Temporally averaged radiative cooling of stellar atmospheres as a function of p/g_⋆, representing the column mass density (black lines). The green, red, and blue lines correspond to the cooling rates due to the photospheric, chromospheric, and coronal radiation, respectively (Q_ph, Q_ch, and Q_co in Section 2.3). The thin vertical line shows the position of the transition region (i.e., ﹩{p}_{\mathrm{tr}}/{g}_{\star }﹩).