Trajectory of a shocked fluid particle from the ambient medium to the postshock region obtained in the standard model. The trajectory is displayed in a particle density versus thermal pressure diagram (top panel) and in a proton density versus thermal pressure diagram (bottom panel) to highlight the phase transition from the WNM to the CNM induced by a shock at 200 km s⁻¹. A fluid particle initially evolves through the radiative precursor (pink curve). As it crosses a shock front, it undergoes three successive regimes: an adiabatic heating (red curve) followed by a quasi-isobaric cooling (green curve), and finally a quasi-isochoric cooling (blue curve) as the magnetic pressure becomes dominant in the postshock gas. The black curve indicates the thermal equilibrium state of the diffuse gas obtained for G₀ = 1 (see Fig. 1). Light gray lines are isothermal contours from 1 to 10⁷ K (from bottom right to top left). We note that these isocontours are evenly spaced in the top panel but not in the bottom panel. Tics on the trajectories indicate the time, in log scale, starting from the adiabatic phase, ranging from 10⁻¹ yr to 10⁶ yr.