Cumulus Studies -49- 



"The new probability theory.. ..has served a valuable guide 

 in devising an objective method of evaluating the distribution 

 in space and time of the rain which follows the operation of 

 the silver -iodide generator on the ground or in the airplane 

 flights near Albuquerque. To illustrate the results, we will an- 

 alyze the data obtained on two days, October 14, 1948 (Flight 

 45) and July 21, 1949 (Flight 110). 



"These days were chosen because large amounts of sil- 

 ver iodide were used, but no seeding was done on the imme- 

 diately preceding days. Furthermore, the wind direction on 

 both days was rather similar. On both days the Weather 

 Eureau predicted no substantial amount of rain. Eoth morn- 

 ings were nearly cloudless, and on both days SW winds pre- 

 vailed from the cloud bases at 12,000 feet up to 20,000 feet. 

 At lower and higher altitudes and later in the day there were 

 also winds from the E, W, and NW. On both days, visual 

 effects indicating thunderstorms and heavy rain over wide 

 areas were observed a few hours after the start of the seeding 

 operations. 



"In the July operation our techniques had been improved 

 compared to those of the preceding October. In October ra- 

 dar observations covered only a period of about an hour in 

 the afternoon, for at that ime it was not suspected that the 

 rain that lasted well on to the morning of the 15th had any- 

 thing to do with the seeding. 



"On July 21, 1949, however, we had complete radar cover- 

 age from early in the morning until late at night. Photographs 

 of the clouds were taken not only from planes but from the 

 ground, including lapse -time motion pictures with photographs' 

 every few seconds. 



"Shortly before 8:30 a.m. on July 21, 1949, a single large 

 cumulus cloud began to form about 25 miles S of the field sta- 

 tion near Albuquerque in a sky that was otherwise cloudless. 

 This cloud was located near the Manzano Mountains, and the 

 silver -iodide smoke had been blowing from the N about 10 mph 

 so that it should have reached the position of the cloud. 



"Between 8:30 and 9:57 the cloud grew in height slowly at 

 the uniform rate of 160 feet per minute. At 9:57, when the top 

 of the cloud was at 26,000 feet (temperature -23°C), the upward 

 velocity of the top of the cloud increased quite suddenly, so that 

 the cloud rose 1200 feet per minute until at 10:12 it had reached 

 44,000 feet (temperature -65°C). 



