An attempt is made to study, by means of a detailed synoptic analysis, the three-dimensional air-motion in a typical outbreak of a polar air-mass. The mean vertical velocities at different levels over the entire cold-air region south of latitude 45°N are computed by use of the principle of continuity of mass. The maximum mean subsidence is found to be nearly 2 km per day at about the 600-mb level. In addition, the distribution of vertical velocity in different parts of the cold tongue is computed by means of trajectories on isentropic surfaces, and some examples of characteristic three-dimensional air trajectories are given. The results show that a typical outbreak of cold air can be regarded as an effective cell for meridional exchange of mass, heat and angular momentum.