Comparison of the “X-ray” and “Y” jumps for a shock in a gas with adiabatic index γ = 5∕3 as a functionof the shock Mach number M. Rankine–Hugoniot conditions provide jumps in thermodynamic properties (ρ, T or P). The ratio of P∕ρ² approximately characterizes the ratio of the jumps in effective volume emissivities for X-rays and tSZ upstream and downstream of the shock. For M ≲ 2.4, the jump in Y is smaller than in X-ray volume emissivities, implying that the shock appears slightly less prominent in Y than in the X-ray surface brightness. The minimum of the ratio (ignoring the temperature dependence of X-ray emissivity) is at M ≈ 1.6 and is equal to 0.87.