ge
limited by the tropopause but only the great bulk of if (consisting largely of condensation
‘ In entrained air. condensed steam from the sea surface and coral and other debris from
the destroyed Islands). There is indirect evidence that the initial central turbulent pil-__ .
lar pierced the tropopause at obout H plus 3 minutes but its further ascent would be
hidden by the great lateral development of the vortex ring. (mushroom cloud). ((Develop~
ment of plume was seen aboard LST 213 miles from zero)} This expansion seemed to
be symmetrical and very rapid in its initial stages but began to diminish sensibly by H
plus 10.minutes. By H plus 15 minutes, if had in all appec:ances ceased and the mush-
room cloudat that time was 65 miles In diameter. In direct sunlight it was of a creamy
white color but the shadows were intense orange. The first evidence of precipitation
from the mushroom cloud occurred ot H plus 6 minutes. Dark trails were seen descend:
Ing from it in the ‘vicinity of the stem. I suggest that these trails consisted largely of
mud and rain. The first rainbow was seen below the top and to the Jeff of the sfem at
H plus 7 minutes and rainbows were seen in various positions under the cloud as the
ship maneuvered between H plus 30 minutes and H plus 45 minutes.
.
oe
After H plus 15 minutes. the intrinsic cloud motions having virtually ceased,
the whole structure began to move'and to be deformed by the winds near Eniwetok. Up
to that time. however, it can be considered to be Independent of the wind. the explosion having set up its own local symmetrical ci-culation ove: an area af least 65 miles
in diameter. The mushroom cloud during the period of deformation was transformed
into a densesheet of altostratus topped by cirrus. Many cumulus shower clouds formed
beneath it, penetrating it in places: rain also continued to fall from the altostratus but
much of if was in the form of virga. The deterioration of the local weather prevented useful
observation of the cloud after H plus 1} hou: 30 minutes.
At sunset, the weather having improved considerably, opportunity again occurred
of observing distant and high parts of the cloud. The sky color from 10 to 25 minutes
after sunset was extraordinarily brilliant. Low in the west there appeared a chevronshaped. brilliantly illuminated tenuous cloud which | suspect. since it was higher than
natural cirrus in the vicinity. lay in the st:atosphere. The tentative suggestion may be
made that it was derived from a narrow filament that pierced the tropopause at H plus
3 minutes. Its position in the west suggests that it moved with the stratospheric winds
between 60,000 and 80,000 feet.