  The work is devoted to the investigation of properties of water vapor. The
main attention is focused on the physical nature of its effective
polarizability and heat capacity at constant volume. We show that the specific
temperature dependencies of these characteristics are mainly caused by thermal
vibration excitations of water dimers. In connection with this we determine
normal coordinates for the dimer vibrations. We investigate fluctuations of the
dipole moments of dimers in details and consider their contributions to the
dielectric permittivity of the water vapor. We also took into account the
contribution of the interparticle interactions on the heat capacity. Analyzing
the behavior of the effective polarizability and heat capacity we defined the
temperature dependence of the dimer concentration on the coexistence
vapor-liquid curve. The noticeable dimerization in the saturated water vapor
takes place only for temperatures T/Tc > 0.8 (Tc is the critical temperature).
It is important to note that the successful reproduction of the effective
polarizability and heat capacity can serve good test for the correct
description of the dimer concentration in different approaches.
