Settings  -  A t m o s p h e r e

Fog Density: Sets the density of the volumetric fog used for volumetric lights.
The number of direct illumination samples in the volume.

Attenuation: Sets the light blocking ability of the fog.

(Sample Type Pulldown): Choose the type of scattering algorithm.   Basically (very basically), Rayleigh scattering concerns the scattering of light in the atmosphere due to atmospheric gases, whereas MIE scattering deals with particulates in the lower atmosphere, such as from smoke, dust, etc.  HG also deals with dust scattering.  As for which one you should use, really it's a matter of taste.  Try a couple of renders and go with the one you like.  It's a very subtle thing.

Scattering:  Turn this on to enable light rays to be scattered.  It's sort of like turning on Global Illumination for the atmosphere, letting the atmosphere itself get light that's been bounced off of objects.

Scattering Samples: The number of global illumination samples for the volume.  Think of it as Global Illumination within the fog.

Eccentricity:  This setting is specific to the HG sample type.  There is no way to explain this (or the sample types) without an entire chapter being written about it.  If you have the interest and the time, play with it.

Noise Octvs: Sets how many "disturbance" waves to send through.  The higher the number the more detailed the noise will be.
Noise Amp:
This sets the contrast of the noise.

Noise Gain: This sets the intensity of the noise.
Noise Scale:
This sets the size of the grain.

Additional info about scattering types (links to external sites):  Rayleigh   Rayleigh & MIE   And Another One   And One More

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