equator. The temperate latitude peak evident in the surface air in the northern hemisphere is virtually absent in the southern hemisphere despite the similarity of stratospheric concentrations. Whether this is seasonal or the result of differences between the hemispheres will become evident when results of the next series of obser- vations, made in November 1960, becomes available. The source of high altitude Sr-90, and its distribution above 100,000 feet are both in doubt. The peak may be at or near the top of the balloon collections as a result of injections made in the older Castle (1954) and Ivy (1952) series, which resulted in clouds extending to higher altitudes than most later tests. It is also possible that the concentrations increase above the highest altitudes sampled as a result of the Teak and Orange events. As with C-14, the Sr-90 concentrations can provide invaluable clues to atmospheric circulations. The uncertainties inherent in using particles as atmospheric tracers are offset in part by the fact that there are no natural sources of Sr-90 and therefore no variable background. Cesium-137. expected. Figure 3 shows that the Cs-137 results are very similar to the Sr-90 values, as would be A possible additional benefit as a tracer may result from the fact that Cs-137 is also a y-emitter and the possibility exists that instrumentation to measure it in situ can be developed. Lead-210. Figure 4 shows the distribution of Pb-210, a long-lived particulate daughter of 3.7-day radon emanating from the surface of the earth. Pb-210, as far as can be determined, is produced only from this source. The variability in the Pb-210 concentration tends to be small (the two high values at 32°N in the troposphere are believed to be erroneous) when compared to the distribution of artificial radionuclides. It is seen that the equatorial stratosphere has somewhat higher values than are found elsewhere in the stratosphere. This may mean that the entry of radon (or its decay products) into the stratosphere occurs in equatorial regions, although the data as yet is too scanty and conclusions at this time would be premature. Further information on Pb-210 distribution would be valuable since this constitutes an important tracer for the movement of air from low altitudes to high. Tritium. H-3 in the atmosphere results from three significant sources. Cosmic ray bombardment, neutron capture by nitrogen in the vicinity of thermonuclear explosions and releases from nuclear reactors. It may exist in the atmosphere as hydrogen gas or as a constituent of water vapor or other gases (e.g., methane). No stratospheric samples of H, are available, but an estimate for the tritium content to be expected can be made from pre-bomb ground-level measurement. This was of the order of 1.6 x 107 tritium units (T.U.). One T.U. is defined as a concentration of 1 tritium atom in i018 hydrogen atoms. The values for tritium in Hy gas given in Table 1 are based on this pre~bomb surface air concentration and an assumption that the abundance of Hy in the stratosphere is 0.5 parts per million. The meteorological interpretation of such tritium measure- ments at high altitudes will be difficult, since there is little information on natural background and the contribution due to nuclear testing is unknown; however, a knowledge of the concentration would be of interest for future research. Tritium is present in the atmosphere as a constituent of water vapor. able to differentiate the natural background. As with C-14, it is important to be One attempt at establishing this value consists of comparing the