An Etymological Dictionary of Astronomy and Astrophysics
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A type of cloud in the → interstellar medium with low molecular content that is fully exposed to the → interstellar radiation field, and therefore nearly all its → molecules are quickly destroyed by → photodissociation. Hydrogen is mainly in → neutral atomic form (→ neutral hydrogen), and atoms with → ionization potentials less than that of hydrogen (most notably → carbon) are almost fully → ionized, providing abundant electrons. The paucity of molecules implies that very little chemistry occurs in such clouds. Many → sightlines with low → extinction seem to pass exclusively through → diffuse atomic gas. Such sightlines typically have a → column density, N_H, less than about 5 × 10²⁰ cm^-2, and are sufficiently → optically thin to be observable by means of → visible and → ultraviolet → absorption line measurements. Diffuse atomic clouds typically have a fairly low → density (~ 10-100 cm^-3), and → temperatures of 30-100 K (Snow & McCall, 2006, ARA&A 44, 367).

→ diffuse; → atomic; → gas.