Here's the result of my first sort-of-successful "plugging the P-node into the root Atmosphere node". Poser 9 render with auto settings (halfway between draft and final):

And here is the associated Poser atmosphere setup:

I used just one light - an infinite light at 50% intensity, xRotate = -26, yRotate = about the same as the camera, the light's atmosphere strength setting is 0.3

I think that if 'Volume_Color' is black then there is no interaction between the light and the volume.

I think that the output of the Color_Ramp is Color1 if Input=0 (or less), Color4 if Input=1 (or more), and some sort of blend if Input is between 0 and 1.

(I don't know why I have the Math_Function brick there - I think I could take it out, change the 'Y' value of the 'P' node to 0.006,  plug the 'P' node directly into the 'Input' of the 'Color_Blend' and get exactly the same result.)

With the 'P' node I think that setting x and z to zero and only y to non-zero means that only the y (height) value is used to set the Input value for the Color_Ramp. The 0.002 value I used was basically trial and error.

So I think I can see why this does what it does, but I'd really love it if somebody who knows would explain...  :)

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The 3Dcheapskate (also available in DAZ and HiveWire3D flavours) occasionally posts sensible stuff. Usually by accident.
And it usually uses Poser 11, with units set to inches. Except when it's using Poser 6 or PP2014, or when its units are set to PNU.