Descriptions of the Atomic Clouds On the opposite page are show outline sketches of the three clouls as they appeared at 15 uinutes after H-hour, When seen at ‘thie stage, the olouwis each had the features by which they are most ensily identified, Prior to thie time, the clouds had somewhat the same char- acterintios. That is, they consisted of « globular sass cua thick stem. Afterwards, they were in various atages of diffusion; tut at 15 minutes past H-hour the differences between the clouds were unuistalable. With al] three clouds, the part of the clowl referred to as the * mass," or the “primary portion," is that part of the cloul which seamed to have come from the initial hot ges bubble, which had been either tha head of a sushrouwr cloud or which had risem because of its om high temperatures. At 15 minutes past H-hour, the primary meas of the INAT Day cloud was globular and somerhat resembled the top of a swelling cumlus cloud; thet of TOKE Day seomed to consist mostly of smoke and dust and was not casy to distinguish froe the stem because of the irregularities of the clouds and that of ZEBRA Day wer something Mike the remains of an oversized smoke ring. Observers were instructed to ais thelr theodclites at each aide of the primary sass in order that the diameter of the radioactive cloud sould be roughly calculated, = The words "highest point" also require explanation, When the primry mepes were rising, the highest point was simply what appeared to be the top of the globular mass, or mushroos. However, unexpected changes which confused the observers, tock place in the tope of all of the clouds, The IRAT cloud developed a wing-shaped sheet of cirrus which is referred to Immediately following the flash, the growth of the hemispherical candensatian clowl begins. It grew ot approrimtely the speed of sound as the drop of atgospheric pressure behind the explosion wave caused condensation of water vapor, The determination of the ayact dimansions of these special clouds were not part of the meteorological program as their sizes are important considerstions in energy calculations; however, for the purposes of comparison, it will be stated that the IRAT and the ZEBRA Day condensation clouds were about 5 miles in dis- weter and the TOKE Day cloud was nearly 6 miles in diasster, In each case, the condensation clouds had nearly amsooth white sides and were brilliantly lighted from within, They appeared to be nearly bomispherical, but the YOKE condensation cloud was distinctly flattened on the top side, It was difficult to see the bottom of the TRAIT Day condensation cloud becaune of distance and natural clouds, ani the base of the TORE Day cloud seemed to rest nearly on the eurface; however, the base of the ZEBRA Day condenaation cloud was at about 2,000 feet, so that it was possible to see under it and to see the flames around the base of the fireball. The condensation clouds disappeared from the middle outward sc that to a surface observer these clouds seemed to vanish instantaneously, Frog the air, the clouds bad the shape of a doughnut, with the incandescent gases of the fireball in the middle, (See Note Below) After the condensation cloud disappeared, the incandescent gases which formed the atouic cloud were seen. The colors of these bot gases changed from nearly white, to yellow, to crange, and then to red in about 20 seconds. The closer the test site, the brighter the flames in the fireball appeared and the longer they secmed to last. The quantity of the mate- as a “plume;" the TORE clow spread at the top so that it had the "anvil" shapes of a cumulonimbus; and the ZHU Day cloud had a projection, or "finger," which pushed out through rial in the YORE Day cloud was noticeably greater than that of the XRAY Day teats and on TOKE aiming point from the primary mas to the near edge of the plune, since the elevation angle flattened tops, and they seemed to rest on a pedestal of amoke and duat which hed bean swept up bohind than as they began their repid rise. the primary meas, In the case of the IRAT Day cloud, some of the observers shifted their of the edge of the plume was larger. They also shifted to the end of the finger on ZEBMA Day for the same reason. It ia only through the use of the original sketches that it is possible to tell how tha theodolites were aimed. The point referred to as tha “highast point" 1a meant to be the highest part of the primary waas; however, in the case of the TOKE Day cloud, the Primary meses 18 a0 poorly defined that it ia impossible to determines whether or not the elevation angle for the top of the anvil is ths same as that for the top of the primary nass. Ta the case of the ZEBRA Day cloud, the top of the finger-like projection is actually the part of the cloud highest in elevation, but the plume and the anvil of the FRAY Day and the TOKE Dey clouds are only signs that the top parte of the clouds are epreading out against the base of the stratosphere. As one of the purposes of the théodolite observatias was to obtain the true altitude of the mdioactive clouls, the fact that some cheervera ahifted to minor features of tha clouds was widesirable, However, where data can be obtained for the alnor featuros without the loss of information for the primary masses, mich data should be collected. That part of the atomic clouds which is referred to as the "stem" of the cloud (not la- beled on the sketches) is what is below the primary man, The atea of the clouda consisted of material swept up from the surface or left behind as the primary mesa cooled in rising. The XRAY Day cloud bad ancther feature berides the plume not common to the other two clouds. That is, it had a distinot "hreek" between the primery portion and its stam, Phenomena common to all air bursts of atomic weapons which should be amtioned az part of a mateorologioal report on atomic olouda are dincusned below in the approximate order of their occurrence, H-hour was marked by the indescritably brilliant flash, On al] three tests, observers actually felt the radiant heat om their faces for a brief instant, but the heat was of too short duration to be recarded on any ordinary meteorological equipment. (Ko observer should look directly at the test site at H-hour without a suitable dark filter for bis eyes, and no ordinary instrument should be pointed at the flash.) Day, the flameo seomed to spread over the island and linger momentarily before starting up- werd, At about Hehour plus 10 seconds, the firetnlle were nearly spherical ameept for their da the brighter colors of the firaballa faded, the soft, blue-violet luminescence of the clouds was revealed. This glow seemed to occur both within the cloade and in the eir sbich was closely surromding. The intensity of this luminescence and its duration is greatly dependent upon the distance of the obeerver fro the clowl, The luminescence of the IRAT Dey cloud faded gradually end disappeared in about two mimites, whereas the glow of thn YORE and the ZEBRA Day clouds lasted about four minutes. The fact that the clouds were self-lumrinous is important to visual observations as features of the clouds could be seen even before the morning twilight became effective. It is presumed that the ABIE Day clowl at Bikini also possessed thin property of Juminsscence, but that the strong mmlight prevented the glow frou being sean. While the lusinessence was being observed, the presgure wave associated with the scund of the explosion arrived. On IRAY Day, the pressure wave arrived at the obeerving shipe in about one and one-half mimtes, on TOKE Day, the wave arrived in about one minute, and a ZEBRA Day, the elapsed time was 45 or 50 seconds. There was no feeling af physical push frow any of the pressure waves and no feeling of discomfort. Ordinary meteorological inatruments were not significantly affected by the pressure wives, The ordinary aicroberograph is designed to resiat sudden pressure changes, Hovever, specially built microbarographs on the 0.3.5. Mt, McKinley without damping mechanism showed pressure chenges for the threes tests of 2.5, 7, and 9 millitars, respectively, The sounds of the three weapons were as follows; On XRAT Day the sound was a deep rumble resembling heavy thundcr, whereas on IOKE Day, the sound was a resounding "pop" which wee much different from the samd of the XRAT Day weapon, The soured on the TOKE Day weapon wan similar to the sudden report, comsiderably magnified, of a paper bag which is forcefully burst in a amall room. Of possible interest is the fact that the sound did not seem to come from any particular direction, An observer not knowing the direction of the test alte vould not have been able to point to ite source. The 2BRA Day explosion sounded much like the "bang" of an eight-inch gm, if the gum were heard from several lamired feet amy. Although the TRAY Dey explosion bad a rumbling sound, there were no reverbarations on YORE and ZEBRA Tnys. Note: The condensation cloud grew te ful] size and disappeared in 5 or 6 seconds.