MANY RAINFALL. STATIONS NECESSARY. 



9 



are felt more by the dry farmers because they are working upon a 

 closer margin. Crops in the East are often lost through drought and 

 not infrequently under conditions where the up-to-date dry farmer 

 would have produced a good crop. On account, then, of the fluc- 

 tuations in rainfall in any locality from year to year, it is necessary 

 in judging the rainfall of a region to know the average annual rainfall, 

 or, as it is called, the normal rainfall. This is the only safe basis 

 upon which the rainfall of any region can be judged, and the longer 

 the records the greater is the dependence that can be placed upon 

 the normal. While two years' or three years' observations are safer 

 than a single year's records, it is not uncommon for two or three wet 

 years or for two or three dry years to follow one another in succession, 

 so that the yearly rainfall obtained at such times would be too high 

 or too low. Records for at least five years, and preferably for ten 

 years, are necessary to obtain the normal rainfall of a region, and 

 the longer the records the more reliable will be the result obtained. 



MANY RAINFALL STATIONS NECESSARY ON ACCOUNT OF LOCAL 

 VARIATIONS IN RAINFALL. 



In studying the rainfall of a region too much dependence must not 

 be placed upon the records of a single station, especially if this station 

 is located in a mountainous region. Under such conditions marked 

 differences in the annual rainfall are often found between stations 

 located on the highlands and those in the valleys. The cooling of 

 the moisture-laden air through expansion as it rises over a range of 

 hills causes precipitation, so that the windward sides of mountains 

 have a greater rainfall than the leeward sides. This is well illustrated 

 in the very heavy rains which occur all along the west slope of the 

 Cascade Range in Washington, Oregon, and northern California, 

 while the rainfall on the east side is very much less. (See the rain- 

 fall maps for Washington, fig. 22, and Oregon, fig. 17; also the chart 

 shown as fig. 1.) This is also well illustrated in Riverside and San 

 Bernardino counties in southern California (see map, fig. 7), where 

 a great change in rainfall occurs as we go eastward a few miles. 



These sudden changes in normal rainfall are not, however, confined 

 to the mountainous regions; marked instances of this kind occur in 

 the Great Plains, where changes in elevation are comparatively slight. 

 For example, the rainfall in the district around Aberdeen, in Brown 

 County, S. Dak., is nearly 5 inches above that of the surrounding 

 sections. (See the South Dakota rainfall map, fig. 18.) This differ- 

 ence is based upon observations extending over twenty years oi 

 more; and the effect upon the crops has been so marked that the 

 farmers generally recognize that good crops may be found in this 

 section when the surrounding regions are suffering from drought. 

 It is important, then, to have as many rainfall stations as possible 

 in order to measure accurately the annual precipitation. 



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