be the case if the deposition were uniform over the entire globe. Within the band of tropospheric fallout there is patchiness as well. Not only is there a general decrease downwind and in a north or south direction from the source but also variations which depend on rainfall. Thus outside of the first, say, 600 miles from Nevada Test Site, one does not find the highest individual deposition immediately beyond 600 miles but rather in the Albany-Troy region of New York State about 2000 miles away. Here, a rapidly moving nuclear cloud at 40,000 ft was scavenged by an intense thunderstorm. The probability of a second such coincidence in the samepiace is, of course, very smail. We find that precipitation scavenging is the main mechanism for the deposition of small particles. The ratio of deposition in rain to that in nonrain varies from 2 to 20. Rapid deposition in a matter of 30 to 60 days of intermediate fallout allows some of the shorter-lived isotopes to contribute to the hazard, whereas the delayed fallout, taking years to come down, involves potential hazard from only those fission products whose half lives are of the order of years. It is also worthwhile to note that the intermediate fallout is all deposited, whereas much of the delayed fallout still, literally, hangs over our heads. 4 DELAYED FALLOUT Delayed fallout is of interest because it represents widespread deposition of a very sizeable amount of the fission products. In megaton explosion it contributes about 15 to 20 per cent for land shots and over 95 per cent for air bursts of the total fission yield. This fallout originates exclusively from particles that were initially injected into the stratosphere. Perhaps a word of explanation about the use of the terms troposphere and stratosphere is in order. In 1899 Teisserenc de Bort first flew a balloon to high altitudes. His ascent probably looked like that on the left-hand side of Fig. 4. The temperature first decreased with altitude and then abruptly remained constant or increased with height. The point of discontinuity in the ALTITUDE, FT IH 6¢.000 ———+ H———- 50,000, =——— -——— 40,000. ——— H———_ 30,000 —___] +H 20000. ———+ H———— 19000. ———+ 6 N TEMP —» | | STRATOSPHERE | TROPOPAUSE | | | TROPOSPHERE | | | | | Ll EQ LATITUDE S Figure 4 vertical temperature gradient is now called the tropopause and separates the troposphere below from the stratosphere above. Many of you have seen smoke emitted from a stack on a windy afternoon. It clearly reflects the turbulent nature of the atmosphere for the case of temperature decreasing with height, typical of the troposphere. On the other hand, you have also seen smoke during quieter, nonsunny periods (evenings, for example), when, near the ground, the temperature increases with height. This nonturbulent evening-like condition, we think, typifies the stratosphere. We are fairly sure that a pollutant near the ground will mix throughout the vertical extent of the troposphere in a matter of days with a few exceptions. It is suspected that the vertical mixing of the stratosphere is very much Slower, being similar to the nearlaminar evening mixing. Contrary to the views of some nonmeteorologists, the prolonged 315