232 
In experimental meteorology, such factors as the sta- 
bility of the atmosphere, the concentration of ice nuclei, 
the absolute humidity, the circulation of the air, and 
the heating by the sun are important. 
The absence of clouds, the presence of strong 
temperature inversions, large variations in wind 
velocity with altitude, low absolute humidity, small 
lapse rate, weak circulation, large quantities of cirrus 
clouds, and other similar atmospheric situations present 
serious, if not absolute, limitations to cloud-seeding 
operations. Conversely, if large cumulus clouds occur 
without the formation of precipitation, if there is 
abundant moisture, large lapse rates, low concentrations 
of ice nuclei, an absence of wind shear, and few inver- 
sions, the possibilities of cloud modification are great 
and some unquestionable effects may follow the use of 
proper seeding techniques. However, even under ideal 
conditions, it is still possible to have poor or inconclusive 
results unless intelligent seeding techniques are fol- 
lowed. 
With evidence now at hand it is possible to draw some 
fairly definite conclusions with regard to the possible 
effects of seeding methods under the following general 
conditions. 
Thunderstorms. Thunderstorms generally occur im 
clouds of large vertical thickness contaming large 
amounts of supercooled droplets and some ice crystals. 
By dissipating or overseeding all clouds in a thunder- 
storm ‘‘breeding area”’ [28] shortly after they pass above 
the freezing level, it should be possible to prevent the 
formation of a thunderstorm. This may be accomplished 
by local seeding with dry ice from an airplane or from 
projectiles from ground stations, but eventually 1t may 
be accomplished most effectively and over a larger area 
by the proper use of silver iodide generators at the 
ground. Initial success may be expected from orographic 
type thunderstorms rather than from the much more 
complex storms associated with frontal systems. 
Hailstorms. Since it is believed that damaging hail- 
storms are due primarily to the presence of a deep 
supercooled layer and a relatively small number of ice 
nuclei, techniques similar to those employed in pre- 
venting thunderstorm development should be effective 
in preventing hail. Smee such storms often possess 
special characteristics im regions where they are com- 
mon, a careful study will suggest the best method of 
dealing with them. If certain features of the local 
topography favor their development, it may be possible 
to locate silver iodide generators so that their effluent 
will be carried into the most active part of the cloud 
system. It is important to provide so many ice nuclei 
that the competition for the available moisture is too 
great for individual particles to grow large. It would be 
more effective, however, to prevent the development 
of a deep supercooled layer and for this purpose a 
program of early seeding should be more effective. 
Increased Precipitation from Cumulus Clouds. As sug- 
gested earlier, large cumulus clouds sometimes form 
without the development of a precipitation phase. High 
wind shear at upper levels in the atmosphere or a low 
CLOUD PHYSICS 
concentration of ice nuclei are two of the factors respon- 
sible for such occurrences. 
Proper seeding by artificial means may have economic 
value in several ways. Besides forming precipitation in 
the vicinity of such clouds, the process would prevent 
the formation of false cirrus streamers which sometimes 
reduce solar radiation to such an extent [30] that the 
formation of other convective clouds downwind is se- 
riously affected. The formation of precipitation also may 
lead to increased air circulation which, if accomplished 
on a large enough scale, could change the weather 
pattern. 
As suggested recently by Langmuir [14], the seeding 
of a given portion of the atmosphere with a sufficient 
concentration of ice nuclei to start a chain reaction 
serves primarily to increase the probability of precipita- 
tion coming from any given synoptic situation. De- 
pending on the “triggering” nature of the seeding effect, 
the effectiveness of any particular seeding operation 
will involve many complex physical relationships in the 
atmosphere not yet understood in enough detail even to 
consider at this time. : 
The Prevention of Windstorms and Torrential Rains. 
Local windstorms of the type characterized by high 
gustiness and turbulence are often caused by down- 
drafts induced by heavy precipitation. By preventing 
the formation of thick cumulus clouds through judicious 
seeding techniques, this type of local storm damage 
may be prevented. Whether these techniques can be 
expanded to affect larger storm systems must await 
further experimental studies. 
The Elimination of Ground Fogs. Under special con- 
ditions supercooled ground fogs may form by radiational 
cooling, contact of cold air with warm open water, or 
some similar condition. Seeding of such fogs may be 
effective in dispersal if care is exercised that they are 
not overseeded. An initial concentration not greater 
than 100 particles per cubic centimeter must be 
achieved. If the concentration is much greater, the fog 
will be overseeded and the visibility will become reduced 
from that existing in the supercooled fog. 
No methods are known at present which would lend 
themselves to the effective dissipation of warm fogs. 
The reduction of ice-crystal fogs which form at very 
low temperatures is also a difficult condition to over- 
come by seeding methods, since these fogs commonly 
form at temperatures below —39C. Under such con- 
ditions, any surplus of moisture exceeding the frost 
point comes out in the form of ice crystals. In the 
temperature range from OC to —39C, the air may 
become supersaturated with respect to ice. Under such 
conditions, it may be possible to put out an optimum 
quantity of ice nuclei to precipitate the moisture as 
snow. 
The Elimination of Supercooled Clouds in the Atmos- 
phere. It is now feasible, using the mechanisms referred 
to in the preceding sections, to limit the formation of 
supercooled clouds in the atmosphere. This control 
over supercooled clouds is not necessarily limited to 
local regions; if desired it could be achieved throughout 
