420 
WALLACE E. HOWELL 
Fra. 5—Photograph of a Cumulus cloud 13 min after seeding with dry ice (Courtesy of P. Squires) 
a 
3 
“ 
Arbitrary scale of average cloud aging or 'senority 
ny 
a 
roy 
° 
20 40 60 80 100 
Percentage of clouds surpassing critical limit 
Fie. 6—Ogive illustrating an assumed typical 
relationship between the mean aging of a group of 
clouds in the field-of-competition model and the 
probability of precipitation onset somewhere in 
the group 
And now let us consider the effect on our 
model of seeding one of the clouds with AgI. No 
effect is to be expected until the top of the cloud 
reaches a temperature of about —5°C. But when 
this temperature is reached in the cloud, some 
ice crystals will appear and some of the rudi- 
mentary precipitation particles will freeze, either 
because of infection with AgI or through collision 
with an ice crystal. The volume of air within 
which these frozen particles will continue to grow 
encompasses all that within which the air is satu- 
rated with respect to ice and therefore is con- 
siderably larger than that within which the un- 
frozen particles can grow; and even in dry air 
the frozen particles evaporate more slowly. Fur- 
thermore, as Douglas [1960] has shown, the fro- 
zen particles may enjoy as much as a two-times 
advantage in growth rate by accretion over their 
unfrozen neighbors. The result is that the rate 
at which the cloud loses rudimentary precipita- 
tion particles is diminished and the rate at which 
these particles grow or ‘acquire seniority’ is in- 
