Evolution of the /N_{H γ} specific reddening (top panel), N_{H γ} opacity (second panel), relative variation in the ratio (third panel), and in total gas column density N_{H γ} derived from interstellar γ rays (bottom panel), in 0. °375-wide bins, as a function of the colour temperature, T_dust, and spectral index, β, of the thermal dust emission. The tilde quantities have been convolved with the LAT response for an interstellar spectrum. The variations in the ratio are given with respect to the mean value of 1.49 × 10⁴ mag found from quasar colours through diffuse atomic clouds (Planck Collaboration XI 2014).