is small compared with the hot gas volume and the derived shock radius is very similar.

![Evolution of the mean shock radius for different $C = 10$ runs. This illustrates the variations that are possible with different cloud realizations. We note that the shock evolutions for the 3D cases are consistent with the 2D cylindrically symmetric cases as is the 2D case using the full volume (positive and negative $z$). The inset shows a detail to illustrate more clearly the differences during a medium age period. \[fig:rs\_evol\]](comp_C=10_shocks3){width="49.00000%"}

![Comparison of the evolution of the mean shock radius for runs with different values for $C$, the ratio of ISM mass in clouds to that in the intercloud medium. The density of the clouds and the intercloud medium is the same for all of the runs, $25$ cm$^{-3}$ and $0.25$ cm$^{-3}$, respectively. Only the filling factor of the clouds is varied. As expected larger $C$ values lead to slower shock expansion. The dashed curve labeled “$t^{2/5}$ fit” is a fit to the shock radius evolution assuming that $R_s \propto t^{2/5}$ for the $C
        = 10$ case, where the fitting was done only for the range in $t$ as plotted. This shows that the 