate are: (i) solar radiation; (ii) atmos-
pheric temperature and humidity; (iii) wind;
and (iv) rainfall.
The movement of the Earth around the Sun
determines the year's seasons. When the
plane perpendicular to the ecliptic which
contains the Earth's axis passes through the
centre of the Sun, which happens twice a
year, the summer and winter solstices occur
(Figs 2.2, 2.3 and 2.4).
Between the two solstices we have the
spring and autumn equinoxes (Fig. 2.5),
moments in which an imaginary line link-
ing the centre of the Earth to the centre of
the Sun is perpendicular to the Earth's
axis. During the equinoxes neither of the
two poles is inclined towards the Sun
(Fig. 2.5).
The winter solstice in the northern
hemisphere (Fig. 2.3) corresponds to the
summer solstice in the southern hemisphere.
At the winter solstice at noon the Sun
reaches an apparent elevation (maximum of
the day) of around 30° in the south of Spain
(latitude 37°N) and 90° in the Tropic of
Capricorn. On the summer solstice (Fig. 2.4)
the apparent maximum Sun elevation is
around 76° in the latitude 37°N (south of
Spain) and 90° in the Tropic of Cancer.
2.2.1
Introduction
The Earth's axis, around which our planet
turns on itself, maintains a fixed inclination
with respect to the plane of the Earth's orbit
around the Sun, called the ecliptic (Fig. 2.1).
The axis also maintains a fixed direction,
that is, the Earth's axis points continuously
to a fixed point in the sky. The 23° 27′ angle
formed by the Earth's axis and the perpen-
dicular to the plane of the ecliptic is called
the obliquity (angle) of the ecliptic (Fig. 2.1),
