of two, but a prescription for distributed sources is necessary in order to model the ionized component.

Conclusions {#secConclusions}
===========

The present [[work]{}]{} describes an implementation of gravity in the well-tested photoionization code [Cloudy]{}. For an atmosphere in hydrostatic equilibrium, the gravitational force has to be balanced by a pressure gradient according to equation (\[eqHydro\]).

By way of example, we consider a plane-parallel model of the Galactic ISM in the solar neighbourhood to illustrate the influence of the gravitational acceleration on the physical properties of the gas. The main result, valid in the general case, is that gravity sets the scale height of the system. If self-gravity is not accounted for, there is nothing that prevents the gas (mostly molecular at large radii) from extending to infinity. When gravity is present, the pressure has to decrease in order to maintain hydrostatic equilibrium, and the gas becomes less dense. The precise structure depends on many other variables; we have investigated the effect of [[the incident radiation field, the presence of an external mass, and]{}]{} a tangled magnetic field, but other ingredients, such as turbulent motions, cosmic rays, dust grains, or variations in the atomic rate coefficients [see e.g. @Abel+08], as well 