The Atomic Hydrogen

The atomic phases of interstellar medium of our galaxy can be distinguished into a cold neutral medium and a warm neutral medium. A great part of the mass resides in the cold neutral medium, distributed in dense sheets or filaments (the HI clouds) occupying $1-4\%$ of the total volume of the Galaxy disk. These clouds, which are not gravitationally bound, have a number density of about $20-60$ $cm^{-3}$ and a temperature of $\sim 100 K$.
The warm neutral medium fills a substantial fraction ( $\sim30-60\%$) of the available volume. This material, located mainly in photodissociation regions, has a density of about $0.3 -1 cm^{-3}$ and a temperature of $\sim 6000 K$. These two phases exist in pressure balance with each other, which is a necessary condition for the existence of a two-phases medium.