Lesson 12
Hess, McKnight’s Physical Geography, 10 ed.
pp. 70, 80-84, and Fig. 4.17 on p. 78
From lesson 11 we now know that the sun’s
direct rays strike the earth at different locations
throughout the year.
These differences give us our seasons and
influence the amount of average daily insolation
(incoming solar radiation)
◦ Average daily insolation is the rate or intensity of the
sun’s radiation that strikes the surface over a 24-hour
 Measured in watts per square meter (W· m-2)
 The average insolation hitting the Earth’s upper
atmosphere is ~1372 W· m-2. This is known as the solar
However, the amount of insolation hitting the
surface of the Earth varies widely due to three
1. The angle of incidence
2. The length of day
3. Atmospheric obstructions
We will discuss each of these next…
Insolation, cont.
Angle of incidence: the angle at which the Sun’s rays
strike the surface of the Earth (solar altitude)
◦ This can be directly related to the intensity of radiation that
reaches the surface.
Areas that have a high angle of incidence have a
given amount of radiation concentrated on a
small area
◦ Therefore, radiation is higher in intensity
While areas with a low angle of incidence have
that same amount of radiation concentrated on a
larger area
◦ Lower intensity radiation
Angle of Incidence
Angle of Incidence, cont.
Angle of Incidence, cont.
We all know that the length of daylight influences
how much solar radiation is received (e.g. longer
days generally mean warmer days)
Even if it is cloudy, longer days generally mean a
significant increase in solar radiation received
Take a look at Fig. 3 and Fig. 4 on pages 58 and
59. These provide the hours of daylight and
daily insolation, respectively, for location at the
equator, 45° N and 90 N°.
Length of Day
The amount of atmosphere that radiation has to
travel through affects the total amount of
radiation received.
◦ e.g. If the angle of incidence is low, then solar radiation
has to travel through more atmosphere, thereby
reducing the amount received when it finally reaches the
Water droplets (clouds) and other atmospheric
particulates also affect the amount received.
The percentage of solar radiation reaching
Earth’s surface through the atmosphere is listed
in Fig. 5
Atmospheric Obstruction
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