66 



SCIENCE 



[N. S. Vol. XXXVIII. No. 968 



PREVAILING BIKECTIOHS OF THE SORTAOE WIltDS ABB THE lEAH PKECIPITATIOH 

 m THE UHITKD STATB3 DUEIHG JAMUAHY 



the direction of the prevailing winds, based 

 on twenty years of continuous records, and 

 by lines the mean precipitation for the 

 months of July and January. The map 

 for the month of July is typical for 

 the summer period and the one for the 

 month of January is typical for the winter 

 period. These maps show, very clearly, it 

 seems to me, that the eastern half of the 

 United States is under the influence of 

 two prevailing winds ; one, which originates 

 in the Gulf of Mexico and in the Atlantic 

 Ocean, is mild and humid ; the other, which 

 comes from the interior of the continent 

 and from the Rocky Mountain region, is 

 dry and continental in character, that is, 

 dry and cold in winter and dry and hot in 

 the spring and summer. 



Another important fact which the rec- 

 ords of precipitation and wind direction 

 establish is that there is a most intimate 

 relation between the prevailing southerly 

 winds and precipitation in the eastern half 

 of the United States. It is during the sum- 

 mer period when the entire eastern half of 

 the United States is under the influence of 

 the southerly winds, that most of the pre- 

 cipitation falls over it. On the plains east 

 of the Rocky Mountains the summer rain- 

 fall forms from three fourths to four fifths 

 of that of the entire year. In July 

 when the southerly, southwesterly and 

 southeasterly winds extend far into the 

 interior of the continent as far north 

 as North Dakota, and as far west as 

 the foothills of the Rocky Mountains and 



