Model brightness of the atmosphere at frequencies around the 183 GHz water vapour line for six values of PWV along the line of sight: 0.3, 0.6, 1, 2, 3 and 5 mm (from lowest red line to highest blue line). The model was computed using the ATM program by Pardo et al. (2001), using the source code the model as used by ALMA (this is available for public download at http://www.mrao.cam.ac.uk/~bn204/alma/atmomodel.html).