336 
the hailstones may reach. The value of such a 
forecast hes im the fact that in general the up- 
per air winds have already been present for 
several hours at least before convective activity 
becomes important. 
This last paragraph and Figure 3 represent 
only a preliminary version of the suggested 
hailstorm structure. It is to be hoped that the 
different studies at present beimg pursued in 
various parts of the world will enable us in 
the near future, to reach some more definite 
conclusion as to the value of this suggestion. 
On the other hand there is need for a theoretical 
explanation of the fact that the strong hori- 
zontal wind at high levels tends to accelerate 
and stabilize the updraft m Cumulonimbus 
clouds. 
REFERENCES 
Dessens, H., Sur l’apparition des particules glacées 
dans les Cumulus, Bul. Obs. Puy de Dome, pp. 
20-23, 1953. 
DISCUSSION 
Dessens, J.. Niveau de congélation des nuages 
convectifs équatoriaux, Bul. Obs. Puy de Déme, 
pp. 73-80, 1959. 
Donatpson, R. J., Analyses of severe convective 
storms observed by radar, J. Met., 15, 44-50, 
1958. 
Lupa, F. H., The production of showers by the 
coalescence of cloud droplets, Q. J. R. Met. Soc., 
77, 402-417, 1951; also Nubila, no. 1, pp. 12-96, 
1958. 
Sounace, G., Réponse A la note de E. K. Bigg: 
Freezing nuclei in the atmosphere, Bul. Obs. Puy 
de Dome, pp. 80-85, 1956. 
Soutace, G., Influence de l’aérosol atmosphérique 
sur la formation de la gréle. I—Pouvoir gla- 
cogene de lair & proximité d’orages A gréle, 
Bul. Obs. Puy de Déme, pp. 125-146, 1958. 
WeIcKMANN, H., Observational data on the forma- 
tion of precipitation in Cumulonimbus clouds, 
Thunderstorm Electricity (H. R. Byers, ed.), 
Univ. Chicago Press, pp. 66-138, 1953. 
Wicumann, H., Ueber das Vorkommen und Ver- 
halten des Hagels in Gewitterwolken, Ann. der 
Met, 4, 218-226, 1951. 
Discussion 
(This discussion pertains to the two 
immediately preceding 
papers.) 
Dr. Helmut Weickmann—I call attention to 
the fact that another plausible explanation 
exists as to why there occurs little or no hail 
in the tropics. It has been found (Fawbush and 
Miller, Bul. Amer. Met. Soc., 34, 235-244, 1953) 
that no hail falls from hailstorms which have 
a wet-bulb freezing level above 11,000 ft, be- 
cause then even the biggest stones melt before 
they reach the ground. I suspect that this is 
the case in these tropical hailstorms. 
Dr. H. Dessens—I think that the greater 
height of the O0°C level cannot explain the 
scarcity of the hail in the equatorial regions. In 
France, in summer, the 0°C level is near 3500 
m, and the top of the hailstorms near 14,000 
m; at this top, the temperature is about —55°C. 
In the Congo Basin, the 0°C level is near 
4500 m, and the top of the thunderstorm near 
17,000 m; at this top, the temperature is about 
—so°c. 
In both of these regions, during the heavy 
showers, the temperature near the ground is 
of the same order: +17°C. 
Thus, the difference in the 0°C level is of 
the order of 1000 m. The time of fall of big 
hailstones through this layer is shorter than 
one minute. This time seems inadequate to melt 
the hailstones noticeably, the more so as the 
mean temperature between the 0°C level and 
the top of the cumulonimbus is much lower 
near the equator than in the temperate regions. 
Mr. Jerome Namias—In the United States 
one of the atmospheric characteristics looked for 
in the prediction of severe storms or tornadoes 
is the existence of a mid-tropospheric jet. No 
one has yet given a satisfactory explanation 
for this association. Nevertheless, it 1s a pretty 
good indicator, provided the lower thermody- 
namic air-mass structure is conducive. I would 
like to suggest two points which may have 
something to do with this. Springtime is the 
most favorable for tornadie activity. Then we 
frequently have conditions which lead to the 
development of deep layers of very dissimilar 
air masses—very cold air from the Pacifie over- 
riding moist and warm, tropical gulf air. This 
differential flow often leads to convective in- 
stability and, simultaneously, the presence of 
jets which are found in the middle troposphere. 
Perhaps there is something about the circulation 
in and around these jets that might encourage 
convection; this would be enhanced by the 
thermodynamic instability. 
