EXPERIMENTAL METEOROLOGY 307 
probably close to the cloud base at 0800. Observations of visible 
smoke plumes in the Rio Grande Valley show that, while there is 
a tendency for an early morning inversion to stabilize the air, this 
condition rapidly changes as the sun begins heating the valley 
floor. Together with the orographic effect of the mountains, it is 
very likely that the stability of the air in the early morning would 
have prevented a rapid dilution of the stream of silver iodide, so 
that an effective seeding reaction probably occurred just above 
18,500 feet, where the temperature was colder than —4°C. Assum- 
ing a rise within the cloud of 120 feet per minute, this would have 
the silver iodide producing an effect at about og4o. 
Our other experiences in New Mexico have shown that seeding 
effects may be expected to produce an initial radar echo within 
15 to 25 minutes. Thus, the initial radar echo at 1006 would be in 
very good agreement with the mechanisms that might be expected 
to control the precipitation cycle. The initial precipitation area 
covered about one square mile and was deep within the cloud. It 
is very unusual for precipitation to develop at such low altitudes 
in New Mexico. Within four minutes the precipitation echo had 
increased to seven square miles, and two minutes later it had ex- 
tended upward to 34,000 feet, where the temperature was — 43°C. 
The rapid vertical growth with an average of 1,160 feet per minute, 
which was first observed at 0957, continued until to12. The out- 
ward manifestation of this upheaval shows a remarkable linearity. 
Extrapolation of this line shows an excellent relation of the 
probable triggering effect of the silver iodide. 
This single local storm developed continuously into a large zone 
of precipitation. 
In the early afternoon, Flight 110 was activated with two Proj- 
ect Cirrus B-17’s. Twelve separate seeding operations were carried 
out between 1445 and 1530, utilizing from one-third to four and 
a half pounds of crushed dry ice, depending on the size of the cloud 
masses seeded. During the afternoon, 1.2 inches of rain fell at the 
radar site. A remarkable fan-shaped pattern of precipitation 
developed in the region downwind from the seeded region. This 
has been analyzed in considerable detail by Langmuir (15), who 
concluded that much of the rain that fell in New Mexico on July 
