Conclusion -77- 



Fair Weather Cumulus . Foremost of these restrictions is the factor 

 of cloud type and size. Certain clouds, such as the fair-weather cumulus, 

 have such a small volume and restricted area that, even though they are 

 easily modified when supercooled, their total liquid-water content is incon- 

 sequential. Another complicating factor is that the air below larger clouds 

 is sometimes so dry that a considerable amount of precipitation evaporates 

 before it reaches the ground. 



Warm Ground Fog. Another type of cloud which is difficult to modify 

 is the warm ground fog formed by radiation or advection. Such fogs are 

 often extensive and of considerable economic importance, especially from 

 the standpoint of airplane traffic control. Eut the natural structure of a 

 fog precludes any simple method of modifying it. Generally, the vertical 

 thickness is not more than 100 meters or so, with a cloudless sky above. 

 This rules out the possibility of modifying from above by forming precip- 

 itation in higher clouds to "rain out" the fog. (But supercooled ground 

 fogs may be modified, as explained on page 75.) 



Drought . Another weather situation where no method of relief is 

 now apparent is in the case of drought. This condition generally results 

 from the stability of a complex weather pattern in a manner which, at 

 present, is not very well understood. Drought is generally accompanied 

 by either cloudless skies or clouds of small vertical and horizontal devel-- 

 opment, because of strong inversions or thick layers of dry air. 



Convergence. The development of convergence is an important 

 feature in the formation of appreciable amount of rainfall in many parts 

 of the world. As a rule, such developments are generally accompanied 

 by the occurrence of natural precipitation, which continues so long as 

 the convergent movement is present. About the only thing that artificial 

 modification of clouds might do under such atmospheric conditions is 

 to initiate the precipitation cycle a few hours before it would start nat- 

 urally, or under some conditions, to delay the onset of precipitation by 

 oversee ding. 



CONTROVERSIAL ASPECTS 



As is so often the case with the proposal of striking or revolu- 

 tionary new concepts in science, the validity of the observations and 

 conclusions of the members of the Research Group, both before and 

 after the establishment of Project Cirrus, was challenged by many. 

 As a result, quite a school of opposing thought has been built up. This 

 is a normal, healthy condition of affairs in a free economy, and the 

 results would be of no particular consequence were it not for the fact 

 that the possibilities inherent in artificial weather modification have 

 such great economic and military significance. 



