Temperature - Agroecology: The Ecology of Sustainable Food Systems - page 61

Atmosphere

D 2

Incoming solar
radiation

D 1

Incoming solar
radiation

A 1

Equator

FIGURE 5.1 The effect of latitude on solar gain. The higher the latitude, the greater the distance that solar radiation must travel
through the atmosphere ( D 2 > D 1 ) and the greater the surface area over which a certain amount of solar radiation is spread ( A 2 > A 1 ).

S EASONAL V ARIATION

Earth's surface, as shown in Figure 5.1. In addition, the
sun's rays must pass through an increasingly thick atmo-
spheric layer at higher latitudes, resulting in a loss of
energy to reflection and scattering by materials in the
atmosphere, such as water droplets and dust. The overall
effect is a regular decline in the intensity of solar radiation
per square unit of surface as one moves away from the
equator. This latitudinal variation in solar gain is one of
the major causes of latitudinal variations in temperature.

Seasonal differences in temperatures over the surface
of the earth are the result of changes in the orientation
of the earth in relation to the sun as it revolves around
the sun on its tilted axis. Through the course of the
year, a belt of maximum solar gain or insolation moves
back and forth across the equator in relation to the angle
of incidence of the sun's rays and the length of the day.
Longer days lead to more solar gain. This swing in
insolation is the direct cause of a seasonal swing in
temperature. The degree of seasonal variation in aver-
age temperatures increases with increasing distance
from the equator.

A LTITUDINAL V ARIATION

At any latitude, as altitude increases, temperature
decreases. On the average, for each 100 m of elevation
gained, ambient temperature drops approximately 0.5°C.
In locations where increased cloud cover during the day
is associated with this elevation gain, temperature differ-
ences can be even greater due to reduced solar gain. At
the same time, the increasing thinness of the atmosphere
at higher altitude results in a greater loss of heat from both
the soil surface and the air just above it by reradiation at
night. This phenomenon contributes significantly to lower
nighttime temperatures at elevations much above the sea
level. In mountainous regions at high elevations in the
tropics (above 3000 m) and at progressively lower eleva-
tions as one moves toward the poles, reradiation at night
is so intense that wintertime temperature conditions are
encountered almost every night when the sky is clear.

Northern Hemisphere Winter
Southern Hemisphere Summer

Northern Hemisphere Summer
Southern Hemisphere Winter

FIGURE 5.2 Seasonal variation in the sun's angle of inci-
dence. The tilt toward the sun that occurs in summer increases
both the length of the day and the intensity of solar radiation
striking the ground.