Average dust temperature as a function of Galactocentric radius for the atomic (blue) and molecular (red) components. The dust temperature in the ionised phase and the dark gas phase are shown by the dashed line and dot-dashed line, respectively. The solid line shows the behaviour of the ISRF with Galactic radius. Horizontal bars show the range in distance over which the mean temperature applies. The temperature of dust mixed with atomic Hi decreases with radius, consistent with the fact that this component is heated by the ambient ISRF. Dust in the molecular component is heated by embedded star formation.