Distribution as a function of Galactic longitude of the median brightness (full lines) and its rms (dashed lines) in regions within each Hi-GAL tile where the brightness levels are below the 10 percentiles of the brightness distribution for that tile. Hi-GAL bands are colour-coded as follows: blue for PACS 70 μm, cyan for PACS 160 μm, green for SPIRE 250 μm, yellow for SPIRE 350 μm, and red for SPIRE 500 μm. Ticks on the right margin of the figure mark the values of the theoretical sensitivities predicted by the official PACS/SPIRE time estimator (available in HSpot) for observations in pMode with 60′′ s^-1 scanning speed.