Slope values k = τ(250 μm)/τ(J) of Fig. 2 as a function of estimated distance (frames a), b)), galactocentric distance (frames c), d)), and galactic height (frames e), f)). Left frames: original slopes, right frames: slopes after the bias corrections of τ(250 μm) and τ(J). The black, blue, and red colours correspond to the full τ(J) range and to data below and above τ(J) = 0.6, respectively. The solid curves with the same colours are the weighted moving averages (window sizes 30% in distance, 800 pc in Galactocentric distance, and 100 pc in Galactic height). All τ(250 μm) values are calculated with SPIRE bands alone.