68 



SCIENCE 



[N. S. Vol. XXXVIII. No. 



inates in the Pacific Ocean drifts over the 

 tops of the mountains and during winter is 

 drained of its moisture by the excessive 

 cold. This moisure may be precipitated in 

 the form of snow over such states as North 

 Dakota, but the amount can not be very 

 great. 



The central interior region of the United 

 States is thus the battleground of two 

 titanic forces, of which one is harmful and 

 the other is beneficial. The beneficial force 

 takes its origin in the Gulf of Mexico and 

 the adjoining ocean, the harmful in the 

 interior of the continent and the Rocky 

 Mountain region, and whether it comes as 

 the wann chinook winds which blow out 

 of the northern Rocky Mountains, or as the 

 dry westerly winds of the upper Missis- 

 sippi Valley and the western Lake region, 

 occurring especially in the spring and 

 early summer, it always carries in its wake 

 serious injury to orchards and fields. 



The central states and the prairie region 

 are geographically at the point where the 

 battle between the two forces is fiercest and 

 the victory is now on the one side and now 

 on the other, being dependent upon the 

 cold and humid, and the warm and dry, 

 climatic cycles as well as upon the seasons 

 of the year. 



When the humid southerly winds extend 

 their influence far into the interior of the 

 continent, and overpower the dry conti- 

 nental winds, the central states and prairie 

 region, the granary of the United States, 

 produce large crops. When the dry winds 

 overpower the humid southerly winds there 

 are droughts and crop failures. 



The southerly winds on their way from 

 the Gulf of Mexico do not meet any me- 

 chanical obstructions. Since the Appa- 

 lachian Mountains, running in a north- 

 easterly and southwesterly direction, do 

 not hamper their passage, they are capable 

 of penetrating far into the interior of the 



country and, therefore, determine the 

 amount of precipitation, even in such 

 states as Minnesota, Nebraska, North and 

 South Dakota. The moisture-laden winds 

 from the gulf, as soon as they reach the 

 land and encounter irregularities, are 

 cooled and begin to lose part of their mois- 

 ture in the form of precipitation. 



As long as the air currents are saturated 

 with moisture the slightest cooling or ir- 

 regularity of the land that causes them to 

 rise will cause precipitation. But as they 

 move inland and become drier the remain- 

 ing moisture is given off with diificulty and 

 precipitation decreases. The sooner the 

 humid air currents in their passage over 

 land are drained of their moisture the 

 shorter is the distance from the ocean over 

 which abundant precipitation falls; the 

 longer the moisture is retained in the air 

 currents the farther into the interior will 

 it be carried and the larger will be the 

 area over which precipitation will be dis- 

 tributed. 



If precipitation over land depended only 

 on the amount of water directly brought 

 by the prevailing humid winds from the 

 ocean, the land would be pretty arid and 

 rainfall would be confined to only a narrow 

 belt close to the ocean. Fortunately, not 

 all the water that is precipitated is lost 

 from the air currents; a part runs off into 

 the rivers or percolates into the ground, 

 but a large part of it is again evaporated 

 into the atmosphere. The moisture-laden 

 currents, therefore, upon entering land at 

 first lose the moisture which they obtained 

 directly from the ocean, but in their far- 

 ther movement into the interior they ab- 

 sorb the evaporation from the land. Hence 

 the farther from the ocean the greater is 

 the part of the air moisture contributed by 

 evaporation from the land. At a certain 

 distance from the ocean practically all of 

 the moisture of the air must consist of the 



