Periodic Seeding -57- 



f In 15 states west of the 95° W meridian (excluding Texas) 

 about 550,000 square miles of 37 per cent of the total area of 

 these states were under seeding contracts during 1951. 



"Maps for the months from December 1949 through July 

 1950, taken from the Monthly Weather Review, illustrated the 

 distribution of abnormally large rainfalls over the United 

 States. The heavy rains nearly always occurred in a band ex- 

 tending from the southwestern to the northeastern states. 



"An analysis of the periodicity in the rainfall induced by 

 periodic seeding was presented in a paper read October 12, 

 1950 before the National Academy of Sciences. The areas 

 having a high weekly periodicity were generally the same as 

 those showing the highest abnormalities in rainfall. Such 

 heavy rains can only occur if the winds and the barometric 

 pressures cause an adequate supply of moisture to flow from 

 the Gulf of Mexico. The periodicities in the pressure differ- 

 ences between Corpus Christi and Jacksonville were studied. 

 During the first 140 days after seeding began, there was a 

 highly significant weekly periodicity indicating a periodic air 

 flow from the Gulf. 



"The upper air temperatures, even up to the stratosphere, 

 showed a high weekly periodicity over more than half of the 

 United States. Nine stations representative of an area of 

 1,300,000 square miles gave 950 mb temperatures having CC(28) 

 greater than 0.5. These data were published, in detail for 

 Chicago and in summary for eight other stations, in the Dec- 

 ember issue of "The Bulletin of the American Meteorological 

 Society', and a statistical analysis was given which proved that 

 these periodicities were highly significant. Mr. William Lewis 

 and Mr, E. Wahl, Bull.Amer.Met.Soc. 32:192-3 (1951), and Mr. 

 Harry Wexler, Chem.Eng. News 29:3933 (1951), maintained, 

 however, that these data on the periodicities in temperature 

 were not truly significant and similar weekly periodicities have 

 frequently occurred in the past. 



"The degree of periodicity in upper -air temperatures ob- 

 served in 1950 during April, July, and November shows a stat- 

 istical significance of a much higher order of magnitude than 

 those referred to by Lewis, Wahl, and Wexler. To illustrate 

 this, an analysis has been made of the temperatures at the 

 700 mb level at nine stations in the United States at the inter- 

 sections of the 80, 90, and 100° W meridians with the 35, 40 ; 

 and 45° N parallels. 



