July 18, 1913] 



SCIENCE 



65 



Comparing the same latitudes in Europe 

 the drop for each degree of latitude is less 

 than half of that for the North American 

 continent. Between the Canary Islands 

 and northern Scotland the decrease in the 

 mean annual temperature for one degree 

 of latitude is only 0.8 of a degree. 



Climatically the North American conti- 

 nent can be divided into three parts : 



1. The narrow strip along the Pacific 

 Ocean, which is separated from the interior 

 of the continent by mountain ranges. This 

 narrow strip from the Peninsula of Cali- 

 fornia to the southern shore of the Penin- 

 sula of Alaska, from the 32d to the 60th 

 degree of sorthern latitude, is under the 

 influence of the Pacific Ocean, as it is open 

 to the west, while in the east high moun- 

 tains separate it from the interior of the 

 continent; and as western winds are, as a 

 rule, the strongest winds in the northern 

 hemisphere, it is only natural that west- 

 erly and northwesterly winds prevail in 

 this part of the country both in summer 

 and winter. 



2. The region of mountains and plateaus 

 to the east of the Cascades and Sierra 

 Nevada ranges. This extends not onlj^ to 

 the Rocky Mountains, but beyond the 

 Rocky Mountains to the 100th meridian. 

 The high plateaus and the low valleys of 

 this region are characterized by extreme 

 dryness and only in the mountains does 

 the snow and rain fall in any abundance. 

 The dryness is due to the fact that the pre- 

 vailing westerly winds give off the mois- 

 ture on the western slopes of the Sierra 

 Nevada and Cascades, and become dry 

 winds on the leeward side of these moun- 

 tains. During the winter the prevailing 

 winds are from the west and northwest, 

 but in the summer the direction of the 

 wind changes considerably, becoming 

 southwesterly. This change in the direc- 

 tion of the wind in summer has been ob- 



served even on Pikes Peak, but is still 

 more pronounced in the valleys and on the 

 plateaus. 



3. Since the Appalachian Mountains do 

 not offer a climatic boundary, the entire 

 eastern part of the North American conti- 

 nent east of the 100th meridian can be 

 considered climatically as one unit. This 

 climatic region is the largest of the three, 

 including the Atlantic plain, the Missis- 

 sippi Valley, except the upper part of its 

 western tributaries, and the Lake Region 

 to the Hudson Bay. During winter and 

 partly in the fall and in the early spring 

 the winds in this region come from the 

 west and northwest. These prevailing 

 winds bring cold and comparatively dry 

 air from the interior of the continent. In 

 the spring and early summer these winds 

 are hot and dry. In summer the prevail- 

 ing winds are from the southeast in Texas, 

 and farther north and east they come from 

 the south and southwest. Professor Henry, 

 in his ' ' Climatology of the United States, ' ' 

 says that in midwinter northwesterly 

 winds prevail uniformly over the Missouri 

 Valley and the upper and middle portions 

 of the Mississippi Valley. As the spring 

 advances the region of southeast to south 

 winds spreads northward and eastward 

 from the Texas coast, so that by April it 

 embraces the states of Texas, Oklahoma, 

 Arkansas, Mississippi, Louisiana, Alabama, 

 western Tennessee, Missouri, Kansas, south- 

 eastern Nebraska and Iowa. By June the 

 northwest winds of midwinter have been 

 supplanted by southerly winds over prac- 

 tically the whole of the country east of the 

 Rocky Mountains. In autumn the north- 

 west winds become more frequent and as 

 autumn shades into winter they gain the 

 ascendency in the Missouri and Mississippi 

 valleys and the plains states. 



The periodicity is well illustrated on the 

 two maps, on which is indicated by arrows 



