Lesson 12 Insolation Hess, McKnight’s Physical Geography, 10 ed. pp. 70, 80-84, and Fig. 4.17 on p. 78
Dec 16, 2015
Lesson 12Insolation
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, cont.
However, the amount of insolation hitting the surface of the Earth varies widely due to three factors:1. The angle of incidence2. The length of day3. Atmospheric obstructions
We will discuss each of these next…
Angle of Incidence
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, cont.
Angle of Incidence, cont.
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°.
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