**1.1 Albedo**

140 Solar Radiation

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Reflection of radiation is one of the mechanisms by which solar radiation is depleted in the atmosphere and it is mostly done by clouds. By its definition, reflected radiation is lost to space completely(i.e.100%).

Albedo is related to reflection of solar radiation at a surface and therefore defined in terms of it, as the ratio of the reflected solar radiation to the incident solar radiation at the surface, i.e., Hr/Ho,in this chapter. It is assumed however that the reflected radiation, Hr, is both diffuse and specular in nature, that is, it is diffuse if the reflected radiation is uniform or isotropic in all angular directions, and specular if the surface of reflection is smooth with respect to the wavelength of the incident radiation such that the laws of reflection are satisfied (Igbal, 1983). It was said by Gutman (1988) that the observed albedo assumed that the radiation field is isotropic. The extraterrestrial radiation, Ho at the edge of the atmosphere, from the sun, is considered the incident solar radiation.

Albedo is also known as reflectance or reflectivity of a surface; by this, the surface albedo of the Earth is regarded the same as planetary albedo by many scientists (Igbal, 1983). Albedo, as a property of a surface, therefore, can be used to determine the brightness of a surface. According to Prado et al (2005), materials with high albedo and emittance attain low temperature when exposed to solar radiation, and therefore reduce transference of heat to their surroundings. Thus albedo is an important input parameter or quantity in evaluating the total insolation on a building or a solar energy collector. It is also important in the studies dealing with thermal balance in the atmosphere.

Several other definitions of albedo are given based on the source of the reflected radiation; only some are mentioned here.

The reflected radiation measured at several portions of the electromagnetic radiation is used to estimate the spectral surface albedos (Gutman et al,1989); the linear combination of them constitutes the broad band surface albedo (Wydick et al, 1987; Brest et al, 1987; Saunders et al, 1990). The broadband or total wavelengths surface albedo is simply surface albedo. Prado et al (2005), however, gave an encompassing definition of albedo as the specular and diffuse reflectance, integrated over 290 and 2500nm wavelengths range, which corresponds approximately to 95% of the solar radiation that reaches the Earth's surface. The albedos of the individual surfaces on the Earth, such as water, vegetation, snow, sand, surfaces of buildings, dry soil, that of the atmosphere, etc, all constitute the surface or planetry albedo**.**

We estimated the surface albedo of the earth's surface at Ilorin by using equ.3 to simulate the daily and monthly averages of the shortwave solar radiation reflection, Hr and reflection coefficient, Hr/Ho, and studied the daily and seasonal variation characteristics of Hr/Ho used to define the albedo. This is the objective of this chapter, which is a report from BSRN station in Ilorin, Nigeria.
