We examine the hypothesis that hydrostatic pressure alone determines the ratio of atomic to molecular gas averaged over a particular radius in disk galaxies .
The hypothesis implies that the transition radius , the location where the ratio is unity , should always occur at the same value of stellar surface density in all galaxies .
We examine data for 28 galaxies and find that the stellar surface density at the transition radius is indeed constant to 40 % at a value of 120 M _ { \sun } pc ^ { -2 } .
If the hypothesis can be confirmed at all radii within a large range of galaxy types and metallicities , combining it with the observed constancy of the star formation rate with H _ { 2 } surface density may enable a physically motivated star formation prescription with wide applicability .