64 



SCIENCE 



[N. S. Vol. XXXVIII. No. 



tablish a relation between the forests of 

 the coastal plain and the southern Appa- 

 lachians, on the one hand, and the humid- 

 ity of the central states and prairie region, 

 on the other. 



There are three fundamental facts upon 

 which, in my judgment, this relation is 

 based. 



1. In the eastern half of the United 

 States there is a marked periodicity in the 

 wind direction. In winter the prevailing 

 winds are from the north and northwest; 

 in summer the prevailing winds are from 

 the south. When the prevailing winds 

 come from the south the entii'e eastern 

 half of the United States is wet. When 

 the prevailing winds are from the north- 

 west and west the precipitation decreases. 

 Therefore, the precipitation of the eastern 

 half of the United States depends largely 

 upon the prevailing southerly winds which 

 come from the Gulf and penetrate far into 

 the interior of the continent. 



2. The evaporation from the ocean plays 

 a comparatively unimportant part in the 

 precipitation over the land; seven ninths of 

 the precipitation over land is supplied by 

 evaporation over the land itself and only 

 two ninths is furnished by the evaporation 

 from the ocean. Therefore, the greater the 

 evaporation from the land which is in the 

 path of the prevailing southerly winds, the 

 more moisture must be carried by them 

 into the interior of the continent. 



3. The forest evaporates more water 

 than any vegetative cover and much more 

 than free water surfaces. Therefore, for- 

 ests enrich with moisture the winds that 

 pass over them and contribute to the hu- 

 midity of the regions into which the pre- 

 vailing air currents pass. 



PERIODICITY OP WIND DIRECTION IN THE 

 EASTERN HALF OP THE UNITED STATES 



After Asia, North America is the largest 

 continent in the world. One of the most 



striking physiographical features of North 

 America is that the mountains run along 

 the meridians and not along parallels. 

 The entire northern part of the American 

 continent has no high mountains except in 

 the western part. As the result of this the 

 central part of the continent does not offer 

 any obstruction to winds from the 30th to 

 70th degree of northern latitude, that is, 

 from the Gulf of Mexico to the Arctic Sea. 

 Even the Asiatic continent does not have 

 such a large continuous area free of moun- 

 tains extending along the meridian. There 

 the greatest extension is from the 38th to 

 the 73d degree of northern latitude, that 

 is, from the southern border of the plain 

 of Touran to the northern shores of west- 

 ern Siberia. To the south of the 30th 

 degree extend the waters of the Gulf of 

 Mexico. The mountains oh the southern 

 shore of the gulf begin only at 19 degrees 

 of north latitude. The North American 

 continent, therefore, together with the in- 

 terior lakes forms an expanse for the move- 

 ment of the air between the tropical and 

 Arctic regions, such as is found outside of 

 it only on large oceans, in the northern 

 hemisphere, on the Atlantic Ocean. 



Another climatic peculiarity of the east- 

 ern United States which has a bearing 

 upon the question under discussion is the 

 rapid decrease in temperature from south 

 to north. Take, for instance, Labrador; it 

 is entirely an Arctic region where agricul- 

 ture is impossible. Yet it lies in latitudes 

 at which in Europe and Asia agriculture is 

 still flourishing and large populous cities 

 are found (in 53d to 60th degree northern 

 latitude are found Christiania, St. Peters- 

 burg). Florida, on the other hand, be- 

 tween 25th and 30th degree of north lati- 

 tude, is almost a tropical country. Be- 

 tween Florida and Labrador the drop of 

 temperature for each degree of latitude 

 (60 miles) is for January 2.9° F., for July 

 1.08° F. and for the entire year 1.7° F. 



