il and rock, all of which become entrained
in an expanding, fast-rising fireball. As the fireball rises, it
expands and cools, producing the distinctive mushroom cloud, signature
of nuclear explosions.

The altitude reached by the cloud depends on the force of the
explosion. When yields are in the low-kiloton range, the cloud will
remain in the lower atmosphere and its effects will be entirely local.
But as yields exceed 30 kilotons, part of the cloud will punch into the
stratosphere, which begins about 7 miles up. With yields of 2-5
megatons or more, virtually all of the cloud of radioactive debris and
fine dust will climb into the stratosphere. The heavier materials
reaching the lower edge of the stratosphere will soon settle out, as
did the Castle/Bravo fallout at Rongelap. But the lighter particles
will penetrate high into the stratosphere, to altitudes of 12 miles and
more, and remain there for months and even years. Stratospheric
circulation and diffusion will spread this material around the world.

RADIOACTIVE FALLOUT

Both the local and worldwide fallout hazards of nuclear explosions
depend on a variety of interacting factors: weapon design, explosive
force, altitude and latitude of detonation, time of year, and local
weather conditions.

All present nuclear weapon designs require the splitting of heavy
elements like uranium and plutonium. The energy released in this
fission process is many millions of times greater, pound for pound,
than the most energetic chemical reactions. The smaller nuclear
weapon, in the low-kiloton range, may rely solely on the energy
released by the fission process, as did the first bombs which
devastated Hiroshima and Nagasaki in 1945. The larger yield nuclear
weapons derive a substantial part of their explosive force from the
fusion of heavy forms of hydrogen--deuterium and tritium. Since there
is virtually no limitation on the volume of fusion materials in a
weapon, and the materials are less costly than fissionable materials,
the fusion, "thermonuclear," or "hydrogen" bomb brought a radical
increase in the explosive power of weapons. However, the fission
process is still necessary to achieve the high temperatures and
pressures needed to trigger the hydrogen fusion reactions. Thus, all
nuclear detonations produce radioactive fragments of heavy elements
fission, with the larger bursts producing an additional radiation
component from the fusion process.

The nucle