FLOODS BROOKS AND THIESSEN 333 



Furthermore, the channels through which the discharges take place 

 tend to be maintained for weeks or months. This seems partly to be 

 owing to the locations of the accumulation areas but is also the result 

 of conditions favoring the transport of cold air without too much 

 warming en route. The first outbreaks of polar air lay down a carpet 

 of snow, which thereafter neither becomes warm itself nor permits 

 much heat from the ground to reach the top of the snow blanket; 

 portions of the polar air masses continuing over the snow are less 

 warmed than if they passed over bare ground ; pressure over the snow- 

 covered regions continues high; and further progress of the cold mass 

 is facilitated. 



The Arctic outpourings in the winter of 1936-37 established a 

 favoring lane southward through the western half of North America 

 and, with few interruptions, dominated this region, continuing drought 

 on the Great Plains, elevating Pacific air to lay down deep snows in 

 the far West, and freezing the oranges of southern California. 



A heavy snow cover became established in the upper Mississippi 

 Valley and upper Lakes region, where the eastern margin of the polar 

 air mass frequently elevated the much-chilled western margin of the 

 tropical air mass. This snow-covered region, like that over the western 

 plateau, became a secondary booster of polar air masses, strengthening 

 them for combat along the tropical front to the southeast (fig. 2) . 



The weather maps for January 14, 21, and 24, when the heaviest 

 rains were in progress, show the principal fronts and trough of low 

 pressure between the Bermuda and northern highs (figs. 3-5). The 

 weather map of January 14 (fig. 3) indicates in typical form the 

 Atlantic high and another high over the eastern slope of the Rockies. 

 A cold front extends in a general northeast-southwest direction and 

 lies just west of the Appalachians. It moved slowly from the morning 

 of the 14th to the evening of the 15th. West of the frontal line was 

 polar Pacific air, and east was tropical air. Heavy rains fell in a 

 narrow belt extending from Lake Erie in a southwesterly direction to 

 Arkansas . 



The succession of formations from January 14 to 25 presents most 

 interesting examples of frontal interactions, as indicated in some 

 measure by the successive daily positions of the tropical front, shown 

 in figure 10. Table 2 shows the daily rainfalls at selected stations, as 

 published on the Washington weather maps. 



