PLUME FORMATION IN THUNDERSTORMS 99 
Fic. 7—Horizontal sections at 5 kft through Storm 2 indicate motion 
and development of core; below each sketch, shaded areas are vertical sec- 
tions, again along bearing 340°; outlines of plumes are shown dashed (2- 
fold vertical exaggeration); at about 17h00m, the storm core decayed, and 
the horizontal sections thereafter (shown dashed) were taken through the 
plume at height 10 kft 
plumes, which at any level was formed by the 
fastest particles, fell faster than the rest. For 
lack of replenishment, the northern end of the 
plume at any level should therefore have dis- 
appeared faster than the rest—in agreement 
with observation. It is tempting to estimate the 
fastest particle fall speeds associated with the 
northern tip of the contracting plume. For the 
times 18h00m, 18h14m, and 18h24m such esti- 
mates are 23, 13, and 11 ft sec” respectively, 
based on the plume contraction at these times. 
These estimates, though plausible, are not 
unique. Slower particles originating at levels 
lower than 35 kft could have accounted for the 
same observed pattern. Thus if the northern end 
of the plume consisted of particles falling at up 
to 10 ft sec* which had been released at about 
30 kft, the retraction of the plume at 25 kft 
would also proceed at the observed rate of about 
0.5 mile min™. The supposition that these lower 
levels also contributed is supported to some ex- 
tent by the change in direction of the plume, for 
a gradual clockwise rotation of the plumes was 
observed. All the plumes, those of Storms 2 and 
4 even more notably than Plume 3, had their 
longest dimension in a N-S direction in their 
later phases. A glance at Figure 5c shows that 
such rotation is best accounted for by supposing 
that while plumes formed initially at 35 kft, in 
their later stages, more and more of the material 
in them derived from lower levels of the shower. 
There is no evidence in our records, however, of 
particles stemming from levels lower than 30 kft. 
Comparisons of the kind outlined were made for 
three of the plumes at several levels; the conclu- 
sions were essentially the same, and will now be 
summarized. 
The storms were tall precipitation clouds, 
which in the face of very strong relative winds 
5 
° 25 MILES 50 a 
L \ N 
\ 
7 \ 
RADAR 
ti ' 
CA TOAOY |. 
1630 ® 
1642 % 
NY oxy 
WAN 
vis 
25kft Sections wy 
H 
(Storm 3) 1657 cS NY 
Ni ~ 
1708 Me lees 
1720 1734 100 ‘'8I4 
1747 
Fie. 8—Horizontal sections through Storm 3 at 
25 kft, showing development of plume; storm core 
disappeared at about 18h 00m EST 
