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Lesson 12. Insolation 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.

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Lesson 12

Lesson 12

Insolation

Hess, McKnight’s Physical Geography, 10 ed.

pp. 70, 80-84, and Fig. 4.17 on p. 78


Insolation

  • 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 period

      • 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 constant

Insolation


Insolation cont

Insolation, cont.


Angle of incidence

  • Angle of incidence Earth varies widely due to three factors:: 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. Earth varies widely due to three factors:


Angle of incidence cont1
Angle of Incidence, cont. Earth varies widely due to three factors:


Length of day

  • 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


Atmospheric obstruction

  • 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 surface

  • 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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