RADIOACTIVITY AND POTENTIAL VORTICITY Fig. 7——-Northern-hemisphere isentropic chart for 310°K, Apr. 1963, at 00Z (see Fig. 3). 445 22, Both WB50 airplanes, following the paths shownin Fig. 9, traversed the stable layer from left to right then retraversed the layer at the Same altitudes from right to left. Radioactivity samples were taken on the return flights. Their beta values are plotted in Fig. 9 at 21,000 and 23,000 ft. The aircraft operating at the lower altitude then descended to 19,000 ft, and the aircraft operating at the higher altitude ascended to 26,000 ft. Additional samples were taken at these altitudes. Outside the layer the beta values are low, between 1 and 3 dis/min per standard cubic foot. These values correspond to those measured in tropospheric air on April 18 and 19, Inside the layer the beta values vary from 20 dis/min per standard cubic foot at 19,000 ft to 87 and 92 dis/min per standard cubic foot at 24,000 and 26,000 ft, respectively.