CLIMATIC CYCLES. 253 



been excessively wet and excessively dry years, the annual rainfall having 

 ranged from 13.30 inches to 47.53 inches; there have been groups of wet years 

 and groups of dry years succeeding one another in a rather irregular manner. 

 Thus the 47 years may be grouped into five periods as follows: The first 10 

 years were mostly wet years, only one of them, viz, 1852, having a rainfall 

 less than normal; the next 9 years, 1859 to 1867 inclusive, constituted a period 

 of scant rainfall, including particularly the exceedingly dry years of 1863 

 and 1864 and the scarcely less dry years of 1859 and 1860; the 9 years from 

 1868 to 1876 inclusive included years of plenteous and years of scant rainfall 

 succeeding each other in a quite irregular manner; then followed 10 years, 

 1877 to 1886 inclusive, of rainfall generally above the normal; and finally the 

 last 5 years have been, with the exception of 1891 and 1892, years of deficient 

 rainfall, with the year 1894 the driest of the whole 47. 



" But if we divide the entire series of 47 years into two periods of 24 and 23 

 years respectively, the average rainfall of the first period will exceed that of 

 the last by only about an inch. The first year of the series, 1849, was one of 

 excessive rainfall, not only as shown by the record made at Fort Kearney, 

 the only station in Nebraska at which records were kept, but also as confirmed 

 by records in the adjacent Territories. This difference of a little more than 

 an inch between the mean rainfall of the first 24 years and that of the last 23 

 years of the 47 would almost disappear if this year of 1849 were omitted from 

 the series; the mean precipitation for the 23 years from 1850 to 1872 is 23.55 

 inches, while that of the 23 years since is 23.46 inches. 



"The conclusion, therefore, seems to be a safe one that the average rainfall 

 of Nebraska, although subject to great fluctuations from year to year, yet 

 in the long run remains substantially unchanged, so far as we can discover 

 from the records of nearly haK a century. " 



Prediction of drought periods. The sun-spot cycle furnishes a ready method 

 of predicting the occurrence of dry and wet periods. The sun-spot numbers 

 are recorded with the greatest accuracy and detail, and the number for each 

 month and year is readily obtainable. These numbers, taken in conjunction 

 with the length of the recent cycles, make it possible to forecast the date on 

 which the next maximum and minimum will fall, as well as something of their 

 intensity. During the past century the average of 11.4 years for the cycle 

 has been strikingly evident, practically all the cycles being from 10 to 12 years 

 long. The accuracy of the correlation between the sun-spot cycle and the 

 climatic cycle as recorded in the growth of trees is 85 per cent, according to 

 Douglass's results (1919). This compares favorably with the accuracy 

 of the daily forecasts of the Weather Bureau, and still more favorably with 

 that of the weekly forecasts. However, there is one essential difference, in 

 that the latter are actual forecasts, while the prediction of dry and wet phases 

 has been attempted as yet only for the dry and wet phases of the current cycle 

 (Clements, 1917, 1918). The close correspondence between the sun-spot 

 cycle and the curve of tree growth strongly suggests a similar degree of 

 accuracy in actual prediction, but repeated trial can alone determine this, 

 as well as disclose the reasons for failure. 



While it is thought and expected that the use of the sun-spot cycle will 

 permit the prediction of drought periods for the West in general, as well as 

 the occurrence of intervening but more diffuse wet periods, the prediction for 

 a particular locality is subject to more uncertainty. In this respect, cycle 

 predictions resemble the daily and weekty weather forecasts. The failure of a 



