CHAPTER I. 



THE MONSOON CLIMATE OF ARIZONA AND NEW MEXICO. 



The climate of Ai'izona, New Mexico, and northern Sonora is of a peculiar, transitional 

 type. It may be defined as a subtropical continental climate of the monsoon variety. 

 It resembles that of the provinces of the Punjab, Rajputana, and Sind in northern India 

 more closely than that of any other part of the world. As the region extends from about 

 north latitude 28° in Mexico to 37° in northern New Mexico and Aj-izona, its subtropical 

 position brings most of it within the great world-zone where high pressure and consequent 

 aridity normally prevail. Here the main movement of the air is downward and outward; 

 and here the northeasterly winds of the trade-wind zone and the southwesterly winds of 

 the zone of prevailing westerlies find their origin. 



If the climatic zones of the earth were not interfered with by the different rates at 

 which land and sea are heated and cooled, seasonal changes would bring this region within 

 the range of the prevailing westerlies and their rain-bringing cyclonic storms in winter, 

 and within the trade-wind belt in summer. In winter the country would receive a fair 

 amount of rain for a few months, the edges, so to speak, of the great storms which whirl 

 across more northerly regions not only in winter but in summer. During the rest of the 

 year it would be rainless, for in spring and fall it would be in the subtropical zone of high 

 pressure and descending air, while in summer the trades would blow across it from the 

 northeast. Inasmuch as the trades would come from a dry interior, they would bring no 

 rain. As a matter of fact, however, the trade-winds are never well developed in Ai'izona 

 and New Mexico, and herein lies the explanation of the most peculiar characteristics of the 

 climate. The cyclonic storms of the westerlies in winter and the descending air of the 

 subtropical "horse latitudes" in spring and autumn give rise respectively to the rain and 

 the aridity which would be expected. In summer, however, because of the great size of 

 the continent of North America, the trade winds which would be expected do not appear; 

 their place is taken by relatively moist winds which blow in general from the south, send 

 may be called monsoons for lack of any more appropriate name. 



In order to give definiteness to our discussion of the climate of the southwest, let us 

 recall the familiar general principles of the effect of continents upon temperature, pressure, 

 winds, and rainfall. Land masses, as is well known, become heated or cooled much more 

 quickly than expanses of water. Hence, in winter the continents become much colder 

 than the oceans, and are therefore the seat of centers of high barometric pressure, a con- 

 dition exactly the reverse of that prevalent over the comparatively warm oceans. From 

 the continental areas of high pressure the winds tend to blow outward, especially toward 

 the east and south. Thus the cold waves of the Eastern and Southern States arise, for 

 on the western side of ordinary cyclonic storms the indraft of air occasioned by the storms 

 themselves is strengthened by the general high pressure prevaihng in the cold interior of 

 the continent. Inasmuch as Ai-izona and New Mexico, unlike the parts of India with which 

 we have compared them, are not protected by an east-and-west range of mountains such 

 as the Himalayas, chill winds from the north sweep over the country in winter, producing 

 frequent frosts. Even as far south as Tucson, in latitude 32° and at an elevation of only 

 2,300 feet above sea level, the thermometer occasionally falls to 16° F. Except in the 

 warmest and lowest places, such as the Gila Valley around Phoenix, or at Yuma on the 

 Colorado River, this hability to sudden cold prevents the growth of subtropical fruits, such 



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