Atomic hydrogen fraction vs. the total column density of hydrogen (bottom scale) and visual extinction (top scale). Observations from Li & Goldsmith (2003) are shown as solid orange circles. Coloured stripes represent our results for the case of photodissociation only (purple), and models (black) and (grey). Dashed lines refer to the average value of the total volume density of hydrogen (suggested by Li & Goldsmith 2003).