Severe Hailstorms Are Associated with Very Strong 
Winds between 6,000 and 12,000 Meters 
H. Derssens 
Observatoire du Puy de Déme, Clermont-Ferrand, France 
Abstract—Both physical and statistical studies indicate that the factor which trans- 
forms thunderstorms into severe hailstorms is a very strong horizontal wind at levels 
between 6,000 and 12,000 m. A structure for the hailstorm is suggested, in which the 
‘chimney’ updraft of the Cumulonimbus is considered as forming a lnk between two 
main sources of energy: in the lower part, the instability and the latent heat of conden- 
sation and of freezing; in the upper part, the kinetic energy of the strong wind. 
In any given region, on certain days thunderstorms cause a very light or moderate 
damage by hail; on other days the damage is widespread and greatly increased. Fur- 
thermore there are regions of the world where thunderstorms are very frequent and 
violent but where hail is extremely rare. We have tried here to delineate a factor or 
factors which transform an ordinary thunderstorm into a destructive hailstorm. 
The influence of atmospheric aerosol particles 
has not been demonstrated—Following a sug- 
gestion of Ludlam [1951], we have investigated 
if the formation of hail is not hindered by an 
abundance of giant hygroscopic nuclei in the 
air. Some trials at hail prevention were under- 
taken in 1951 using rockets which took crystals 
of sodium chloride up to cloud base. Neither 
the results of these trials nor air sampling have 
indicated that there is any correlation between 
hail formation and the concentration of giant 
nuclei in the air. 
I have observed on several occasions that de- 
structive hail occurs more frequently when the 
lower atmosphere is very clear; if these obser- 
vations are confirmed, they will mean that hail 
forms less easily when there is a high concen- 
tration of condensation nuclei. Remember that 
the concentration of giant nuclei does not nec- 
essarily vary in the same manner as the small 
condensation nuclei. Soulage [1958] has studied 
the variations of the ice-forming power of the 
air during two thunderstorm seasons. He has 
not found any strong diminution of the ice- 
forming power during destructive hailstorms. 
These measurements, made at ground level, have 
so far failed to reveal any significant correlation 
between hailstorms and the concentration in ice- 
forming nuclei. 
Finally I call your attention to the fact that 
hailstorms are extremely rare in the Congo Ba- 
sin as observed at Lukolela (1°S, 17°E). The 
observations at ground level made by Soulage 
[1956] indicate, at least during the dry season, 
there is an almost total absence or at most a 
very low concentration of freezing nuclei in the 
area. Furthermore, the photogrammetric meas- 
urements of clouds according to a method de- 
scribed [H. Dessens, 1953] show that in the 
rainy season the effective freezing level in the 
clouds is observed at approximately —85°C [J. 
Dessens, 1959]. This tends to show that there 
are very few freezing nuclei active in the clouds 
in that area. These data therefore contradict 
rather than support the hypothesis that de- 
structive hail is connected with an absence or 
low concentration of ice-forming nuclei. 
A strong vertical development appears to be 
a necessary but not a sufficient condition for the 
formation of hailstorms—Since 1949 we have 
measured the altitude of Cumulonimbus tops in 
the region north of the Pryenees by photogram- 
metry, using a base lie of 8-km length. Meas- 
urements are possible only when the Cumulo- 
nimbus are from 30 to 120 km distant from the 
observation stations and if the sky is clear in 
the neighborhood of the stations. For the 14 
thunderstorm days without heavy hail, the max- 
imum height reached by the Cumulonimbus on 
each day, was as follows: 6,000 to 9,000 m, four 
days; 9,000 to 12,000 m, eight days; and 13,000- 
13,200 m two days. 
We have been able to make similar observa- 
tions on days of destructive hail on only three 
occasions. The corresponding summits’ heights 
were: 12,500 m, 14,000 m, and 14,500 m. 
The two storms which reached 13,000 m with- 
out giving hail were formed over peaks of the 
