**4. Discussions**

The results obtained confirm that the atmospheric constituents responsible for reflecting solar radiation back to space are clouds, aerosols and dust particles of different sizes, of which cloud is the chief (McIIveen,1992). When therefore an atmosphere is clear and clean, that is, cloudless and dustless, the values of Hr/Ho are expected to be relatively small and that of H/Ho to be relatively large. The implication of this is that, when the value of H/Ho is large, most of the radiation on such days is expected to reach the ground surface not deviated and not scattered, and the reflection of radiation to space on such days is expected to be small and mostly from the surface of the Earth, because the atmosphere is cloudless. In general, it can be said that reflection of radiation back to space by the planet would be mostly that of clouds and aerosol in the atmosphere. That is, the shortwave radiation reflection by the Earth's surface alone is comparatively small to that by its atmosphere. Thus it can be safely said that the atmospheric conditions that influence reflection of shortwave radiation back to space most are clouds and aerosol particles, particularly those of molecular size.

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

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 expected to rise (Babatunde et al, 2009).

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

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 of it at other locations.
