Frequency dependence of the main components of the submillimetre sky in temperature (left) and polarization (right; shown as ). The (vertical) grey bands show the Planck channels, with the coloured bands indicating the major signal and foreground components. For temperature the components are smoothed to 1° and the widths of the bands show the range for masks with 81–93% sky coverage. For polarization the smoothing is 40′ and the range is 73–93%. For steep spectra, the rms shown here is dominated by the largest angular scales. But as shown by Fig. 5, on much smaller angular scales in regions far form the Galactic plane, the foreground signals fall far below the cosmological signal (except at the lowest ℓ, in polarization).