Top: Evolution of radiation temperature, T_γ (dash-dotted line), gas temperature, T_k (solid line) and 21 cm spin temperature, T_s (dotted line) for a collapsing 10⁸ M_⊙ cloud, as a function of the time to collapse. We highlight on top of the figure the times when the dominant thermal channel changes. Bottom: Evolution of the associated 21 cm brightness temperature from a collapsing 10⁸ M_⊙ cloud.