Formulations of albedo making a variety of simplifying assumptions about the nature of the diffuse 
irradiance field do not appear to introduce significant errors in the calculation of albedo, except at low 
solar elevations. In particular, it would appear that effects due to the directional nature of the sky 
radiance have little overall effect on albedo, and it is sufficient to assume that the sky is isotropic. 
The albedo does not appear to vary much between light- and heavy-aerosol concentration atmospheres, 
except at relatively low solar elevations, as the influence of the diffuse irradiance is still relatively minor. 
A parameterization of albedo through the definition of an albedo anisotropy factor (R H (0 sun )) should 
be further investigated, as it appears to be relatively invariant with wavelength. This would lead to a 
parameterization of spectral albedo based on a wavelength-dependent ’overcast sky’ albedo (e.g. the 
bihemispherical reflectance) and a potentially wavelength-independent parameter R H (9 sun ) which would 
be applicable for a wide range of illumination conditions. 
Significant proportional errors can be introduced when making the various assumptions given above for 
low solar elevations, and whilst this will generally be offset in absolute terms by the low values of 
irradiance at these angles, such errors may be important for some applications. 
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