The spectral atmosphere model also refers to sunlight and how it scatters within the atmosphere and clouds by determining how colors are produced by various factors like density, moisture,  atmospheric curvature, light angle, etc.  This is different from the old model, which was mainly    determined by fog, haze and light decay.  The problem with that model is that the sky color was directly generated from the haze and fog effects.  In reality all 3 are somewhat separate, since each volume effect has its own set of physical characteristics that determines how light scatters within it.