RADIOACTIVITY AND POTENTIAL VORTICITY 439 this form the potential vorticity is the product of the stability and the vertical component of the total vorticity, where the total vorticity is the sum of the relative vorticity (computed from the winds on a © surface) and the vertical component of the earth’s vorticity. In the simplest idealized concept of stratosphere and troposphere, the stability and baroclinity change rapidly at the tropopause, as shown in the upper left diagram of Fig. 1. If, also, the tropopause coincides with the maximum wind on each © surface, the vorticity also changes rapidly at the tropopause. The © surfaces drawn in Fig. 2 satisfy these conditions. Recognizing that the wind shear or spin is positive inthe stratosphere (i.e., £,>0 and f is positive everywhere), we see that a large stability is multiplied by a large absolute spin in the stratosphere, Conversely, in the troposphere a small stability correlates with a small absolute spin, £, < 0. The positive correlation between stability and spin makes the change in potential vorticity at the tropopause larger in magnitude than the change in stability. Recalling also that the potential vorticity is conserved for isentropic processes, we can app~eciate its importance in both identifying and tracing the movement of stratospheric air. When the stability of tropospheric air is greatly increased as in frontogenesis, the spin is correspondingly decreased. Tropospheric fronts can therefore be recognized by large negative values of ¢, and small values of P, . The stratospheric layers will have positive values of £, and large values of P,. Fig. 2— Explanation of large potential vorticity in the stratosphere and small potential vorticity in the troposphere.