I have been using this basic formula below...

min(1,pow(exp(_base_scale_radius-_atm_scale_radius*sqrt(x*x+y*y+z*z)),
_hh_gamma))

First (hidden) parameter is the "Scale Height"... that is the height above
the surface of the planet where the atmospheric density changes by a factor
of e.  (E is 2.718281828...) and is generated by fancy trigonometric
operations.  In POV-Ray, it's exp(1).  In this formula, I set "base scale
radius" to be about 32 * scale height below the surface of earth, and I set
"atm scale radius" to be 32 * scale height above the surface of earth.  I
use "Scale Height" because when you do the research, information is
available... for earth, the scale height is 8.5 km.  A rocket going
straight up when reaching 8.5 km in the sky will experience a pressure of
(surface)*e^(-1), which is about 300 mbars.

However, in POV-Ray, this function may seem broken when applied to shooting
beams through media (atmosphere).  To correct it, I apply a gamma of e.
This seems to correct a bunch of things.

So.. just experimenting with some parameters and colors, I produced this
image...

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