Relation of the FWHM and the central column density N_H(0) through an isothermal self-gravitating cylinder (black) and sphere (grey). The cloud temperature is 10   K and the external pressure is 2    ×    10⁴   K   cm^-3, except for the long-dashed curve where half the pressure is assumed. The column densities corresponding to the maxima in the FWHM are marked with vertical dashed-dotted lines. The asymptotic behaviour for low and high overpressure is shown by short-dashed curves. The dotted curves (almost hidden by the solid curves) are approximations using Eq. (32). The values for a cloud with the overpressure of a critical stable sphere are shown as filled circles.