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Page:Popular Science Monthly Volume 44.djvu/111

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103
ORIGIN OF THE MISSISSIPPI VALLEY RAINFALL.

surface vapor rising off the gulf; this is evident because of the unusual distance to which the sea-breeze penetrates into Texas and the adjoining region of Mexico. In addition, the water of the gulf is not as warm as that of the Atlantic equatorial current—to be noticed presently—by an average of ten or twelve degrees Fahr., and in consequence, in proportion, its evaporation is just so much the smaller. The equatorial current penetrates the gulf about five hundred miles, but does not diffuse itself and thus impart its heat to the adjoining waters, but in a compact body the current turns toward the east and finds its way out through the Florida Strait, and thus becomes the Gulf Stream.

It is estimated that if the "gulf was landlocked and evaporation checked," the volume of water poured into it by the Mississippi alone would "raise the level of this great area one and a quarter feet each year." (Appletons' Physical Geography, p. 130.) The height of the surface of the gulf, however, remains uniformly the same year in and year out. It follows from this that the outflow of water and its evaporation 'combined amount each year to only one foot and a quarter. This leaves twelve feet and three quarters to be obtained elsewhere, in order to furnish the rainfall for the great valley. The question is. Where can this be obtained?

The Atlantic equatorial current may furnish an answer. This vast stream is about four thousand miles long and about three thousand wide. Taking its rise in the Gulf of Guinea, it flows westwardly, but, dividing on Cape St. Roque, the much greater portion moves along the north shore of South America, and just before entering the Caribbean Sea it unites with the northern counter current. (See Appletons' Physical Geography, pp. 50, 51.) These currents are both under a broiling tropical sun, and their water is heated from 80º to 82º; "the evaporation is rapid in the equatorial regions, and most of all in the warm belts constantly swept by the trade winds." Thus, when the warm, saturated air next the surface rises, it is rapidly carried away by the wind, and cooler air flowing from the north takes its place, to be in turn heated and floated upward. Says Captain Maury, U. S. N. (Geography of the Sea, p. 102), "Off this ocean belt there is, in the form of vapor, annually floated up into the higher air fifteen feet of water." Says Prof. Arnold Guyot, in Earth and Man, p. 85, when speaking of the same, "The sun causes these invisible vapors to rise, which, being lighter than the air itself, increasingly tend to soar into the upper atmosphere, filling it and constituting within it another aqueous atmosphere." This vapor is carried by the trade winds steadily westward at the rate of about thirty or thirty-five miles a day, and meets its first obstruction in the plateau of Mexico, which is five thousand feet above sea-level. On the west coast of Mexico stand the Sierra Madre Mountains, whose altitude