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.
Absolute Nature Texture Pack update that includes new hi-res grass textures, sligtly different for each level to add some variety.
There are 3 seasonal options to install: early spring, summer, autumn.
Get it from Addons section.