Temporal evolution of a H ii region expanding in a volume of (40 pc)³ filled with inhomogeneously distributed (initially neutral) gas. The top seven rows show cross sections through the center of the volume and show the temperature and the absolute as well as relative number densities of the ions H ii, O ii, and O iii. Lower two rows: synthetic images of the line intensities of [O ii] 3726 Å + 3729 Å and [O iii] 5007 Å + 4959 Å. In these models, both the central ionizing star (a 40 000 K dwarf) and the gas have a metallicity of 0.1 Z_⊙.