DISCUSSION 337 
Dr. J. Smagorinsky—It might be possible to 
explain the convective instability underneath 
a jet as a gravitational shearing instability. 
Dr. Weickmann  (communicated)—I may 
make an attempt to explain this phenomenon 
from purely cloud-physics reasons. We know 
from the Byers-Braham thunderstorm model 
that a storm’s life span can be subdivided into 
a developing stage, a mature stage, and a dis- 
sipating stage. The dissipating stage begins with 
the development of a downdraft in the rain 
area which slowly gains momentum and extends 
upward in altitude. Downdrafts also develop 
aloft and within the cloud because of entrain- 
ment of drier outside air. 
In conditions of great instability the updraft 
velocities in the inner core of the storm may 
be sufficiently high as to prevent even larger 
particles from descending against the updraft. 
They too end up in the anvil where the updraft 
finally ceases. The particles, however, continue 
to grow until they are large enough to descend 
against the powerful updraft and to initiate 
the downdraft mechanism. The hailstones thus 
formed have a conical shape in the ideal case. 
Suppose now the existence of a strong jet in 
the upper levels. It will displace the anvil down- 
wind from the mother cloud, so that large par- 
ticles cannot fall back into it and cannot stimu- 
late the formation of downdraft which would 
lead eventually to the dissipating stage. Of 
course, from the downwind spreading anvil large 
particles will fall mto other up-coming clouds 
and may form hailstones in these. The lifetime 
of the original mother cloud depends now only 
on the correct adjustment of external factors, 
such as inflow characteristics, migration velocity, 
and air consumption in the updraft. The in- 
creased lifetime of this updraft may be a deci- 
sive factor for tornado formation as it will per- 
mit the encounter with an already existing 
micro-low, or it will even assist in the organiza- 
tion of the correct low-level cyclonic circulation. 
Then the tornado may form. 
Dr. W. E. Howell—Of course, in the tropics 
there are very seldom high-level winds of ve- 
locities that are anywhere comparable with mid- 
dle latitude high-wind velocities. However, on 
the Pampa de Junin, the high plain in central 
Peru, there is often a strong easterly wind at 
high levels across the main range of the Andes 
while the Pampa lies, protected, between two 
ranges. Here the air is relatively slow moving 
and in fact often undergoes a reverse circulation 
so that there is a west wind near the ground 
beneath the east wind aloft. On this Pampa it 
is not unusual to have rather strong showers of 
graupel. Of course, since the elevation of the 
ground is 14,000 ft, there is much less depth 
of cloud in which hail could grow, but we do 
have the first phase that Mr. List has shown. 
Dr. E. Kessler—Referring to this problem in 
a purely intuitive way, I have been struck also, 
as many others have, with the association of 
thunderstorms and high winds aloft, and their 
occurrence together in the spring. In Corpus 
Christi, Texas, where I used to live, numerous 
severe thunderstorms occur in the spring; later, 
in the summer they are not as severe. I have 
thought at times, that the explanation for this 
may he in the transport of heat away from the 
region of active convection by the strong up- 
per-level winds, which are weak or absent in 
summer. The unstable thermal stratification 
which is associated with the development of the 
springtime storms may then persist locally for 
a comparatively long time, with a correspond- 
ingly long time available for an organized, strong 
circulation to develop in the lower troposphere. 
Dr. D. Swingle—I just have a quick question 
for Dr. Cunningham. Do you have proof this 
is the same storm that made hail near Cheyenne? 
Dr. R. M. Cunningham—I do not know ex- 
actly what you mean by proof, but the track 
of the storm is right for that conclusion, shall 
I say. 
Dr. Swingle—You do not have the continuity 
that actually ties it back? 
Dr, Cunningham—Radar showed it coming 
from that area, but I do not believe we have 
the radar echo right over Cheyenne. 
Mr. Alan Faller—I would like to ask if the 
hail was falling through the zone of strong shear 
from the region of high winds into the lower 
region where the wind is not so strong? 
Dr. Cunningham—The visible hail coming 
out of the side of the cloud was all lower than 
the strong winds. The strong winds are up in 
the Cumulus part of the cloud and the hail was 
down where we only had a wind of 20 knots. 
Of course, I do not know where it came from 
inside. 
Dr. Tor Bergeron—Concerning Dr. Dessens’ 
paper, I must confess that I on the whole agree 
with the first remark made by Dr. Weickmann. 
On the other hand, it is not my intention to deny 
the possible effect of the shear. But I want to 
remind you of the fact that in the United States 
