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Show 153 that this report may furnish some interesting points, as the greater portion of the Mohave Desert was until a comparatively recent date a terra incognita. This report treats of the winds, rain- fall, cloud- bursts, sand- storms, electrical phenomena, hot winds, temperature of air, river, and soil, relative and absolute humidity, and ozone. Numerous comparisons with other countries have been added, in order to elucidate more clearly the specific character of this region. Very respectfully, your obedient servant, OSCAR LOEW. Lieut. GEO. M. WHEELER, Corps of Engineers, in charge. There exist probably but few regions on earth where two totally different climates are separated by a single mountain range, as in Middle and Southern California, where the uniform sea- climate of the coast- strip, with comparatively small variations in temperature, forms a most remarkable contrast with the climate on the other side of the gigantic chains of mountains, the Sierra Nevada and its southern continuations, the San Bernardino and Jacinto ranges, whioh represent the backbone of the State of California. The state of humidity, the direction of wind, the electrical condition, the daily oscillations in temperature, how far different are all these conditions on the west and on the east side of those mighty elevations which separate the Mohave Desert from the fertile coast! In regard to the winds of the Mohave Desert, it is a striking fact that southeast winds are by far the most prevalent in summer time. It is also easily observed that the clouds and summer rains come from that direction. This observation was confirmed, on our arrival at Fort Mohave, by the hospital- steward of that post, who attended to the meteorological observations for the past three years. The great prevalence of these winds in Northern Texas, Southern New Mexico, and Arizona, was recalled to my memory ; these same winds producing there the summer rains. That this moisture- carrying current comes from the Gulf of Mexico, and is the continuation of the great equatorial current which, like the equatorial marine current, is a consequence of the earth'B rotation, and flows from east to west in a width from 28° south latitude to 28° north latitude, cannot be subject to doubt. By the development of the high summer temperature of those countries, the air- current deviates, upon its arrival in the Mexican Gulf, from its original westerly directiou to the northwest, and appears therefore as a southeast wind. That this stream of air must contain a great deal of moisture, may be inferred from its passage across the warm and extensive oceanic area between Africa and Central America. Although deprived of the greater part of moisture after its flowing across Texas, New Mexico, and Arizona, it still carries enough moisture as far west as the Mohave Desert, to produce occasional summer rains there; but at the backbone of California this influence is completely broken ; the wind, now deprived of its moisture, is forced into a different direction, and no summer rains fall on the California coast. Here, on the contrary, prevails the northwest wind, whioh saturated at a comparatively low temperature with moisture, and passing over the much warmer continent, becomes drier: its relative humidity sinks rapidly; hence no rain can fall. However, when in the winter months the power of insolation is weakened, the land cools off, then the aqueous precipitates become extensive and the coast country covers itself with verdure. In winter time the northwest winds also prevail over the entire Mohave Desert, but having lost most of their moisture on the passage across the high ranges, bring but rarely rain or snow in the desert. The absolute humidity, or rain- producing powers of the northwest wind, must naturally fall far below that of the southeast monsoons, on account of their respective temperatures. While the summer rains in the Mohave Desert fall chiefly in great cloud- bursts that are always accompanied by electrical phenomena, neither is the case on the coast; thunder is at least there a very rare occurrence. The phenomenon of cloud- bursts is doubtless produced by the alternating position of hot, deep, and sandy valleys and steep, high ranges; the reflected heat of the former shifts the tendency of the clouds to a condensation to rain upon the extreme point, and these, approaching the somewhat cooler mountain- chain, must discharge their moisture at once, and sheets of water come down with such force that fragments of still uncondensed clouds are carried along. We witnessed this interesting phenomenon in more than one case} the clouds had the singular appearance of a lake- surface reflecting the light like liquid water. The rains falling principally upon the mountains, rarely in valleyB, it is not surprising that the rain- fall at Forts Mohave and Yuma is exceedingly small, both these posts being situated in the low and hot valley of the Colorado River. The following table, taken from the records of these posts with permission of the authorities, shows the rain- fall in four years: |