size is set by the Bonner-Ebert criterion for self-gravity to become important, $$S_{max}=350{\ T}_4\left({{nT}\over{10^5}}\right)^{-1/2}{\ \rm pc},$$ where $T_4$ is the temperature of the WFC’s in units of 10$^4$ K, and $nT$ is the pressure of the clouds (assumed to be in pressure equilibrium with the RHG).

In the two-phase model discussed by [@LM00], the WFC’s are heated by UV emission from massive stars, in a self-regulated star formation process. For the parameters given above, the total column densities of the clouds range from $5\times10^{19}$ to $10^{22}$ cm$^{-2}$.

In Models 1-2 and 4, we adopt a polytropic equation of state for the WFC’s with a power index $\gamma_w = \gamma_h$. In Model 3 we attempt to maintain $\gamma_w\approx1$ throughout the evolution. This is done by allowing the cloud to cool, but turning cooling off below a set minimum temperature, which we select as $10^4$ K. The high cooling efficiency in this temperature regime ensures that the cloud temperature cannot significantly exceed the cutoff temperature. Cooling is not allowed to proceed in the background gas.

Because we are using a single-fluid code (see below), zones where cloud and background gas mix are a concern. In order to prevent significant cooling of the 