There have been extensive calculations of model atmospheres of stars over the past few decades. Almost all of these calculations have assumed radiative equilibrium. Except for convection, very little work has been done on the stability of the surface layers of stars at small optical depths. Although there has been a considerable amount of research concerning the stability of stars, this work has assumed the diffusion approximation for the transport of radiation and has been primarily concerned with the envelopes of stars rather than their atmospheres.