We characterize the column density probability distributions functions ( PDFs ) of the atomic hydrogen gas , H i , associated with seven Galactic molecular clouds ( MCs ) .
We use 21 cm observations from the Leiden/Argentine/Bonn Galactic H i Survey to derive column density maps and PDFs .
We find that the peaks of the H i PDFs occur at column densities ranging from \sim 1 – 2 \times 10 ^ { 21 } ~ { } \mbox { cm$ { } ^ { -2 } $ } ( equivalently , \sim 0.5 –1 mag ) .
The PDFs are uniformly narrow , with a mean dispersion of \sigma _ { HI } \approx 10 ^ { 20 } ~ { } \mbox { cm$ { } ^ { -2 } $ } ( \sim 0.1 mag ) .
We also investigate the H i -to- H _ { 2 } transition towards the cloud complexes and estimate H i surface densities ranging from 7–16 M _ { \odot } pc ^ { -2 } at the transition .
We propose that the H i PDF is a fitting tool for identifying the H i -to- H _ { 2 } transition column in Galactic MCs .