Abstract
It is suggested that in order for molecular clouds to form they must be in a high pressure. Expressed in terms of the system's pressure and chemical abundances, the requirement is P0 ≥ 10^-12 (Z⊙/Z)2 dyn cm^-2 and is just barely satisfied at the solar circle. This criterion would have significant impact on radial trends in spirals and distinctions among dwarf irregulars. This paper has been submitted to the Astrophysical Journal (1985).
