Cycles of Rainfall 5 



in our leading crop area; secondly, as the existing 

 meteorological records are of unequal lengths and of 

 varying reliability, it is necessary to take the best long 

 records that can be found within the limits of the crop 

 area. 



The principal region of grain production in the United 

 States is in the Mississippi Valley, but the meteoro- 

 logical records of the Middle West do not extend through 

 a long period of time. In order to achieve the two ends 

 of having a long record of precipitation and of having 

 the record typical of the conditions in the grain area, 

 the device has been adopted of investigating rainfall in 

 the Ohio Valley — which affords the longest record ob- 

 tainable in the neighborhood of the central Mississippi 

 region — and of showing that the rainfall of our lead- 

 ing grain state, Illinois, follows the same law as the 

 rainfall of the Ohio Valley. 



The stations in the Ohio Valley with long rainfall 

 records are Marietta, Portsmouth, and Cincinnati. 

 Their mean annual rainfall since 1839 is given in 

 Table I 1 of the Appendix to this chapter. The graph 

 of the course of rainfall in the Ohio Valley since 1839 

 is traced with other graphs on Figures 4, 5, and 6. The 

 problem that must now be faced is the question as to 

 whether the sequence of annual rainfall in the Ohio 

 Valley follows a simple law, and if so, to give a quanti- 

 tative formulation of the law. 



1 The data were taken from Bulletin }V of the Weather Bureau of 

 the United States and from the A7inual Reports of the Chief of the 

 Weather Bureau. 



