  We apply the Sternberg et al. (2014) theoretical model to analyze HI and H2
observations in the Perseus molecular cloud. We constrain the physical
properties of the HI shielding envelopes and the nature of the HI-to-H2
transitions. Our analysis (Bialy et al. 2015) implies that in addition to cold
neutral gas (CNM), less dense thermally-unstable gas (UNM) significantly
contributes to the shielding of the H2 cores in Perseus.
