in the nearby fallout (within several hundred miles downwind) being
partially depleted in strontium-90 while that at more distant areas
will be enriched.

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.

The activity placed in the stratosphere circles and recircles

of:
— uf

the earth, first at the same general latitude as the burst and then slowly
spreading laterally.

At the same time there will be a slow diffusion

into the tropopause.

Initially, there will be more deposition in the

same hemisphere (northern or southern) in which the burst occurred
but after many months the rate of deposition may become more generally
uniform over the entire earth's surface.

In terms of strontium-90

about 10 to 20 percent of the activity remaining in the stratosphere
may descend each year.
The distribution of the nearby fallout (up to several hundred
miles downwind) from high yield weapons detonated near the earth's
surface will be determined principally by particle size, initial position in the steam and cloud, and by the wind structure at various
altitudes.

The particle sizes and the distribution of these particles

within the stem and cloud are principally functions of the yield of
the bomb, the nature of the surface over which the burst occurs and
the quantity of material vaporized.

There are uncertainties in our

knowledge but Figure 1 presents one generalized concept of such an
initial distribution.

Although the cloud may be 100 miles in diameter,

the activity probably is not uniformly distributed, but rather is more

concentrated near the central and lower portions of the cloud:

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