Compared with the immediate colossal destruction suffered in target areas,
the more subtle, longer term effects of the remaining 10 percent of the
energy released by nuclear weapons might seem a matter of secondary
concern. But the dimensions of the initial catastrophe should not
overshadow the after-effects of a nuclear war. They would be global,
affecting nations remote from the fighting for many years after the
holocaust, because of the way nuclear explosions behave in the atmosphere
and the radioactive products released by nuclear bursts.

When a weapon is detonated at the surface of the earth or at low altitudes,
the heat pulse vaporizes the bomb material, target, nearby structures, and
underlying soil 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.