  HI 21cm-line self-absorption (HISA) reveals the shape and distribution of
cold atomic clouds in the Galactic disk. Many of these clouds lack
corresponding CO emission, despite being colder than purely atomic gas in
equilibrium models. HISA requires background line emission at the same
velocity, hence mechanisms that can produce such backgrounds. Weak,
small-scale, and widespread absorption is likely to arise from turbulent
eddies, while strong, large-scale absorption appears organized in cloud
complexes along spiral arm shocks. In the latter, the gas may be evolving from
an atomic to a molecular state prior to star formation, which would account for
the incomplete HISA-CO agreement.
