Molecular clouds form out of the surrounding diffuse gas through the conversion of atomic (HI)
to molecular hydrogen (H2), and previous Galactic and extragalactic observations have suggested
that this HI-to-H2 transition is a major bottleneck process toward star formation.
Among two main flavors of HI, the cold and warm neutral medium (CNM and WNM),
the cold component is expected to play a critical role.
For example, the CNM is far more effective at forming and shielding H2 thanks to its higher density.
In addition, numerical simulations have found that CNM structures have physical properties that resemble those
of molecular clouds, implying that the initial conditions for star formation are set before the gas becomes molecular.
Despite its vital importance for the HI-to-H2 transition and ensuing star formation,
however, the CNM in and around molecular clouds remains largely unexplored.
In this talk, I will describe my attempts for the last few years to understand the roles of the CNM in the formation
and evolution of molecular clouds and present some preliminary results from these attempts.