CHAPTER 3 DISTRIBUTION OF RADIOACTIVE DEBRIS METEOROLOGY A nuclear detonation creates a large hot bubble of air containing most of the radioactive debris. The buoyance of this bubble causes it to rise rapidly, and in the process of rising, it is cooled by adiabatic expansion and by mixing with large quantities of ambient air. The resultant rapid cooling of the rising mass brings it to equilibrium with the atmosphere in about 5 to 10 min, andit stops rising at this time. Theparticles in the bubble fail, under the influence of gravity; very large particles rapidly reach the ground; very small particles are so mixed by the atmospheric turbulence that they may be considered to act as a gas. The large mass of air containing the small particulate debris is subjected to the same atmospheric motions as any other large mass of air. The first step, therefore, in con- sidering the spread of atomic debris is to inquire into the motions of the atmosphere. The picture that one gets of the motion of the atmosphere is very much a function of the sensitivity of the measuring instruments and of the frequency and spacing of the measurements. The established networksindicate that in mid-latitudes there is a mean motion of the air from west to east and that superimposed on this motion there are cyclones and anticyclones ranging from 100 to 1000 km in diameter. Special studies on a very small scale give ample evidence that there are turbulent elements of the order of a few meters, but the evidence for motion in the range scale from 1 to 100 km is not adequate. These different scales of motion all have their effect on the atomic a in the early history of the cloud, much larger than the cloud, so that the effect of these motions is to transport the cloud. In general, then, the centroid of the cloud will move toward the east and will be shifted to 23