the Rocky Mountains, and in running along their eastern slopes are deflected to the southeast, and become the northwest winds of the valley of the Mississippi. . . . These cool winds meet the surplusage of the moist return trade winds, and by their coolness condense still more the latter's vapor, which descends in rainstorms that are sometimes quite violent, but furnish water for the head streams of the Missouri and its branches" (Prof. Guyot, Earth and Man, p. 100).
It has been suggested that this warm air, thus saturated with vapor, loses the latter when it floats aloft, because of the cold in the higher regions of the atmosphere, and consequently such air, floating north, could not deposit moisture when it reached the earth. That theory is not consistent with the fact that vapor often becomes visible in the form of clouds, which frequently float higher than the altitude of the Sierra Madre. In this special case it is worthy of notice that the plateau of Mexico is five thousand feet above sea-level, and it is also under a tropical sun, and therefore the incumbent air is so much the more heated. In such circumstances the vapor-loaded winds would not be likely to lose so much of their warmth and moisture as under conditions wherein there was no similar elevation. The great valley being free from mountain barriers at both ends, the winds flowing either way are unobstructed. In consequence, the comparatively warmer and vapor-loaded winds off the equatorial current meeting those coming from the north that are nearer the surface and also cooler and drier, the moisture of the former is condensed into mists and clouds, and finally descends to the earth in copious rains.