HAIL STORMS 
TEMPERATURE °C 
HEIGHT IN km 
0100 200 500 
ASSOCIATED WITH STRONG UPPER WINDS 
99r 
2090 
WIND SPEED 
kmh 
Fie. 2—A suggested structure for the hailstorm: (left) wind velocity 
versus height; (right) the chimney inclined to the vertical 
prevailing wind. In the cloudy edifice, a chim- 
ney is born, develops, and quickly dies to give 
place to another. Such sporadic activity is not 
favorable to the formation of large hailstones 
[Weickmann, 1953]. 
On the contrary, under conditions of strong 
windshear, as in Figure 2, the chimney is in- 
clined to the vertical. On the right side of 
Figure 2 is included a chimney of very restricted 
transversal dimensions, in accordance with the 
observations of Wichmann [1951]. It is certain 
that in a case such as this, a part of the air 
rising in the chimney is entrained horizontally 
at the top of the chimney at a speed of the 
order of SO m/s. Therefore I envisage a coupling 
taking place above 6000 m between the updraft 
in the chimney and the very strong horizontal 
current and that this current tends to stabilize 
and prolong the life of the chimney, which now 
receives, in addition to the energy of thermo- 
dynamic origin at low levels, kinetic energy of 
the strong wind at high levels. The chimney 
may, in fact, be considered as forming the lnk 
between these two main sources of energy; the 
lifetime of an individual chimney appears to be 
prolonged under these conditions for periods 
of more than 30 min sufficient to make possible 
the formation of very large hailstones. 
Arguments in favor of the suggested struc- 
ture—In addition to the data which led me to 
suggest this structure of the hailstorm, there is 
the following argument in its favor. It is the 
correlation, which has been observed, between 
the maximum size of hailstones and the maxi- 
mum wind velocity between 6,000 and 15,000 m. 
DIAMETER ,mm 
w 
) 
VELOCITY ,m/s 
Fic. 3—Correlation between diameter and _ ter- 
minal velocity (curves after Wichmann) of hail- 
stones and the maximum wind velocity between 
6,000 and 13,000 m (crosses) 
Figure 3 shows the correlation between these 
two parameters for seven hailstorms which oc- 
curred in the region within a 200 km radius of 
Bordeaux. The curves showing the variation of 
terminal velocity of hailstones with height and 
with diameter are taken from Wichmann. The 
fact that the crosses representing observations 
lie fairly close to the curve corresponding to 
11,000 m, shows that the maximum velocity of 
the updraft tends to approach that of the hori- 
zontal wind as measured outside the storm- 
cloud. Thus the maximum size of the hailstones 
would appear to depend on the velocity of the 
horizontal wind. 
It is because of this that a hailstone cannot 
remain indefinitely within the ascending chim- 
ney when this is inclined to the vertical; and 
therefore we can obtain from this picture of the 
hailstorm a forecast of a maximum size which 
