100 WALTER HITSCHFELD 
above 29 kft showed little tendency to bend. 
Instead the winds succeeded in eroding these 
storms by sweeping material away from them. 
This erosion was most pronounced at 35 kft, the 
level of the strongest relative wind, but occurred 
also to a slight extent above that level, and grad- 
ually spread down towards 30 kft. The evidence 
is clear that the plumes detected by the radar 
consisted of particles whose fall speeds were in 
excess of 3.6 ft sec’, and are thus probably pre- 
cipitation, rather than cloud, particles. The 35- 
kft temperature was —50°C. At 30 kft, it was 
—37°C, and it therefore is unlikely that any 
liquid material was involved. Fall speeds of pre- 
cipitation particles vary with height of course. 
Douglas, Gunn, and Marshall [1957] used the re- 
lation 
Vv x 702% 0-4 
where 7 is the viscosity and p the density of the 
surrounding air. On this basis the factors of Ta- 
ble 1 were derived. Using an average factor of 
1.55 for the trajectory of the particles explicitly 
considered above would indicate fall speeds (cor- 
rected to 1000 mb and 0°C) ranging from 3.6/ 
1.55 = 2.3 to 23/1.55 = 14.9 ft sec”, or in metric 
units from 0.75 to 4.9 m see”. 
Using Langleben’s [1954] measurements of the 
fall speeds of snow crystals, the lower speeds 
could be associated with dendrites of melted di- 
ameter about 0.8 mm; the fast speeds (around 
4 m see") were too high for aggregates, and 
probably indicate frozen rain or small hail. Us- 
ing Weickmann’s [1955] compilation for hail par- 
ticles of specific gravity 0.8, the melted diameter 
TaBLe 1—Increase of terminal fall speeds of 
particles with height in the atmosphere 
Height Pressure Temp f = v/vo*® 
kft mp | Se 
40 190 —58 1.9 
35 240 —50 iS 
30 300 =3i 1.5 
25 | 375 —22 1.4 
20 470 —138 1.3 
15 | 570 —3 1.3 
10 | 700 +6 1.2 
5 840 15 ii 
surface 1000 20 1.0 
® The factor f = v/vo , where vo is the fall speed 
of the particle at 1000 mb and OC, is evaluated 
here for July 1, 1956, but its value is not very de- 
pendent on the particular air-mass stratification. 
would be about 5 mm; for graupel particles of 
specific gravity 0.2, the melted diameter might 
be as great as 1.2 em. It may be pointed out that 
scattered hail was reported at the ground, which 
(from its timing and location) was shown to 
come from the showers under study. (Hail at 
the ground is indicated by the arrows on the 
5-kft pictures of Figure 2.) 
Newton [1960] has recently discussed the pres- 
sure field surrounding a storm moving in a wind 
field with vertical shear. Considering the storm 
as a rigid structure moving with the winds ap- 
propriate to its middle levels in a typical case, 
he finds ahead of the storm an excess pressure 
near its base, and a pressure deficit near its top. 
On this basis, Newton concludes that enhanced 
lifting, and so formation of new cloud, should 
take place at the leading edge of the storm. Con- 
versely, at the rear of the storm the pressure 
field is modified to lead to downdrafts and conse- 
quent cloud decay. These conclusions are not in 
agreement with the more common notion that 
the strongest activity is near the trailing edge 
of the storm. But cloud development near the 
leading edge and remaining separate from the 
rest of the storm could account for the plumes 
observed by us. For when this cloud reaches the 
layer of high winds, it would presumably be ecar- 
ried away from the storm in exactly the same 
way as the plumes described. On this model, the 
plume would not be the result of storm erosion, 
though erosion may well play an important part 
in maintaining the storm upright in severe wind 
shear. Such cloud moreover would surely com- 
pete severely with the main storm for the mois- 
ture supply which the storm draws in to an ap- 
preciable degree from its leading edge. 
Yet another interpretation of plumes was 
given by Imai [1957] who made observations by 
PPI as well as by RHI (vertical) radar sections. 
His beautiful records of bright-band forming in 
the falling plume allowed him to identify the 
plume particles as ice, origimating in the storm. 
On the basis of his somewhat sparser records he 
concluded however that the radar plume comes 
into being only after the decay of the convection, 
and that it consisted entirely of very small crys- 
tals. The only fall speed quoted is 40 em sec™. 
It is noteworthy that our observations and 
analysis do not require the existence of cloud par- 
ticles in the plume. Conceivably cloud is also 
swept out by the wind, but remains undetected 
