Temperature evolution of gas with different densities (from n = 10⁻² cm⁻³ to n = 10³ cm⁻³), starting from different initial temperatures (T = 10⁵, 10⁶, and 10⁷ K). The gas has metallicity Z = Z_⊙ in the upper panel, and Z = 10⁻³ Z_⊙ in the lower panel. The gray shaded area marks the temperature T = 3 × 10⁴ K, which in our work is defined as the threshold temperature between cold and hot gas.