Page:Popular Science Monthly Volume 44.djvu/112

104 is five thousand feet above the plateau (Appletons' Physical Geography, p, 23). The latter furnish the second impediment to the onward progress of these winds, since they run southeast-northwest; but the trades blow directly west, and thus impinge upon them at an angle which deflects the winds themselves toward the north.

The Sierra Madre are more than one thousand miles long, and are an insuperable barrier to the progress of these vapor-loaded winds. This is evident, as there is no indication of their presence on the west side, neither on the land nor on the water, as the Pacific trade winds appear to originate about one hundred and fifty miles west of the coast of Mexico. An analogous case is cited by Prof. Orton, in his Andes and the Amazon, p. 118, who says, when speaking of the Andes, "So effective is that barrier that the trade winds are not felt again on the Pacific till you are one hundred and fifty miles from the coast."

These winds appear to be shoved up, strata upon strata, on the Mexican plateau, and when they finally reach the Sierra Madre Mountains, over which they can not pass, they are rolled back upon themselves. They must have an outlet. The rushing wind from the east prevents their moving in that direction, and the force of the main current forbids their flowing toward the equator, and thus their outlet can only be toward the north. They are now so high that they must be beyond the influence of the rotary motion of the earth, and are governed by the force of gravitation alone. In accordance with the latter law they flow, as on an inclined plane, over the colder and more dense air toward the north, and thus restore the equilibrium of the atmosphere that has been disturbed. This disturbance is caused by a continual flow of the cold and heavy surface air from the extreme north toward the equator, because along the tropical belt a partial vacuum is created by the air becoming heated and lighter and in consequence floating upward, and the cold air rushes in to supply that vacuum.

These comparatively warm strata, though high in the atmosphere, have a tendency to reach the earth, but, being lighter than the surface air, they float above it until their respective densities are about the same. The point of contact with the earth of the lower strata of these "return trades" is near 30º north latitude in the summer, but still further north in the winter. This point of contact is near and along the north shore of the gulf, and the blending of the moisture of the "return trades" with that off the gulf may account for the unusually large rainfall of sixty inches near that line; meanwhile the main and higher strata blow on and reach the earth further north.

"The polar winds, seeking the equator, strike obliquely against