**2. Determination of solar radiation reflection coefficient, Hr/Ho (albedo)**

Hr/Ho is a ratio of short wave reflected radiation Hr, towards the space, to the extraterrestrial radiation Ho

incident on the surface of the earth at the edge of the earth's atmosphere. Here at Ilorin, the location of this work, Hr , the reflected radiation is not measured nor is there a formula in literatures by which it may be predicted or estimated. The apparatus to measure surface albedo or reflected solar radiation is not available here nor in many other under-developed countries. It is therefore determined to produce its data by estimating or simulating it. Therefore the work done on short wave energy balance at the edge of the atmosphere becomes relevant, as it provides a means by which the short wave reflected solar radiation back to space could be estimated. Once the reflected radiation flux is obtained, the solar radiation or short wave radiation reflection co-efficient is easily obtained. It is reasonable to want to know the fraction of the incident radiation Ho is returned back to space on daily, seasonal, and annual basis. Therefore the knowledge of reflection co-efficient, Hr/Ho, used to define albedo, is desirable, and is a very important and relevant radiation parameter in radiative transfer in the atmosphere.

In estimating and studying the characteristics of albedo, global (total) solar radiation H, and diffuse solar radiation Hd, of wavelengths range, mostly from 0.2 to 4.0µm, were used to simulate solar radiation reflection, Hr and the reflection coefficient, Hr/Ho. The radiation fluxes were obtained from the BSRN station, Physics Department, University of Ilorin. The extraterrestrial radiation, Ho, at the top of the atmosphere at Ilorin, computed for year 2000, were used.

The global (total) radiation was measured by Eppley Pyranometer, PSP, with calibration constant of 8.2 x 10-6 V/Wm-2 **,** while the diffuse radiation Hd was measured by the Black and White Eppley Pyranometer model 8-48 with calibration constant 9.18 x 10-6 V/Wm-2.

From the measured and computed radiation fluxes, the daily and monthly averages of the fluxes, and the ratios H/Ho, Hd/H and Hr/Ho were computed. Thus, the sw- solar radiation reflection, and total wavelengths reflection co-efficient or reflectance simulated using the data of year 2000 at Ilorin were used for the study. In compliance with the world WRR, sampling rate of 1-second duration of the radiation fluxes was done every minute with integration time of 3-minutes maintained for averaging and recording.

In the work on shortwave solar energy balancing at the edge of the atmosphere carried out in 2003 by Babatunde (2003; 2003), the relation

$$\text{HI/}\,\mathrm{H}^{\circ} + \mathrm{H}^{\circ}\,\mathrm{/}\,\mathrm{H}^{\circ} + \mathrm{H}^{\circ}\,\mathrm{/}\,\mathrm{H}^{\circ} = \mathrm{I} \tag{1}$$

was used to establish the sw-solar radiation energy balance at the edge of the Earth's atmosphere. H/Ho is the fraction of the extraterrestrial radiation, Ho, transmitted through the atmosphere to the ground surface, and called clearness index (Babatunde and Aro,1995; Udo,2000), Ha/Ho is the fraction absorbed, called the absorption co-efficient or absorbance, and Hr/Ho is the fraction reflected back to space called the reflection co-efficient or reflectance (Babatunde,2003). Further in the work, Ha/Ho was found to be very small in value compared with the other ratios and therefore negligible, i.e. Ha / Ho < < 1

Hence equation (1) becomes

142 Solar Radiation

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

**2. Determination of solar radiation reflection coefficient, Hr/Ho (albedo)** 

Hr/Ho is a ratio of short wave reflected radiation Hr, towards the space, to the

incident on the surface of the earth at the edge of the earth's atmosphere. Here at Ilorin, the location of this work, Hr , the reflected radiation is not measured nor is there a formula in literatures by which it may be predicted or estimated. The apparatus to measure surface albedo or reflected solar radiation is not available here nor in many other under-developed countries. It is therefore determined to produce its data by estimating or simulating it. Therefore the work done on short wave energy balance at the edge of the atmosphere becomes relevant, as it provides a means by which the short wave reflected solar radiation back to space could be estimated. Once the reflected radiation flux is obtained, the solar radiation or short wave radiation reflection co-efficient is easily obtained. It is reasonable to want to know the fraction of the incident radiation Ho is returned back to space on daily, seasonal, and annual basis. Therefore the knowledge of reflection co-efficient, Hr/Ho, used to define albedo, is desirable, and is a very important and relevant radiation parameter in

In estimating and studying the characteristics of albedo, global (total) solar radiation H, and diffuse solar radiation Hd, of wavelengths range, mostly from 0.2 to 4.0µm, were used to simulate solar radiation reflection, Hr and the reflection coefficient, Hr/Ho. The radiation fluxes were obtained from the BSRN station, Physics Department, University of Ilorin. The extraterrestrial radiation, Ho, at the top of the atmosphere at Ilorin, computed for year 2000,

The global (total) radiation was measured by Eppley Pyranometer, PSP, with calibration constant of 8.2 x 10-6 V/Wm-2 **,** while the diffuse radiation Hd was measured by the Black and White Eppley Pyranometer model 8-48 with calibration constant 9.18 x 10-6 V/Wm-2.

From the measured and computed radiation fluxes, the daily and monthly averages of the fluxes, and the ratios H/Ho, Hd/H and Hr/Ho were computed. Thus, the sw- solar radiation reflection, and total wavelengths reflection co-efficient or reflectance simulated using the data of year 2000 at Ilorin were used for the study. In compliance with the world WRR, sampling rate of 1-second duration of the radiation fluxes was done every minute with

In the work on shortwave solar energy balancing at the edge of the atmosphere carried out

H/Ho + Ha /Ho + Hr /Ho = 1 (1)

integration time of 3-minutes maintained for averaging and recording.

in 2003 by Babatunde (2003; 2003), the relation

station in Ilorin, Nigeria.

extraterrestrial radiation Ho

radiative transfer in the atmosphere.

were used.

$$\text{H}/\text{H}\_{\text{0}} + \text{H}\_{\text{t}}/\text{H}\_{\text{0}} \approx 1 \tag{2}$$

From this, an expression for estimating Hr / Ho was obtained as

$$\text{H}\_{\text{t}}/\text{H}\_{\text{o}} = \text{1-H}/\text{H}\_{\text{o}}\tag{3}$$

A similar equation was obtained by Babatunde and Aro (1995) for cloudiness index, Hd/H, i.e.

$$\mathbf{H}\_{\mathrm{d}}/\mathrm{H}=\mathbf{1}\cdot\mathbf{H}/\mathrm{H}\_{\mathrm{o}}\tag{4}$$

Thus, can these two parameters be said to be twins of the same physical quantity?

Both Hr and Hr/Ho could be and were estimated using eqn.3.

The sw- reflected solar radiation, Hr, is understood to include the reflected radiation from the Earth's surface, reflected radiation back to space by clouds, and the scattered radiation back to space by atmospheric particles and clouds. Reflection, with regards to solar radiation, is redirection of radiation by 180o after striking a surface or any atmospheric particle; it is a lost radiation to the space. The fraction,

Hr /Ho, called total wavelengths (0.2 - 4µm) reflection co-efficient or reflectance from all the surfaces on the Earth's surface defines generally the Earth's surface albedo (Igbal,1983).The monthly averages of Hr and Hr/Ho produced are shown in columns 8 and 7 respectively of Table 1.
