For plane-parallel atmospheres
or ZEN flags),
the Curtis-Godson equivalent p,T and partial pressure for
each path can be expressed as explicit functions of the profile values
(see notes at the top of module vrtsum_sub.f90).
However to calculate
the absorber amount in each layer requires a factor g.M
where g=gravity, M=molar weight of air.
The RFM default is to use internal values of
g=GRAVTY and WGTAIR
(see *PHY for how to
adjust these from their default values),
the plane parallel atmosphere calculations completely independent of
the altitude grid
With the HYD flag the internal values
GRAVTY and M=WGTAIR are ignored and the product g.M
is inferred from the altitude grid spacing assuming hydrostatic equilibrium.