P UNIVERSITY OF CALIFORNIA ~ Y Los Angeles 24, California uaa Air Research and Development Command Post Office Box 1395 EuU-7 ‘| a February 2, 1953 ] . Institute of Geophysics elit lL) INFORMATION : SLB 411347 wee I ' Baltimore 3, Marylend THIS DOCUMENT cONSISES or7_prsesy LOS ALAMOS 02016 314 20 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- RESTRICTED DATA This | Gocuns't € t as dplins 1954, ; , ~vladtive and ~ em ed data ale ee Criminal gy Act of subjcot meee Sanctéons. --— A Lf (tL/ 4 ~ C - Jf rot i OFGNERGY DECLASSIFICATIONRELEW DIPARTVENT 921-4 7 DETERMINATION (CIRCLE NUMEERS)) {istmeviewoats: Of-¢2- TCLRSSIPICATION RETAINED © AUTHORITY: DAOC BIADC GADD 2) GrassiFICATION CHANGED TO: NAMES fe 3. CONTAINS NO DOE CLASSIFIED INFO 2ND REVIEW-DATE: 10-22. : ALTHORITY: ADD NAME: < aoa WITH: ABSIF'‘CATION CANCELLED & CA:5SieikD INFO BRACKETED 7.Crih a (SPECIFY): 3-5352 : - _ , ee