  A convenient model for estimating the total entropy ({\Sigma}Si) of
atmospheric gases based on physical action is proposed. This realistic approach
is fully consistent with statistical mechanics, but uses the properties of
translational, rotational and vibrational action to partition the entropy. When
all sources of action are computed as appropriate non-linear functions, the
total input of thermal energy ({\Sigma}SiT) required to sustain a chemical
system at specific temperatures (T) and pressures (p) can be estimated,
yielding results in close agreement with published experimental third law
values. Thermodynamic properties of gases including enthalpy, Gibbs energy and
Helmholtz energy can be easily calculated from simple molecular and physical
properties. We propose that these values for entropy are employed both
chemically for reactions and physically for computing atmospheric profiles, the
latter based on steady state heat flow equilibrating thermodynamics with
gravity. We also predict that this application of action thermodynamics may
soon provide superior understanding of reaction rate theory, morphogenesis and
emergent or self-organising properties of many natural or evolving systems.
