suspension of contaminants in the stratosphere is due to the slowness of vertical mixing throughout the lower stratosphere and not because the tropopause is some kind of a semipermeable barrier. The right-hand side of Fig. 4 shows the change of the tropopause height with latitude. It is highest in the equatorial region, lowest in the polar regions, and, on many occasions, shows a break in the temperate latitudes coinciding with the jet stream (a rapid west to east river of air in the upper troposphere). Less is known about the stratosphere than the troposphere, mainly because it is harder to get at. Certain evidence of atmospheric motions on transport phenomena in the stratosphere which bear on the question should be reviewed. Where should the stratospheric radioactive particles reenter the troposphere? The residence time in the stratosphere is also of concern, but, since our interest revolves around Sr”or C387 both with 28-year half lives, no significant decay will occur in the stratosphere if the residence time is much less than 28 years—which appears to be the case. 9 ATMOSPHERIC TRACERS For more than eight years, the British have been making measurements of humidity by specially instrumented aircraft to about 48,000 ft or about 13,000 ft into the stratosphere.” These flights show, as seen on the left-hand side in Fig. 5, a frost point as low as 190° absolute over England. They find that this low value is amazingly constant in time. A flight in the stratosphere from the Sahara Desert to Iceland confirmed the same constant low frost point. oy Ol ALT. FT |*/ 70,000 SX, S 60,000 I 190% I 200° A — + || _ ee ee ee ee Car 200% 210% FROST POINT _ 490°A ——-— 50,000 }~---— 40,000 -— WARMER THAN 200°A 30,000 20,006 oS ° WARMER THAN 200°A | < —— | — 40,000 -— | _] 0 N POLE 1 50°N WINTER L EQ SUMMER S POLE Figure 5 Where could air originate that has a frost point as cold as 190° absolute? Toattain this low value, the air must have passed through a region with temperatures this cold in order to condense out the excess moisture. The most likely place, as can be seen from the right-hand side of Fig. 5 is the upper troposphere or lower stratosphere of the equatorial region. This probably means that stratospheric air over England at, say, 45,000 ft came from the equatorial tropopause region. It also means that very little tropospheric air was probably transported upward over England since this would bring moisture with it and would raise the humidity values above that which is observed. A second tracer of atmospheric motions is ozone. Ozone is formed by photochemical reactions at about 75,000 ft and above. It is transported into the lower stratosphere by mixing and direct air movement, so that most observations below 75,000 ft show more ozone than should be there from photochemical processes alone. Measurements made primarily in Germany and reported by Paetzold® (Fig. 6) and as yet unreported work of Brewer and col- leagues in England reflect the same seasonal variation in ozone between the tropopause at about 75,000 ft. Between 60,000 and 75,000 ft there is an ozone maximum in late winter anda minimum in late summer. In the 30,000- to 45,000-ft layer, as well as in the troposphere, the 316 Eastin: ‘