Top: distribution of the apparent dust temperature (T_obs) and column density, as derived in Planck Collaboration XI (2014). Bottom: distribution of the polarization fraction (p) as a function of T_obs in regions of the sky excluding the inner Galactic plane (excluding ℓ_II < 90deg or ℓ_II> 270deg, | b_II | < 2°). Both plots are for pixels not masked in Fig. 1. The colour scale shows the pixel density on a log₁₀ scale. The curves show, from top to bottom, the evolution of the upper 1% percentile, mean, median and lower 1% percentile of p. Horizontal dashed lines show the location of p = 0 and p_max = 19.8%.