  We analyze the evaporation and condensation of spherical and cylindrical HI
clouds of the cold neutral medium surrounded by the warm neutral medium.
Because the interstellar medium including those two phases is well described as
a thermally bistable fluid, it is useful to apply pattern formation theories to
the dynamics of the interface between the two phases. Assuming isobaric
evolution of fluids and a simple cubic form of the heat-loss function, we show
the curvature effects of the interface. We find that approximate solutions for
spherical clouds are in good agreement with numerically obtained solutions. We
extend our analysis to general curved fronts taking into account the curvature
effects explicitly. We find that the curvature effects always stabilise curved
interfaces under assumptions such as isobaric evolution we adopt in this
Letter.
