Fallout & the Winds is also a old Civil Defense Technical Bulletin, like others I have posted before. This one is TB-11-21 of October 1955
Radioactive fallout is the surface deposition of radioactive material which has been explosively distributed in
the atmosphere by the detonation of a nuclear weapon. When a bomb is detonated at heights which’ allow the
fireball to come in contact with the ground, great quantities of pulverized and vaporized material are carried up
in the atmosphere. The cloud then contains a vast’ amount of radioactive dust particles of all sizes. from submicroscopic specks to visible grains or flakes. The larger particles settle to the ground rapidly, the smaller more slowly. The particles of earth are not in themselves radioactive, but fragments of bomb materials adhere to them and fall to the ground. This is fallout.
The radioactive particles formed from the bomb materials are themselves very small, and can remain in the
air for a long time before settl~ to the ground. For this reason the cloud from a bomb detonated high in the air
so that’ the fireball does not touch the ground, does not produce dangerous fallout.
Clouds produced by “A-bombs” of the type used in World War II generally do not rise above 50,000 feet. Photographs of the cloud produced by the first thermonuclear bomb at Eniwetok in November 1952 show that it reached a height of 25 miles, or about 130,000 feet. However, this does not mean that dangerous fallout comes from all altitudes up to the top of the cloud. It appears that the uppermost
part may not contribute much to the overall hazard. Still the evidence indicates that debris which rises to altitudes of at least 80,000 feet must be considered in attempting to explain the observed fallout from test detonations of the thermonuclear weapons. Figure 2 shows the comparative size of an A-bomb cloud, H-bomb cloud, and an ordinary thunderstorm cloud.