ALTITUDE, 1000 FT 13 im WINNEMUCCA 40Q ELY LAS VEGAS WINSLOW ALBUQUERQUE Fig. 8&—Vertical cross section for Apr. 22, 1963, at 00Z. Isolines of Gat 4K intervals. Isotachs at 10-knot intervals. Heavy lines are guide lines for Fig. 9. The RB57 samples taken at 28,000 gave readings of 99 and 163 dis/min per standard cubic foot. Once again these large values correspond to those measured in the layer three days earlier. Three of the four values are lower by about a factor of 2. The RB57 aircraft also monitored the lower latitude jet stream as shown in the upper right of Figs, 8 and 9, Continuous records are of course more revealing than point samples. In Fig. 10 accumulated radioactivity and recorded temperatures are plotted vs. time for the traverse and retraverse by both aircraft. The similarity in recordings is beautiful. The only discrepancy (air- craft No. 2 at 2230Z) was caused by the aircraft’s making a 90° turn while taking a sample. The orientation of the layer was changing with time; therefore the flight track did not remain orthogonal to the flow. Instructed to sample along a heading orthogonalto the track,the pilot turned left after reentering the layer. The plane then reapproached the warm boundary of the layer. This caused an increase in tempera- ture and a temporary decrease in the accumulation rate. Both changes attest to the accuracy of the measurements, The average slope of the accumulated radioactivity trace during the time of sampling is a measure of the activity per unit volume ob-