one time varies from place to place because Earth presents a
spherical surface to insolation. Therefore, only one line of latitude
on the Earth’s rotating surface can receive radiation at right
angles, while the rest receive varying oblique (sharp) angles
( â—Ź Fig. 3.15a). As we can see from Figure 3.15b and c, solar
energy that strikes Earth at a nearly vertical angle renders more
intense energy but covers less area than an equal amount striking
Earth at an oblique angle.
The intensity of insolation received at any given latitude
can be found using Lambert’s Law, named for Johann Lambert,
an 18th-century German scientist. Lambert developed a formula
by which the intensity of insolation can be calculated using the
sun’s zenith angle (that is, the sun angle deviating from 90° directly
overhead). Using Lambert’s Law, one can identify, based
on latitude, where greater or lesser solar radiation is received on
Earth’s surface. ● Figure 3.16 shows the intensity of total solar
energy received at various latitudes, when the most direct radiation
(from 90° angle rays) strikes directly on the equator.
In addition, the atmospheric gases act to diminish, to some extent,
the amount of insolation that reaches Earth’s surface. Because
oblique rays must pass through a greater distance of atmosphere
than vertical rays, more insolation will be lost in the process. In
1854, German scientist and mathematician August Beer established
a relationship to calculate the amount of solar energy lost as
it comes through our atmospheric gases. Beer’s Law, as it’s called, is
strongly affected by the thickness of the atmosphere through which
the energy must pass.
Since no insolation is received at night, the duration of solar
energy is related to the length of daylight received at a particular
point on Earth (Table 3.2). Obviously, the longer the period of
daylight, the greater the amount of solar radiation that will be received
at that location. As we will see in our next section, periods
of daylight vary in length through the seasons of th
