It is a measure of the scattering in the backward direction ( i.e. towards the incident direction, at an scattering angle of 180 deg., sometimes also meant for scattering at an angle larger than 90 deg. )
for the light encountering the atmospheric aerosol particles.
In radiative transfer studies, back scattering ratio is defined as:

Where μ is the cosine of the scattering angle
and P is the scattering Phase function.