We have developed a new method to investigate the
importance of the propagation of sunlight through the Earth's
atmosphere in modelling the perturbations due to radiation pressure
from Earth--reflected sunlight --- the so--called {\it albedo effect}.
The atmosphere is considered as a refractive optical medium, where
Rayleigh scattering takes place. Our mathematical
formulation is based on the theory developed in Vokrouhlick\'y et~al.
(1993c), which allows us to model in a realistic way a number of
subtle phenomena. Contrasting with direct solar radiation
pressure, which is fairly sensitive to atmospheric processes during the
penumbra phases, the albedo effect turns out not to be affected
significantly by the atmospheric processes considered here. We have
also reinvestigated the behaviour of the radiation specularly
reflected from the Earth's surface, and confirm by an independent
approach the validity of previous simple models of the corresponding
radiative force, provided a ``dilution factor'' related to the
Earth's sphericity and introduced by Wyatt (1963) and later on by
Barlier et~al. (1986) is properly accounted for.
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