above the tropopause increased in speed
to the highest altitude of the available
wind information for the Bravo shot,
while for Mike the easterly winds decreased in speed and ultimately changed
to westerly winds. The easterly winds in
the trade-wind layer, the moist maritime
air mass lying near the sea, extended up
to about 20,000 feet during the detona-
tion of the Mike device, while for the
Bravo shot they were below 10,000 feet.
Between the trade-wind layer and the
tropopause, one normally finds westerly
winds. During the Mike shot these westerlies were temporarily interrupted and
became southerly winds, while for the
Bravo shot they were toward a more
normal bearing.
In Fig. 2. is found the approximate
area covered during the early days by
that part of the nuclear cloud from the
Mike shot which was located below the
tropopause. The shaded areas in Fig, 2
have been deduced from meteorological
considerations alone, and, in manycases,
are subject to considerable uncertainty.
Shading was discontinued when the
meteorological data no longer warranted
any reasonable estimate of the path. The
light winds and sparsity of upper-wind
observations have made tracing the upper
tropospheric portion of the Mike cloud
particularly uncertain. For this reason,
the time of passage across the North
American mainland is unknown. Tracing
was discontinued on 7 November. The
tradewind portion of the nuclear cloud
appears to have split south of Japan, the
upper portion (near 20,000 feet) curving
around a Pacific high cell and entering
the United States about 9 November.
The estimated meteorological path of
the Bravo cloud is shown in Fig. 3. The
upper tropospheric portion of the nuclear
cloud was traced to the Central Ameri-
of the two nuclear tests, suggests a path
that would travel around the carth at
about the same latitude as the point of
origin. It is interesting to note that in no
case was it imperative to rely on stratospheric transport of the nuclear debris to
account for the earliest arrival at any
point, for the transport of the nuclear
cloud in the troposphere appeared to ac-
count for the first observations of radio-
activity.
An attempt to determine the earliest
arrival time at the ground at each point
of observation has been undertaken. The
results, which are shown in Figs. 2 and 3
as the numberof days after the shot day,
should in many cases be viewed with
caution. First, in many of the stations in
feature unusual for the region during
any season.
There has been no attemptto track the
stratospheric portions of the atomic cloud
because of the sparsity of wind observa-
tions at these altitudes. Evidence from
numerous isolated high-level winds, not
necessarily obtained during the periods
14 SEPTEMBER 1956
The comparatively small values ob-
mage
Sn
+
110,000
“+
+
100,000
Sy
+
+
+
+
+>
+o
+s
70,000
t~,
aa
g 60,000
+n
_,
~
°
£
90,000
Fallout
80,000
It is noted that, in accordance with the
meteorological estimates, the fallout over
the United States progressed roughly
from west to east during the Mike shot.
Fallout from the Bravo event did not appear at the West Coast stations in the
United States until 2 weeks after one of
the cloud protuberances entered the cen-
tral United States. Of perhaps greatest
interest, although also of greatest doubt,
in the Southern Hemisphere. Thus, for
example, a literal interpretation of the
chart reveals that every station in the
Southern Hemisphere showed an earlier
arrival time than did the United States
West Coast stations for the Bravo case.
of
interest
are
the
compara-
tively late arrival times for the mid-Pacific stations west of the Hawaiian Islands during the Mike fallout. These stations were south of one branch of the
nuclear cloud and north of the other.
The actual fallout at each station and
an analysis of the data are shown on Figs.
4 and5. The units are cumulative decayed
beta activity for the first 35 days following each event and are approximately
equivalent to millicuries per 100 square
miles (the values have not been corrected
for the efficiency of the gummed film.)
Several features that differentiate the
two maps should be noted. First, an aver-
age value for all United States and Canadian stations was obtained for the Mike
shot, as opposed to values for individual
+
¢
rival time of the cloud at manystations
coincided with rainfall, suggesting that
the nuclear cloud may have been overhead some time earlier but that precipitation was required to bring its activity
to earth.
Also
tion not far from the Marshall Islands
whichis not typically present in March.
The shallowness of the trade-wind layer
during the Bravo shot is an example of a
marily in an attempt to locate stations
in rainy areas. In manycases, when the
period of record is incomplete or the
data are suspect, parentheses have been
placed around the number. No attempt
has been madeto reconstruct the isolines
for the fallout that occurred within the
first 24 hours of the shot.
despite elaborate precautions,it is likely
that some gummed films were contaminated during handling. Finally, as noted
in the second paragraph the apparentar-
are seasonal and in part due to the spein November the mid-tropospheric westerly winds are not as strong as they are
in March, and they are located farther
north, on the average. Further, in Novemberonefinds an anticyclonic circula-
Finally, as is evident, the network was
expanded between the two events, pri-
activity was so low that it made the arrival date almost meaningless. Second,
are the comparatively early arrival times
cific metcorology for the shot days. Thus,
tained from transport vessels for Bravo.
the Southern Hemisphere, the deposited
can area by about 5 March, and an offshoot extending northward into the
United States at about 20,000 feet was
detected approximately 1 week later.
Differences between the paths of the
Mike and Bravo clouds are evident from
Figs. 2 and 3. In part, the differences
stations during the Bravo shot. Second,
the isolines located between points on the
West Coast of the United States and
points in the Western Pacific Ocean are
also based on fallout observations ob-
z
5
+
LoS
+.
pe
ty
50,000
Vt
or
40,000
T
ot
toa
J
30,000
;
ot
20,000
Ns
web
_K
ae
10,000
ad
KY
0
N
+
N
“te
wre
MIKE
BRAVO.
Fig. 1. Upper winds at shot time. Arrows !
blow with the winds, and barbs indicate
wind speed; full barb, 10 knots; one-half
barb, 5 knots.
475