no. 9 



FORECAST OF U. S. PRECIPITATION — ABBOT 



15 



Table 4. — From three-month running means of precipitation, Helena, Mont. 

 39-month period = p 



Elimination of subordinate periods. Interval 1891-1917 

 Mean percentages of the normal. Original mean and departures 

 removing subordinate periods 



after 



Mean da 29.6 percent. 



Average deviation before clearance 29.6 percent. 



Mean A 2.1 percent. 

 After clearance 2.1 percent. 



Note. — Thus the removal of overriding harmonics reduces the average deviation by 93 

 percent. Of about 10,000 such removals of overriding harmonic periods, probably 4,000 gave 

 fully as satisfactory end results as the 39-month curve at Helena did for the years 1891 to 

 1917. 



