$$F_D = {1\over2}C_D \pi S^2 \rho_h V_w ^2,$$ where $C_D$ is the drag coefficient, and $S$ is the cloud radius (Batchelor 2000). In flows with high Reynolds number, the turbulent wake behind the body provides an effective momentum transfer mechanism, dominating $C_D$. For example, the experimentally measured $C_D$ for a hard sphere in a nearly inviscid fluid is 0.165 (Whipple 1972). For compressible gas clouds, $C_D$ is probably closer to unity.

When $F_D\approx g$, the WFC’s attain a terminal speed $$V_t \approx \left( {8 D_{\rho} {S g} \over 3 C_D } \right)^{1/2}.
\label{eq:vterm}$$ At the launch point, the size of the WFC is set to be $S (y_0) = 0.2$ in models 1-3. If $C_D =1$ and the WFC preserves its integrity, $V_t =
2.3$, which would exceed sound speed of the RHG. Once the Mach number of the WFC exceeds unity, however, shock dissipation would greatly increase the drag relative to the above estimate. Prior to the WFC achieving $V\approx1$, however, Rayleigh Taylor instability causes it to break up into smaller pieces. For smaller fragments, the value of $V_t$ is reduced, as seen in Equation (\[eq:vterm\]). Due to shock dissipation, the sound speed in the RHG is also slightly 