Causes of the United States Floods

Abstract

THE unprecedented floods of the Ohio River in January of this year are the subject of a paper by Prof. C. F. Brooks and Major A. H. Thiessen in the Geographical Bevieiv of April. Floods may occur in the eastern United States at any time of the year, for there is always an extensive warm water surface near by to provide great volumes of water vapour, and not far distant are cold surfaces to furnish moving wedges of cold air to elevate the tropical air. Outstanding floods, however, occur only where a persistent high pressure area over the western Atlantic sends air currents inland for days in succession, and a winter high pressure area over the central or northern interior of the United States sends cold air southward or south-eastward. Along the front between the masses of polar air and the tropical air continuous ascent of the warmer air occurs with resultant heavy rains. In an ascent of two miles, three quarters of the water vapour in the warm air is precipitated. The authors calculate that a mass of air, nearly saturated at 77° F. over the Caribbean Sea, chilled over land in middle latitudes to 68° F., would contain about 80,000 tons of water vapour for every square mile of surface. If this were forced upward to a height of two miles the rainfall would be 0-8 inches. This would probably occur in one day with wind at a velocity of four to eight miles an hour. Since the average speed of the tropical air mass is 30 - 40 miles an hour, it is easy to understand the occurrence of torrential rains of some five inches a day.