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Published online by Cambridge University Press: 14 August 2015
The interstellar HI is best discussed in temperature categories because the requirement of pressure equilibrium leads to gross differences between hot and cold gas. The temperature of the hot (6000K) neutral gas has been measured by five techniques while the cold (60K) gas is visible mainly in 21cm absorbtion. There is evidence for warm (200K to 1000K) gas. Because of emission line blending, the small scale structural morphology is undetermined; however, there is evidence it is filamentary. With few exceptions, spatial sizes observe a lower limit of about 5 pc. On a larger scale, the gas is organized into sheets which reflect the recent history of cloud formation.