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In this paper, we investigate the conditions for the HI-to-H$_{2}$ transitionin the solar neighborhood by analyzing HI emission and absorption measurementstoward 58 Galactic lines of sight (LOSs) along with $^{12}$CO(1$-$0) (CO) anddust data. Based on the accurate column densities of the cold and warm neutralmedium (CNM and WNM), we first perform a decomposition of gas into atomic andmolecular phases and show that the observed LOSs are mostly HI-dominated. Inaddition, we find that the CO-dark H$_{2}$, not the optically thick HI, is amajor ingredient of the dark gas in the solar neighborhood. To examine theconditions for the formation of CO-bright molecular gas, we analyze thekinematic association between HI and CO and find that the CNM is kinematicallymore closely associated with CO than the WNM. When CNM components within COline widths are isolated, we find the following characteristics: spintemperature $<$ 200 K, peak optical depth $>$ 0.1, CNM fraction of $\sim$0.6,and $V$-band dust extinction $>$ 0.5 mag. These results suggest that CO-brightmolecular gas preferentially forms in environments with high column densitieswhere the CNM becomes colder and more abundant. Finally, we confront theobserved CNM properties with the steady-state H$_{2}$ formation model ofSternberg et al. and infer that the CNM must be clumpy with a small volumefilling factor. Another possibility would be that missing processes in themodel, such as cosmic-rays and gas dynamics, play an important role in theHI-to-H$_{2}$ transition.

Probing the Conditions for the H{\i}-to-H$_{2}$ Transition in the Interstellar Medium