CLOUD SEEDING — HOUGHTON 185 



The hope that large-scale effects might result from seeding is based 

 in part on the assumption that large segments of the atmosphere are 

 often in a metastable condition such that a small impulse will suffice 

 to release the instability. A logical corollary of this theory is that 

 the point at which the activity w^ill start spontaneously is indetermi- 

 nate, but that a small artificial impulse may be sufficient to initiate 

 the release of the instability at a predetermined point. If this princi- 

 ple of indeterminancy is correct, it sets a basic limit on the precision 

 with w]iich it is possible to forecast weather. Most meteorologists 

 will agree that this principle often seems to hold in the development of 

 such small-scale phenomena as thunderstorms and tornadoes. This 

 may be due, of course, to the fact that the scale of these phenomena 

 is small compared to the spacing of the observations. In the case of 

 large-scale phenomena there is no real evidence in favor of the inde- 

 terminancy principle. To be sure, many errors are made in forecasts, 

 but these appear to be explicable in retrospect when adequate data 

 are available. The motion of such disturbances seems to be contin- 

 uous without the erratic behavior to be expected from the indetermi- 

 nancy principle. Studies of the motion and development of cyclones 

 strongly suggests a dependence on the large-scale fields of motion 

 and temperature. These large-scale variations appear to be of much 

 more importance than any small-scale, accidental differences. 



SUMMARY AND CONCLUSIONS 



The principal conclusions reached in this paper may be summa- 

 rized as follows : 



1. Seeding of supercooled clouds with dry ice or silver iodide will 

 convert the clouds to ice-crystal clouds if the temperature is below 

 about -5°C. 



2. Seeding of an inactive cloud will not induce an important 

 amount of precipitation. 



3. Seeding a cloud will not accelerate the growth of the cloud 

 unless there is active vertical motion below the freezing level and 

 the environment above the freezing level is moist and only slightly 

 stable. Partial or complete dissipation is more likely due to the 

 lifting-off and drying-out of the top of the cloud and the downward 

 force exerted by the falling hydrometeors. 



4. Seeding of an active cloud which does not quite reach the 

 activation temperature of the natural ice nuclei may release useful 

 precipitation. 



5. It appears possible to advance the onset of precipitation from 

 a cloud that is about to precipitate from natural causes by seeding it. 



6. The possibility of increasing precipitation initiated by natural 

 processes by seeding cannot be determined because of incomplete 



