PLUME FORMATION IN THUNDERSTORMS 101 
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Fic. 9—Derivation of plume pattern in the vertical, along bearing 340°; shaded area at left is a see- 
tion through a typical storm core; solid lines represent projections of particle trajectories in the 340° 
160° plane (variation of fall speeds with temperature and pressure was taken into account); dashed 
lines are isochrones, drawn 15, 30, 60, --- 
min after the beginning of pluming; positions of trajectories 
and isochrones are shown relative to storm (two-fold vertical exaggeration) 
by the radar. At the other extreme, particles 
falling faster than those mentioned may also ex- 
ist in the shower at high level, and be moved by 
the wind, but would remain unidentified by the 
radar, since such particles would fall very close 
to, or within, the intense core of the storm itself. 
This point is among those borne out by Figure 
9 which is a theoretical reconstruction of a plume 
in vertical section, the way the eye might see it. 
The lines sloping down to the right are the 
trajectories of particles of various fall speeds, all 
originating at 35 kft. The fall speeds are in 
meters per second, and are all corrected to stand- 
ard temperature and pressure. To the extent 
that the plume has texture, e.g. unevennesses or 
characteristic knobs or holes, it would give the 
appearance of developing relative to the mother 
storm, in the direction of these trajectories. The 
dashed lines are isochrones, and indicate size and 
extent of the plume after 15, 30, 60, and 90 
min from the incidence of pluming. The earlier 
stages are in excellent agreement with observa- 
tion. The 90-min line would indicate a rather 
broader base to the plume than we observed, but 
it is at such later times, and in the altitudes be- 
tween 25 and 15 kft, that evaporation effects, 
neglected here, are especially important. Espe- 
cially noteworthy is the separation of the par- 
ticles in the plume according to their fall speeds. 
For the case of snow-generating cells and the 
‘continuous’ precipitation resulting from them, 
such sorting of the particles was studied in detail 
by Gunn and Marshall [1955]. 
Acknowledgments—It is a pleasure to ac- 
knowledge the interest in this work of J. 8. Mar- 
shall, who as co-author of a method of analysing 
snow trails, suggested that a similar approach 
might work for plumes. J. L. Galloway con- 
tributed a carefully documented analysis of the 
synoptic weather pattern of the day referred to. 
Though his painstaking work has not been 
quoted explicitly, it is he who has provided confi- 
dence in the validity of the hodograph. 
This work was performed under Contract 
AF19(604)-2065 with the Geophysics Research 
Directorate of the Air Foree Cambridge Research 
Center. 
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