AtmosFear is a mod for S.T.A.L.K.E.R. - Call Of Pripyat that tweaks and expands the vanilla weather system to be more realistic and various.
Making an already dark atmosphere more dark is a great idea. CoP + AtmosFear 3 is still the best stalker experience one can have.
This mod just enhances what makes CoP and S.T.A.L.K.E.R. in general so great, the atmosphere. Download it, you won't be disappointed.