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UNIVERSITY OF CALIFORNIA
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Los Angeles 24, California
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Air Research and Development Command
Post Office Box 1395
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February 2, 1953
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Institute of Geophysics
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Dear ColonelIsbell:
Thank you for your letter of 12 January. Please excuse the delay in answering
it; I am in the middle of moving our research project to the Hawaiian Islands and have
not, until now, had an opportunity of giving undivided attention to the very important
topic raised by your letter. The priority of the topic, indicated in the sécond peragraph of your letter, justifies both the informal style of my answer and its length.
I feel that I should explore as thoroughly as possible those points which, in the
interests of brevity, had to be omitted from my report.
First, I must give the positive evidence which led me to asswne that the mushroca
cloud lay in the troposphere. Within a few minutes of the detonation (I forgot how
many now, and I have kept no tiotes) we made a vertical angle measurement on the edge of
the mushroom of 92°, in other words, we were at that time just under the edge of the
canopy.
The details of the cloud overhead were very plain.
It consisted of liquid
water, at least in all parts which were visible; the edges were hard and bright and, in
places where fragments were detached, it hed the structure of altocumulus mixed with
altostratus. Since it remained in this condition for a considerable time, long after
eny intrinsic upward motion due to differences in temperature between it and its environment hed ceased, it could not have hed its base at 60,000 feet but rather somewhere
in the layer 25,000-45,000 feet. At 60,000 feet, the temperature in the clowl would be
very much below -40° C and the whole cloud would consist of ice particles. The appearanc
would be quite different from that observed. I watched the mushroom cloud for almost
two hours after the explosion; the various parts of the deformed cloud were readily
distinguished from natural clouds, which formed in the vicinity, by their colour,characteristically pink or, rather peach-coloured. Although some parts of the tops of fragments had been transformed to cirrus, most of the derivatives of the cloud assumed typical tropical altostratus form. Moreover, rain continued to fall from parts of the cloud,
and, in profile and except for the colour of the parent cloud, the rain looked similar
to altostratus precipitetion typical of the Marshall Islands in disturbed weather. In
addition to this, both Major Stopinski and myself had observed certain fragments of
‘natural cirrus (certainly lying within the troposphere) which were almost overhead just
before the explosion. The advancing edge of the mushroom cloud stopped in such a position as to underlap part of this natural cloud. Several competent observers agree with
me in saying that the edge of the mushroom was lower than the natural cirrus, which
was, indeed, relatively undisturbed by its advance. So much for the gross observable
structure of the cloud.
,
Now we come to another kind of evidence. The winds for the day are very well
known up to and above the tropopause. If the bulk of the cloud had been in the stratos-
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