332 



ANNUAL REPORT SMITHSONIAN INSTITUTION, 19 3 8 



1931, as compiled by the United States Weather Bureau. The sea 

 temperatures in the general longitude of the Azores and latitude of 

 Bermuda, on the other hand, were 1.7, 1.0 and 2.6° F. below the nor- 

 mals for those 3 months. 



In accordance with the pressure gradients on the south and west 

 of this standing Bermuda high, great tropical air masses moved day 

 after day over the hot waters of low latitudes westward and north- 

 westward toward the United States. Passing the length of the 

 Caribbean Sea, they turned northward into the Gulf of Mexico and 

 entered the continent nearly saturated with water vapor to a height 

 of 2 kilometers or more. Flooding rains were inevitable. The ques- 

 tion was, where? The Arctic held the answer. 



The major interzonal exchanges of air take place in alternating 

 bands of considerable width. In January 1937, there was a northward 



Figure 2.— The depth of snow on the ground, 8 p. m., January 11, 1937. (From Weekly Weather and 



Crop Bull., January 12, 1937.) 



current of tropical air occupying a band about 2,000 miles wide from 

 somewhere east of Bermuda to a short distance west of the Mississippi 

 River. Its western boundary is strikingly suggested by the sharp 

 gradient between the large positive temperature departures east of 

 the Mississippi and the large negative ones to the west (fig. 1). From 

 the plains to the Pacific coast and beyond a southward current of 

 polar air prevailed occupying a band also about 2,000 miles wide. 



The chilling and shrinkage of air continuously in progress in the 

 polar regions in winter leads to an accumulation of air that must dis- 

 charge toward the lightened pressures of middle latitudes. The loci 

 of chief upbuilding of polar air masses vary somewhat from year to 

 year, apparently in relation both to the distribution of cold surfaces, 

 namely ice-covered sea and snow-covered land, and to the proximity 

 of warm surfaces, for the expanded air over warm surfaces readily 

 overflows and feeds the accumulating cold air masses. 



