Info

Figure 3.2 Yearly sunspot numbers for the sun's visible surface for the period 1700 to 2000.

Sources: Reproduced by courtesy of the National Geophysical Data Center, NOAA, Boulder, CO. Data before ad 1700 courtesy of Foukal (1990) and Scientific American.

A EQUINOX B

, 20 MARCH m SFPT

A EQUINOX B

, 20 MARCH m SFPT

Figure 3.3 Perihelion shifts. (A) The present timing of perihelion. (B) The direction of its shift and the situation at 11,000 years bp. (C) The geometry of the present seasons (northern hemisphere).

Source: Partly after Strahler (1965).

4 per cent more radiation than today (Figure 3.3B). This same pattern will return about 10,000 years from now.

Figure 3.4 graphically illustrates the seasonal variations of energy receipt with latitude. Actual amounts of radiation received on a horizontal surface outside the atmosphere are given in Table 3.1. The intensity on a horizontal surface (Ih) is determined from:

Ih = I0 sin d where I0 = the solar constant and d = the angle between the surface and the solar beam.

3 Altitude of the sun

The altitude of the sun (i.e. the angle between its rays and a tangent to the earth's surface at the point of observation) also affects the amount of solar radiation received at the surface of the earth. The greater the sun's altitude, the more concentrated is the radiation intensity per unit area at the earth's surface and the shorter is the path length of the beam through the atmosphere, which decreases the atmospheric absorption. There are, in addition, important variations with solar altitude of the proportion of radiation reflected by the surface, particularly in the case of a water surface (see B.5, this

Table 3.1

Daily solar radiation

on a horizontal surface outside the atmosphere: W irr

-2

Date

90°N

70

50

30

0

30

50

70

90°S

2I Dec

O

O

86

227

4IO

5O7

5I4

526

559

2I Mar

O

I49

2SO

37S

436

37S

2SO

I49

O

22 June

524

492

4S2

474

384

2I3

SO

O

O

23 Sept

O

I 47

276

373

43 O

372

276

I 47

O

Source: After Berger (1996).

Source: After Berger (1996).

90'S

90'S

Figure 3.4 The variations of solar radiation with latitude and season for the whole globe, assuming no atmosphere. This assumption explains the abnormally high amounts of radiation received at the poles in summer, when daylight lasts for twenty-four hours each day.

Figure 3.4 The variations of solar radiation with latitude and season for the whole globe, assuming no atmosphere. This assumption explains the abnormally high amounts of radiation received at the poles in summer, when daylight lasts for twenty-four hours each day.

chapter). The principal factors that determine the sun's altitude are, of course, the latitude of the site, the time of day and the season (see Figure 3.3). At the June solstice, the sun's altitude is a constant 23M° throughout the day at the North Pole and the sun is directly overhead at noon at the Tropic of Cancer (23M°N).

4 Length of day

The length of daylight also affects the amount of radiation that is received. Obviously, the longer the time the sun shines the greater is the quantity of radiation that a given portion of the earth will receive. At the equator, for example, the day length is close to 12 hours in all months, whereas at the poles it varies between 0 and 24 hours from winter (polar night) to summer (see Figure 3.3).

The combination of all these factors produces the pattern of receipt of solar energy at the top of the atmosphere shown in Figure 3.4. The polar regions receive their maximum amounts of solar radiation during their summer solstices, which is the period of continuous day. The amount received during the December solstice in the southern hemisphere is theoretically greater than that received by the northern hemisphere during the June solstice, due to the previously mentioned elliptical path of the earth around the sun (see Table 3.1). The equator has two radiation maxima at the equinoxes and two minima at the solstices, due to the apparent passage of the sun during its double annual movement between the northern and southern hemispheres.

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Renewable Energy 101

Renewable Energy 101

Renewable energy is energy that is generated from sunlight, rain, tides, geothermal heat and wind. These sources are naturally and constantly replenished, which is why they are deemed as renewable. The usage of renewable energy sources is very important when considering the sustainability of the existing energy usage of the world. While there is currently an abundance of non-renewable energy sources, such as nuclear fuels, these energy sources are depleting. In addition to being a non-renewable supply, the non-renewable energy sources release emissions into the air, which has an adverse effect on the environment.

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Responses

  • rowan
    Why does amount of solar radiation absorbed vary with latitude?
    9 years ago

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