Fig. 3—Northern-hemis phere isentropic chart for 310°K, Apr. 19, 1963, at 002. Stream-function isolines are drawn at an interval of 60 10° ergs/g. Within the gray area, the 310°K surface is in the strato- sphere. When the WB50’s arrived over Wyoming, cumulonimbus clouds and snow showers delayed the mission. After an hour’s delay permission was granted to cross the airways and to proceed with the mission. The plane assigned the lower altitude flew the prescribed distance along the track but encountered only clouds. The pilot aborted the mission. At the higher altitude the pilot was just about to turn back when his plane broke into the clear. The plane had entered the stratospheric layer. The pilot traversed the layer, flew well beyond it and returned, but because of limited fuel, he could not repeat this pattern at another altitude. Three samples were taken which gave total beta readings of 2 dis/min per standard cubic foot at two points outside the layer and 136 dis/min per standard cubic foot within the layer. These values are plotted in Fig. 4 along with the total beta readings for the RB57 samples. Note the similarity in values north of the jet at Denver, 114, 136, do