This material certainly consists of large, heavy dirt particles which subsequently fall out of the cloud. Yet most of them somehow fail to come in contact with the radioactive fission products. This peculiar phenomenon can be understood by looking at the details of how the fireball rises. At first the central part of the fireball is much hotter than the outer part and thus rises more rapidly. As it rises, however, it cools and falls back around the outer part, creating in this way a doughnut-shaped structure. The whole process is analogous to the formation of an ordinary smoke ring. In most of the photographs one sees, the doughnut is obscured by the cloud of water that forms, but sometimes when the weather is particularly dry, it becomes perfectly visible. During the rather orderly circulation of air through the hole, the bomb debris and the dirt that has been sucked up remain separated. (See pictures 1-4.) The close-in fallout accounts for only a portion of the radioactivity, ranging from less than a per cent for a high altitude shot to almost complete deposition for some ground shots. For the world-wide fallout we are interested in what happens to the remainder. This depends on how the atomic cloud is carried by the upper winds for long distances. In this connection it is important to distinguish between a big bomb and a little bomb. It is also important to distinguish between the lower and higher portions of the atmosphere called, respectively, the troposphere and the stratosphere. The atmosphere is heated by the sun in an indirect way. The sun’s rays pass through air without warming it. They heat up instead the bottom of the atmosphere, that is, the solid ground. The atmosphere is heated in the same manner in which a boiling pot is heated on the kitchen range. The heat is delivered from below and is carried in rising currents to the top. Only in the case of the atmosphere there is no sharp upper limit. The currents rise to an altitude of thirty to fifty thousand feet, then turn and descend. This boiling part of the atmosphere is called the troposphere or region of heat. Above it there is less vertical motion. The upper region is called the stratosphere or stratified region. For a little bomb the atomic cloud stops rising before it reaches the stratosphere. For a big bomb, above about a megaton of energy (a million tons of TNT equivalent), the cloud pokes right into the stratosphere and keeps going to a height of a hundred thousand feet or so. The most important fact about the stratosphere is this: It has very little weather. Most of the weather phenomena such as clouds, rain, snow, fog, mist, etc., are confined to the lower portion of the atmosphere, the troposphere. The stratosphere, however, contains practically no water.