NO. lO SUN SPOTS AND WEATHER ABBOT 7 



D. THE 23-YEAR CYCLE DURING THE PLEISTOCENE 



Reeds has compared the thicknesses of clay varves near Haver- 

 straw, New York, laid down in the glacial period and measured inde- 

 pendently by Antevs and himself.* From these results I have formed 

 25 consecutive 23-year groups, and have averaged them in groups of 

 five, and also all together. Figure 4 shows the result of this investiga- 

 tion. Owing to a variety of influences, such as warmths of summers, 

 quantity of rainfall, hardness of the soil, and others which would all 

 affect the thickness of the varves, we should not expect close accord 

 in the individual cycles. Yet the five groups, each covering 115 years, 

 show some similarity, and the general mean for 575 years seems to 

 me fairly conclusive that the 23-year period was as influential during 

 Pleistocene glaciation, some 30,000 years ago, as it is now. Eight 

 principal features occur in the general mean, and I am inclined to 

 believe them to indicate that the sun's radiation varied then as now 

 by several periodicities related to 276 months, and that its variations 

 then as now controlled the weather. 



E. FORECASTING WEATHER CONDITIONS 



In some cases the 23-year cycle has features of high or low values 

 prevailingly over the course of several years, and repeated nearly simi- 

 larly during each cycle. Such cases occur, for example, in the Nagpur 

 precipitation cycles from the twenty-first through to the fifth year, 

 during which seven years the precipitation is subnormal. I believe, 

 therefore, that it is probable that subnormal precipitation will be ex- 

 perienced in Central India from 1942 to 1948. Similar indications 

 from studies of records of North Platte indicate subnormal precipita- 

 tion in central Nebraska from about 1939 to 1948, though with partial 

 relief during two separated years intervening. 



When the attempt is made to forecast weather for coming years in 

 more detail than such general statements as these, the embarrassing 

 changes of phase already referred to are encountered. These, though 

 they do not destroy the general sequence of the individual features of 

 the 23-year cycle, produce displacements, sometimes reaching a year, 

 and often several months, in the times of their occurrence. Further 

 research, it may be hoped, will aid in overcoming this difficulty. In 

 order to show the shortcomings of such detailed forecasts if made 

 only with present knowledge, I give in figure 5 predictions of depar- 

 tures from normal monthly temperature and percentages of normal 



* See Ann. Rep. Smithsonian Inst. 1930, pp. 295-326. 



