Thermal instability (TI) is a trigger mechanism, which can explain the formation of small condensations through some regions of the interstellar clouds. The instability criterion for flat geometry approximations has been investigated in previous works. Here, we focus on spherical perturbations in the spherical clouds. Our goal here is to examine the conditions for the occurrence of TI through the thermally dominated (i.e., gravitationally stable) quasi-static spherical interstellar clouds. First, we obtain the profiles of density, temperature, pressure, and enclosed mass of a symmetric spherical cloud. Then, we use the perturbation method to investigate the linear regime of instability and find its growth rate. Considering spherical perturbations on the quasi-static spherical cloud, instead of a thermal and dynamical equilibrium flat cloud, changes the instability criterion so that we can conclude that sphericalness can increase the occurrence of TI. The results show that in the spherical clouds, perturbations with shorter wavelengths have more chance to grow via TI (i.e., greater growth rates).
