  We study how different opacity-temperature scalings affect the dynamical
evolution of irradiated gas clouds using time-dependent,
radiation-hydrodynamics (rad-HD) simulations. When clouds are optically thick,
the bright side heats up and expands, accelerating the cloud via the rocket
effect. Clouds that become more optically thick as they heat accelerate $\sim
35\%$ faster than clouds that become optically thin. An enhancement of $\sim
85\%$ in the acceleration can be achieved by having a broken powerlaw opacity
profile, which allows the evaporating gas driving the cloud to become optically
thin and not attenuate the driving radiation flux. We find that up to $\sim
2\%$ of incident radiation is re-emitted by accelerating clouds, which we
estimate as the contribution of a single accelerating cloud to an emission or
absorption line. Re-emission is suppressed by "bumps" in the
opacity-temperature relation since these decrease the opacity of the hot,
evaporating gas, primarily responsible for the re-radiation. If clouds are
optically thin, they heat nearly uniformly, expand and form shocks. This
triggers the Richtmyer-Meshkov instability, leading to cloud disruption and
dissipation on thermal time-scales.
