The Beer-Lambert law solves the source-free attenuation term of the radiative transfer equation. Extinction includes absorption and scattering out of that ray. Missing terms are thermal emission and scattering into the ray from other directions, including repeated scattering and surface reflection.
A cloud can remove nearly all direct sunlight while still supplying substantial diffuse downward radiation. Consequently the total surface solar flux can greatly exceed the direct-beam answer, although reflection sends some energy back to space and absorption can heat matter. At ordinary atmospheric thermal emission is negligible. No unique total flux follows from extinction alone; the single-scattering albedo, angular redistribution and boundary conditions are needed.[ref]
Radiative transfer in the atmosphere by Met Office. Absorption, scattering and thermal-emission processes
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