NO. 9 FORECAST OF U. S. PRECIPITATION — AfiBOT 1 3 



The mean form of the supposed overriding period of one-half 

 length is: 



a+n b + o c + p k + x l+y m + z 

 , , , ... 1 > 



2 2 2 2 2 2 



When this half-length curve is written twice, and subtracted, we have : 



a — n b — o c — p k — x l — y tn — z 



, , - £- , ... , — — , — - — , 



2 2 2 2 2 2 



and following that : 



n—a o—b p—c x—k y — / z—m 

 , , , ... , — - , - 



2 2 2 2 2 2 



So the last half of the long curve, when cleared of the period of one- 

 half of its length, is exactly like the first half, but with reversed signs. 



Grouping of periods. — All weather influences caused by changes in 

 solar rays are subject to lags. For instance, June and noonday are 

 times of highest solar altitudes, but the warmest months and hours 

 occur later. The lag is longer the longer the period of the solar radi- 

 ation change. These lags are due to atmospheric conditions, and vary 

 from locality to locality, from month to month, from times of great 

 sunspot activity to quiet solar times, and as population and foresta- 

 tion change. Hence, though the family of periods integrally related 

 to 273 months proceeds with perfect regularity in measures of the 

 solar constant, in weather the same family of periods is affected by 

 changes of phase, depending on the locality, the population, the sun- 

 spot frequency, and the time of the year. The periods are the same 

 in weather that they are in solar radiation, but owing to complex 

 atmospheric influences on the lags the weather phases are so altered 

 from time to time that these periods are unrecognizable without a 

 segregation of the data, governed by consideration of these modifying 

 influences. 



It is not possible to anticipate and allow for these phase changes 

 precisely. I content myself as follows : 



(a) The year divided: January to April; May to August; September to 



December. 



(b) Solar activity divided : Wolf numbers > 20; Wolf numbers < 20. 



(c) Secular time divided: first half of tabulated records; second half thereof. 



All these divisions of data hold for periods up to 1 5-1/6 months, 

 or 15 groupings for these periods. The segregation according to the 

 Wolf numbers holds from 18- 1/5 months up to 39 months, but not 

 the segregation for times of the year. 



