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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.

- 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

However, the amount of insolation hitting the surface of the Earth varies widely due to three factors:

- The angle of incidence
- The length of day
- Atmospheric obstructions
- We will discuss each of these next…

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

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°.

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

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