Top: dust colour temperature T_d versus dust emissivity at 353 GHz, two independent observables (Fig. 3), with typical error bars at the top right. The dashed line represents the expected dependency of T_d on the dust emissivity for a fixed emitted power of 3.4 × 10^-31 W H^-1. The blue dots identify data for sky patches centred at Galactic latitudes b ≤ −60°. Bottom: T_d versus dust opacity at 353 GHz, re-expressing the same data in the form plotted by Planck Collaboration XXIV (2011) and Martin et al. (2012).