**6. Summary of the chapter**

150 Solar Radiation

radiation back to space most are clouds and aerosol particles, particularly those of molecular

High values of reflectivity or reflectance indicate period of low altitude and thick clouds, and rains, dominating the sky. The large albedo values, therefore, in June to September must be mainly due to clouds. The implication of this is that there will be the possibility of poor performance of the solar energy systems, particularly solar concentrating devices, poor fruition of crops and plants and low surface temperature of the Earth during this period as most of the sunlight is sent back to space by reflection. According to the value of the albedo of this period, about 60% of the sunlight that strike the Earth-Atmosphere surface is

October to November is a transition period between rainy and dry seasons; it had the lowest average value of albedo of 0.404 (Table 2 above). This indicates about 40% of sunlight being reflected away back to space. This does indicate a period of little or no clouds to reflect radiation, little or no dust to scatter radiation back to space but enhances more sunlight reaching the ground surface; hence performance of solar energy devices is expected to be high, fruition of crops and plants to be enhanced and the Earth's surface temperature is

April-May period is another transition period between the dry and rainy seasons. Changes in the sky conditions were dynamic during this period as the variations of all the parameters were significantly high and frequent. It is therefore relatively cloudy and contained more of hygroscopic particles than dust. The next highest average value of albedo of 0.465 (Table 2 above) was recorded in this period. This value indicates less than half or about half of the sunlight being reflected back. The albedo of the period was however higher than that of the period termed, very dry, cloudless and with high concentration of the harmattan dust, this period is known to be, December to March, a period, with albedo of 0.447(Table 2 above). This analysis indicates that an atmosphere with low altitude and thick clouds will reflect

Since it is possible to use equation 3 to estimate the reflectance of a surface at a location, the values of it, obtainable at Maceio, Brazil (90 40'S, 350 42'W), of coordinates almost similar to that of Ilorin (80 32'N, 40 34'E), are compared. It has a value of 0.53 for H/Ho in the rainy season and 0.59 in the dry season. These correspond to, by computation, reflectance or albedo of 0.47 and 0.41 for the rainy and dry seasons respectively (De Sonsa et al, 2005). Brazil is covered with thick rain forest and also in the tropics, with clouds cover most of the time. These albedos are comparable to the ones obtained here at Ilorin. Hence this method of estimating albedo, though simple, may give a reasonable estimation

The daily and monthly variation patterns of the simulated shortwave reflection co-efficient, Hr/Ho, known as albedo, a surface phenomenon, were compared with the corresponding cloudiness index, Hd/H, an atmosphere phenomenon. They were both found to be mirror images of one another. While the shortwave diffuse radiation is toward the Earth's surface,

reflected back, and was not available to solar energy devices for operation.

more radiation than the scattering one, even with large dust concentration.

its mirror image, the shortwave reflection radiation is back toward space.

expected to rise (Babatunde et al, 2009).

of it at other locations.

**5. Conclusion** 

size.

The expression, Hr/ Ho = 1 – H/Ho, developed by Babatunde,(2003) and Babatunde et al,(2003), at Ilorin (8o 30' N, 4o 34' E), Nigeria was used to simulate short-wave (SW) reflected radiation Hr, and reflection coefficient, Hr/Ho. Hr/Ho was used to define total wavelengths surface albedo. The temporal variations of the simulated reflectance, Hr/Ho, the clearness index, H/Ho, and cloudiness index, Hd/H obtained for year 2000 were studied to establish any inter-relationship between them. It was observed, in the relationship between them, that the clearness of the atmosphere characteristically, is diametrically opposite to that of reflectivity of the atmosphere, while the cloudiness and reflectivity of the atmosphere have the same characteristics. It is thus observed, in the effects on solar radiation, while high value of clearness index will enhance the performance of solar energy devices on earth, high value of reflectance will adversely affect it.

The highest reflectance recorded was 0.644 at the peak period of cloud activity in August, and the lowest was 0.361 in November when it was relatively cloudless and dustless. It was deduced that, characteristically, shortwave solar radiation reflection is a mirror image of shortwave diffuse solar radiation, and that reflectance is a sort of cloudiness index.

The albedo deduced from the study, for the Earth–Atmosphere at Ilorin in 2000, ranged between 0.4 and 0.6. These values were consistent with the possible values of albedo of different surfaces on the Earth's surface. The above equation therefore, may be found suitable for estimating surface albedo at any other place on the Earth's surface.
