328 ANNUAL REPORT SMITHSONIAN INSTITUTION, 19 3 8 



CAUSES OF THE GREAT FLOODS 



Two elements are always involved in the production of a great 

 flood in the eastern United States: a rapid and continuing flow of 

 moist tropical air into the country and a frequent or persistent eleva- 

 tion of this tropical current by a colder air mass over the same region. 



Such a wind situation depends on a persistent high-pressure area 

 in the western Atlantic and another high over the central or northern 

 interior of the United States or farther west. The Atlantic high pushes 

 air from the tropical Atlantic northwestward, northward, or north- 

 eastward, while the western high pushes the polar air southeastward, 

 southward, or southwestward. The low pressure trough between the 

 highs is marked by one or more moving centers. Its position varies 

 in accordance with the relative strengths and locations of the two highs 

 from day to day. The western high or an offshoot from it often moves 

 eastward; thence it may pass out to sea, to strengthen the Atlantic 

 high and, consequently, the landward flow of tropical air. This is 

 what happened January 20 to 24, 1937. Along the front between the 

 polar and the tropical air masses continuous, sometimes violent, ascent 

 of the warmer current is in progress. Either the tropical air is the 

 aggressor (warm front) or the polar air is making gains (cold front). 

 In either event, the tropical air is forced upward, though usually with 

 less violence on the warm front than on the cold. This is why it rains. 



When moist tropical air is forced to rise, it expands in the lower 

 pressure aloft and cools at such a rate that in an ascent of 2 miles 

 three-quarters of its vapor will be precipitated. If such air, originally 

 nearly saturated at 77° F. (25° C.) over the Caribbean Sea, has been 

 chilled over land in middle latitudes to 68° F. (20° C), a layer of such 

 air 2 miles (3.2 km) thick would contain about 80,000 tons of water 

 vapor over each square mile of surface. If it were forced upward over 

 a cold air mass or by convergence to a height of 2 miles, some 60,000 

 tons of its vapor would be precipitated, or the equivalent of 0.8 inch 

 (2 cm) of rain. Since the slope of the upper surface of the cold air 

 mass is commonly only 1 or 2 percent, the lifting of the tropical 

 air mass by 2 miles would require the passage of 100 or 200 miles of 

 wind, which at a velocity of only 4 to 8 miles an hour would complete 

 the entire elevation in 1 day. Since the advance of a tropical air 

 mass commonly proceeds at 30 to 40 miles an hour, it is easy to see 

 how rains of several inches in 1 day may fall when a tropical air mass 

 mounts a polar one. The heaviest fall occurs when the polar front is 

 standing or advancing under the tropical air mass. 



The weather preceding any flood is important in its conditioning 

 of the watershed as regards run-off. If the weather has been rainy 

 before the heavy continuous rains that make the flood and the ground 

 has become soaked, run-off naturally is favored. With well-frozen 

 ground a like condition exists. 



