(on behalf of the Fermi-LAT collaboration)
High-energy cosmic-ray (CR) protons and electrons interact with the interstellar gas or the interstellar radiation field and produce diffuse gamma rays. Since the interstellar medium (ISM) is transparent to these high-energy photons, GeV gamma rays are a powerful probe to study the ISM and Galactic CRs. Indeed, a significant amount of gas not traced properly by standard radio line surveys ("dark gas") was revealed by CGRO-EGRET and has been confirmed by Fermi-LAT. In those studies, dust observations were used to construct a template of the dark gas. Yet, the procedure to convert dust observations into the distribution of the total neutral gas column density (NH) has not been established yet, preventing an accurate measurement of the NH and CR density. In this contribution, we report a new study of the ISM and CRs in nearby high-latitude HI clouds in the third quadrant, for which we used Fermi-LAT data, HI4PI survey data, and Planck dust thermal emission model. In analyzing gamma-ray data using dust as a gas tracer, we examined the possible dependence of the ratio of NH to dust emission on dust temperature, and possible non-linearity between NH and dust emission. Details of the data analysis, results, and implication of ISM and CRs in the local environment will be presented and discussed.