  Internal energy, enthalpy and entropy are the key quantities to study
thermodynamic properties of the moist atmosphere, because they correspond to
the First (internal energy and enthalpy) and Second (entropy) Laws of
thermodynamics. The aim of this chapter is to search for analytical formulas
for the specific values of enthalpy and entropy and for the moist-air mixture
composing the atmosphere.
  The Third Law of thermodynamics leads to the definition of absolute reference
values for thermal enthalpies and entropies of all atmospheric species. It is
shown in this Chapter 22 that it is possible to define and compute a general
moist-air entropy potential temperature, which is really an equivalent of the
moist-air specific entropy in all circumstances (saturated, or not saturated).
Similarly, it is shown that it is possible to define and compute the moist-air
specific enthalpy, which is different from the thermal part of what is called
Moist-Static-Energy in atmospheric studies.
