304 H. DESS 
Pyrenees and their anvils were only slightly dis- 
placed by the wind. These results are generally 
in agreement with those observed by Donaldson 
[1958], using radar, if one assumes that the top 
of the radar echo coincides with the top of the 
anvil. 
Nevertheless, one cannot conclude that a 
strong vertical development constitutes the de- 
termining factor in hailstorm formation. In the 
Congo Basin a vertical development up to 
16,000 m occurs without a single hailstone reach- 
ing the ground. This happens in spite of a 
vertical temperature structure very similar to 
that which is observed in temperate regions on 
those days of heavy hail, and also, in spite of a 
degree of instability and of humidity equally 
favorable for hail formation. 
The transformation of a thunderstorm into a 
hailstorm is determined by the wind velocity at 
high levels—The only fundamental difference 
between hailstorms of temperate regions and 
thunderstorms of the Congo Basin lies in the 
horizontal wind speed at high levels. These ob- 
servations have led us to make a statistical anal- 
\ of the thunderstorms of southwest France. 
In Figure 1, the full line represents the fre- 
quency distribution of maximum tropospheric 
wind velocity for 38 days on which the damage 
by hail, over the region, is estimated not. less 
than 10 million franes. The wind data are ob- 
tained from radiosonde and the value for each 
day is that of the maximum velocity observed 
between the ground level and the tropopause, 
at whatever level this occurs. 
In order to emphasize the significance of this 
1| 
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ry 60 400 140 180 220 300 
MAXIMUM WIND VELOCITY ykm/h 
Fre. 1—Frequency distribution of wind speeds 
at high levels for 38 days with heavy destructive 
hailstorms (full line) and for 38 control days with 
thunderstorm but without significant damage by 
hail (broken line) 
diagram, we have included also (broken line) 
the frequency distribution of wind speeds for 
an equal number of days during the same season 
of the year (April to September) and the same 
period (1951-58), these 38 control days were 
days without significant damage due to hail, al- 
though thunderstorms did occur over the region. 
A comparison of these two frequency distri- 
butions shows clearly the correlation between 
the wind velocity at upper levels and the degree 
of damage caused by hail. It is probable that 
the correlation between wind speed and the in- 
tensity of the hailstorms is actually closer than 
indicated on Figure 1. In effect the correlation 
is probably somewhat reduced by the fact that 
the radiosonde observations are often made at 
places distant both in space (that is to say, up 
to 250 km) and in time (up to 12 hr) from the 
hailstorms. It is worth mentioning in addition, 
that although on five days, winds of more than 
90 km/h were observed without wide-spread 
destructive hail. On four of these days, heavy 
hailstorms did in fact occur but the damage 
done was not very great because they were very 
limited in extent. Finally, to sum up, on a total 
of 76 days with Cumulonimbus activity, there 
is only one case in which the maximum wind- 
speed exceeded 90 km/h without bemg accom- 
panied by destructive hail. 
In conclusion, it appears that the presence of 
a jet stream, or at least a very strong wind at 
upper levels, is the factor which determines 
whether or not a thunderstorm situation will 
transform itself into a heavy destructive hail- 
storm. 
A suggested structure for the hailstorm—In 
air masses with relatively little variation of 
horizontal wind speed with height, the ascend- 
ing currents in Cumulonimbus are approxi- 
mately vertical. The air in the ‘chimney’ up- 
draft takes its energy from the instability of 
the atmosphere, perhaps increased locally by 
differential heating of the soil and from the 
latent heat of condensation and of freezing; this 
energy is liberated mainly in the lower part of 
the ascending air column. Therefore chimneys 
are fed from the low end and receive no im- 
portant energy at high levels; furthermore, if 
air is not removed from the top, then the ‘chim- 
ney’ may fail to draw. Chimneys therefore tend 
to be short lived, depending, as they do in this 
ease, on the hazards of local conditions and 
particularly on the nature of the terrain over 
which they travel, under the influence of the 
