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* 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 or something else.
*soon only to be available in two flavours -  DAZ and Renderosity