THE MONSOON CLIMATE OF ARIZONA AND NEW MEXICO. 



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is upward. Thus in the peninsula of Lower California, in the Mexican states of Sonora and 

 Chihuahua, and in Arizona and New Mexico, the summer is characterized by heavy 

 thunder-showers of the kind commonly known as tropical. These usually occur from about 

 the end of June to the early part of September, beginning earlier and ending later in the 

 south than in the north. Thus the country has two rainy seasons, one in winter deriving 

 its rain from cyclonic westerly storms, and one in the summer deriving rain from southerly 

 monsoon thunder-storms. 



The total rainfall is small, ranging from 5 to 20 inches per year in most parts of the 

 area, as is shown in the map, figure 1, and rising above 20 inches only in the high mountains. 

 The variation from year to year, however, is great, as may be seen in figure 2, where the 

 rainfall of Tucson is plotted by calendar years. The rainfall of the two seasons, summer 

 and winter, is still more variable, a fact evident from figure 3. The average of the winter 

 season at Tucson is 4.5 inches. The amount is small because the moisture comes largely 

 from the Pacific and must cross the high Sierras on the way. The winds, of course, 

 often blow from the east at the actual time of rainfall, but this affords no indication of the 

 soiu-ce of the moisture. In all cyclonic storms of the northern hemisphere the motion of 

 the air around and toward the centers of low pressure is similar, and the southeastern 

 quadrant of a cyclonic area lying in front of the storm where the air has not yet suffered 

 depletion of its moisture, and where the winds move rapidly from warmer to cooler lati- 

 tudes, is apt to have a rainfall more abundant than that of any other quadrant. Much 

 of the rain which accompanies such winds has doubtless come from the oceans to the 

 eastward, but more has probably been brought by the prevaihng westerly winds and is 

 merely caught up and prepared for precipitation by the easterly winds. 



FiQ. 2.— Annu-al Rainfall at Tucson, Arizona, 1868-1912. 



The paucity of the winter rainfall would not be so harmful were it not for its extreme 

 variability. In the winter of 1903-4 the total precipitation at Tucson for the six months 

 from November to April, inclusive, amounted to only 1.08 inches, while in the succeeding 

 year it amounted to 14.74. Records kept at Tucson and at the neighboring army post of 

 Fort Lowell show that in the years from 1868 to 1912 the winter rainfall was less than 2.5 

 inches, or practically useless, in 9 winters; it amounted to from 2.5 to 5 inches, that is, 

 it was fair, in 20 wmters; it ranged from 5 to 7.5 inches, or was good, in 13 winters, and 

 exceeded 7.5 mches only three times. (See table la.) These figures do not show quite 

 the true state of affairs so far as agriculture is concerned, for 3 inches in February and 

 March, after the chief frosts are over, are worth double that quantity in November and 

 December. Still, the figures serve to give an idea of the extreme variability and uncer- 

 tainty of the winter rains. Manifestly, even with the help of irrigation, the prospects of 

 the farmer are not of the rosiest when he may have only one-fourteenth as much rain in 

 one year as in another. 



After the dry spring season — the fore-sunamer, as MacDougal has called it* — the 

 southerly monsoon gradually becomes well established by the strong indraft toward the 

 heated continent, and thunder-showers finally begin upon the mountains. Far to the south 

 in Mexico the first showers may come in May or even April. In southern Arizona they 

 usually begin, as we have seen, toward the end of June or early in July, while farther north 



* D. T. MacDougal: Botanical Features of North American Deserts. Carn. Inst. Wash. Pub. 99, 1908. 



