  Metrology in atomic physics has been crucial for a number of advanced
determinations of fundamental constants. In addition to very precise frequency
measurements, the molar polarizability of an atomic gas has recently also been
measured very accurately. Part of the motivation for the measurements is due to
ongoing efforts to redefine the International System of Units (SI) for which an
accurate value of the Boltzmann constant is needed. Here, we calculate the
dominant shift of the molar polarizability in an atomic gas due to thermal
effects. It is given by the relativistic correction to the dipole interaction,
which emerges when the probing electric field is Lorenz transformed into the
rest frame of the atoms that undergo thermal motion. While this effect is small
when compared to currently available experimental accuracy, the relativistic
correction to the dipole interaction is much larger than the thermal shift of
the polarizability induced by blackbody radiation.
