Upper Wind Vectors To begin a atudy of the behavior of atomio clouds it in necessary to mow what the upper winds were which affected the clouds and to consider how those winds Tha winds which affected the atomic clouds have bean estimated from the representative wind data which were obtained before and after H-hour, The exawina- acted an the visual airborne material produced by the atomic explosidns, tion of a number of wind acundings will give « truer picture of the air flow than a single sounding at the time in question. dn examination of al] of the wind scundings for the three test days shows that the upper winds were of the same general pattern on all of the test days, This is true because there are many approx- imations inherent in any particular scumding which result fros the accepted manner in At which soundings are made and upper winds caloulated, In estimting the wind directions and velocities which acted on the clouds, consideration has been given to the trends low levels the winds were easterly, and at progressively higher levels the winds veered through south into southwest or weat, and averuges for particular levels, and to the winds between the 5,000 foot levels. The abesnce of winds from a northerly direction were a radiological safety requirement, It was not practical to evacuate all personnel from Eniwetok, and it was known that the teste would be greatly hindered if installations on Eniwetok be~ cams radiologically contaminated. For thie reason, an area in the southsastern part of the atoll, including Eniwetok and Parry Islands, was aselected for anchorages of the observing ships; and the teate were conducted on dayé when there wis no northerly companent in the upper wind directions which might oarry radiologically active mte- (See Appendix II) In using these wind data it 1¢ assumed that the estimated winds oocurred at A-bour and endured for the three houre following. It is also assumed that the winds occurred over the entire Eniwetok area and acted on the cloud without regard to the restive distances of the mrts of the clouds or their locatias in the changing wind ds. rial southward. By chance, the winds on the dates originally selected for XRAY and ZEBRA days were operationmilly suitable. However, prior to 30 April, the my originally scheduled for YOKE DAY, there was a high frequensy of northwesterly winds in the anti-tredes at levels between 20,000 and 30,000 feet. In fact, prior to the actual YORE DAY, there wore fourteen impossible firing days because of wind conditions, XRAY DAY An examination of the upper winds shows that wind soundings above 55,000 feet cannot be estimated for H-hour. The winds for levels above 55,000 fret #hown in the second 0800 (iocal time) sounding are not believed to be representative of the winds at fl-hour, On 29 April, the upper winds showed a transitional zone of vartable winds with northerly components between 15,000 and 30,000 feet; and since there was no justification for believing that a rapid change in the wind structuro was imminent, there was considerable doubt expressed at the morning briefing that tha winds would change sufficiently to meet radiological safety requirements, There was, neverthe- less, a reasonable expectation that these awkward winds would tend to veer in such 4 manner that the northerly components would be eliminated within 48 hours. Thus, unfavorable winds caused the postponement of YOKE IMY until 1 May, By the time of the briefing on the morning of the new YQEE minus one day, the upper winds had altered sufficiently to indicate that a new air flog was beginning to predominate, and a forecast of favorable winda could be given, Sinne meteorological conditions were also indicative of suitable cloud conditions, the test was scheduled for, and conducted on 1 May, , The method of formation of the wing-lile plume which grew out of the TRAY DAY cloud has been studied,and it is thought that it was produced after a part of the top of the cloud protruded through the tropopause into the stratosphere, With winds which were nearly calm at the base of the atratosphere, the plume would have been formed as the primary masa moved awsy in the stronger winds. A treil of cirrocumilus type cloud was formed in this region of wind shear at the tropomune. The shear zone between 20,000 and 25,000 feet may be seen on the diagram labeled ProjectionNorth. At this altitude the winds changed from easterly to westerly, ond the yialble cloud separated, This separation of the clowd was thought to be caused by the energy of the weapon, ths altitude of the burst, the character of the surface, and other such factors rather than the wind sheer; however, the wind shear did contribute to the separation, Therefore, in theesetests the upper wind conditiona which determined the shapes of the clouds and the spread of the radioactive material were predetermined by the relative positions of the test sites and the area which had to be kept free YOKE DAY=— the cloud shich formed on the second test was a more or lass cantimous colum frea the surface to about 55,000 feet. The estimated winds on this day must closely approximte the actual winds for photographe of ths atomic cloud show that the cloud was shaped just as would be expected from the wind vectors. of radiclogical contamination, Qn IRAY DAY, nine minutes approximately after H-hour, the atomic cloud sheared and broke apart at about 20,000 feet and the drone airoraft stationed at that altitude could not sake a penetration as there waa no cloud visible at that alt@tude on which the aircraft could be vectored, For that reason, closer atten- tion waa paid to the probable shape of the atomic olouds in the ataff briefings om TOKE and ZEBRA days; and diagrams were presented at the briefinga on those dseysa which pictorially showed how the atomic clouds would look from the observing ahipe, Theso diagrams were drawn in similar manner to the diagrams laboled ZEBRA DAY= tha third atomic cloud reached an altitude between 30,000 and 35,000 feet, = that are in the upper right hand corner of the following three pages except that the dashed line connecting the arrow heads was replaced by a rough outline of an atomio cloud, This led to an incorrect prediction of the spe of the better understanding of the shape may be obtained by exmining the winds at 1,000 foot intervals between 10,000 and 20,000 fest, These intermediate winds were used in drawing the diagram labeled Cloud on ZEBRA DAY cloud because the atomic cloud an that day did not go as high as the AY and TOKE clouds, or to as high an altitude as was indicated on the diagram; however, on YOKE DAY the shape of the cloud was remarkably like the predicted shaps. Therefore, it is not necessary to consider winds above 35,000 feet. when studying thin cloud, The shape of the cloud was approximtely what might be expected from the mind vectors; however, in the case of this particular cloud, a Page dae Further use of these vertical projections for predicting the shape of atomic clouds from upper winds ia believed to be worthwhile. -(4-