EXPERIMENTAL METEOROLOGY 301 
by a factor of a million (25); there are periods of a half day to 
several days when values are relatively high, but there are other 
days when the concentrations are very low. During the six and a 
half years of record, a trend is noticeable toward higher concen- 
trations of moderate counts (§ X 10? to 1 X 104 per cubic meter) 
with lower levels of both high and low counts. It is not easy to 
determine whether this trend is caused by increased cloud seeding 
activities or, what is more likely, an increase in area of the drought 
stricken regions of the Southwest United States. Air trajectory 
studies have shown a high correlation between the high counts at 
Mt. Washington and dust storms and related air movements 
from the Southwest. 
Importance of Ice Nuclei in Controlling Precipitation 
In some of the early flight studies of Project Cirrus, particularly 
during February 1948 when a series of exploratory flights were 
conducted over and near Puerto Rico, we were able to determine 
conclusively that ice crystals were not unique in causing heavy 
precipitation (24). Subsequent studies by Langmuir (13), Wood- 
cock (32), Bowen (5), Mordy (20), d’Albe (1), and others have 
demonstrated that quite often in the subtropical regions of the 
world the precipitation cycle may be controlled, if not completely 
dominated by the presence or absence of large salt nuclei. The 
presence of the “‘trade wind inversion,” which normally limits the 
vertical development of clouds to levels which never cool below 
o°C., together with the relatively long life cycles of individual 
cloud elements, is conducive to the development of this ‘‘warm 
cloud” rain. In almost all cases, however, there are exceptions to 
the rule. Large cumulus clouds break through the inversion or 
develop when the inversion is absent, reach high altitudes, and 
produce excessive rain. 
Thus, although it is likely that a certain concentration of large 
salt nuclei may initiate the precipitation cycle even in such large 
supercooled clouds, there is no reason to believe that the precipi- 
tation pattern will be dominated by the salt particle effect if 
optimum concentrations of ice nuclei are present. Much further 
research remains to be done in separating these two distinctly 
different effects. 
