RADIOACTIVITY AND POTENTIAL VORTICITY 437 POTENTIAL TEMPERATURE AND ENTROPY Both concepts evolved from studies of atmospheric motions on surfaces of constant entropy. These surfaces, more steeply inclined than pressure surfaces, move with the air parcels when the flow processes are isentropic. They are ideally suited for trajectory analyses because entropy is slowly varying except in certain identifiable regions. An isentropic surface is also a surface of constant potential tem- perature, This derived temperature, usually denoted by 0, corresponds to the temperature a parcel of air would attain if compressed to sea level, or 1000 mb. The compression mustbe adiabatic, i.e., no gain or loss of heat by conduction or radiation, In this paper the isentropic surfaces shall be referred to and identified by 6. TROPOPAUSE FOLDING The vertical distribution of © and some of its implications as a coordinate will be clarified in Fig. 1. It illustrates schematically the first concept: a steepening and a folding of the tropopause with the transport of a layer of stratospheric air into the troposphere, In the upper left diagram, the thin lines sloping upward to the north in the troposphere and downward to the north in the gray-toned stratosphere are isolines of 6. Proceeding from upper left to upper right and then to lower left, notice how the 6 lines change their slope and spacing as the tropopause folds, After the tropopause folds the © lines in the stratospheric layer have the same slope and horizontal and vertical spacing as in the tropospheric zone extending from the fold, In other words, the layer has the identical characteristics of a tropospheric frontal zone, i.e., a large horizontal temperature gradient and a large hydrostatic sta- bility (small vertical spacing between © lines), For these reasons the folded tropopause has been misanalyzed by meteorologists as a tropospheric frontal zone in the manner shown at the lower right of the figure. The difference in analyses is not trivial since the two lower diagrams imply a drastically different spatial distribution of radioactivity and ozone, POTENTIAL VORTICITY However, the stratospheric layer can be distinguished from the tropospheric frontal zone if another quasi-conservative property of the atmosphere is utilized. From the equations of motion, continuity, and