FORMATION OF CUMULONIMBUS BY ARTIFICIAL CONVECTION 397 
ments, is that the only falls of rain which we can 
attribute with certainty to our intervention are 
those which have been stimulated by artificial 
convection [H. Dessens and J. Dessens, 1956, 
1957]. 
As far as the cloud seeding is concerned, the 
results are particularly disappomting, because 
the conditions under which we work seem ex- 
ceptionally favorable: (1) the base of the Cu- 
mulus clouds was less than 1000 m above the 
ground, so that the rockets can easily reach the 
cloud base. (2) In this region there was a com- 
plete absence or at most very few natural ice- 
forming nuclei active at temperatures above 
—30°C. This was shown by measurements made 
with an ice-nuclei counter, at ground level, by 
Soulage [1956], by the photogrammetric study of 
the clouds themselves by J. Dessens [1959], and 
also by observations of large subeooled Cumulus 
over the southern Congo and Rhodesia made by 
Schaefer [1958]. A third factor in favor of the 
seeding was the fact that the winds are gener- 
ally light and of very steady direction, which 
facilitated the evaluation of the results of seed- 
ing. The last favorable condition is that the pre- 
cipitation is principally due to local convection 
clouds. 
In contrast with the apparent lack of any ef- 
fect on the precipitation of the seeding, the evi- 
dence for the positive effect on the Cumulus of 
the artificial convection is very striking. Fur- 
thermore, these first experiments have permitted 
us to calculate approximately the amount of 
energy which has been liberated by combustion 
to trigger the formation of Cumulus or Cumulo- 
nimbus in definite local conditions in the dry and 
in the rainy season. 
The ‘Meteotron’: a brief description—I use 
the word ‘meteotron’ to describe any experi- 
mental arrangement capable of modifying the 
local system of convection sufficiently to form 
frequently clouds which may produce rain. Our 
experience with brush fires suggests that the 
energy to be liberated must be comparable with 
that received from the Sun, that is to say of the 
order of 10° kw/km’*. This energy must be im- 
mediately available and ready for application to 
take advantage of the meteorological situation; 
that is, that it is necessary to use some hquid 
or gaseous fuel. We chose to use fuel oil. Our 
‘meteotron’ therefore includes a station for the 
preparation of the fuel with a pump driven by 
a diesel motor. The motor drives also a small 
generator which produces alternating current 
necessary for automatic firing of the fuel at a 
distance. 
We are, at present, thinking in terms of a dis- 
tribution of 100 burners connected to the cen- 
tral post by tubes, these 100 burners situated on 
a circle of approximately 250 m in diameter. 
Each burner will burn 600 kg of fuel oil per hour 
so that the total consumption will be of the 
order of one ton of fuel oil per minute in the 
whole system. 
Site of the experiments—The most suitable 
region for a first trial of this method, would, I 
think, be between latitudes 5°N and 5°S, in a 
region of plains during the rainy season. The 
severe droughts observed in 1958, south of the 
equator, Justify such an experiment for the eco- 
nomic point of view. Regions such as the Ma- 
yumbe, the Niari Valley or the Congo Basin will 
be very suitable. The relatively light winds, the 
absence of cyclonic perturbations, the perma- 
nent presence of a very humid air layer at low 
levels, the low cloud-base level are favorable 
factors. It is important to realize these differ- 
ences between conditions in the tropics and 
those in temperate regions. 
Method of use of the ‘meteotron’—The most 
delicate part of the operation is the choice of 
the moment to start. Our first objective is mod- 
est: just before the moment at which the general 
instability becomes sufficient to permit the natu- 
ral formation of Cumulonimbus, we must light 
our burners, that is to say, anticipating the natu- 
ral formation of Cumulus by a short period. Six 
minutes after lighting the burners, the first arti- 
ficial Cumulus cloud appears. After 15 min the 
development of this cloud is greater than that 
of the other Cumulus in the region. So that, in- 
stead of leaving it to chance the location of the 
first formation of Cumulonimbus over the area, 
we have chosen and imposed this location. In 
brief, what we do, is to add a little artificial 
impulse to the natural instability of the air, 
which is due to the heating of the soil by the 
solar radiation of the whole area. This impulse 
starts half an hour before the time at which the 
normal Cumulonimbus will begin to form gener- 
ally over the whole region in radius, say, of 50 
to 100 km without intervention. 
Despite the fact that the experience of previ- 
ous days gives us some ideas of the moment at 
which Cumulonimbus will begin to form, it 1s 
necessary to be able to make, each day, a short- 
term forecast, in order to know exactly when the 
‘meteotron’ should be put in the action. This 
