64 CLIMATIC CYCLES AND TREE-GROWTH 



burnt stump is a common sight. Stumps were examined in different 

 places and ring samples were collected at Victoria, British Columbia, 

 at Blyn, Washington, and at Toledo, on the Oregon coast, but the 

 growth was so exceedingly complacent that no special effort was made 

 to form a group. However, there is no real doubt that group char- 

 acters will show, if the right tree and location are found. 



WESTERN CONTOURS AND RAINFALL 

 The important mountain ranges of the western States extend in 

 north-and-south rows, whose western slopes precipitate moisture from 

 the westerly winds. The long valley running north from the Gulf of 

 California, with the smaller parallel San Joaquin Valley in central 

 California, is the driest area, because the westerly winds are drying 

 winds as they descend into them. 



Mechanism of Arizona summer rains — The maximum rainfall on 

 the coast is in winter, but the maximum in the northern parts of the 

 dry valleys just mentioned is in late spring, when their warming 

 causes the air to rise and move to the east and "pull" in the wester- 

 lies. In midsummer it is so hot that the moisture is reabsorbed even 

 before it falls and the amount that reaches the ground is small. The 

 same summer "pull" draws moisture-laden air from above the Gulf 

 of California far to the south (whose water temperature at Port 

 Libertad in September 1923 was 87° F.), and perhaps from other 

 warm bodies of water. This air, as it is drawn up over the mountains 

 and plateaus in its northward-moving path, gives up its moisture in the 

 common torrential summer rains of that region, strongest near the 

 Gulf and fading out in Utah. 



Prediction possibility — If this statement of the possible mechanism 

 of our summer rains is correct, it would seem possible to predict their 

 amount, some months at least beforehand, by some formula involving 

 chiefly the mean temperature of the water in the Gulf of California 

 and of the desert areas of the western valleys. 



The Rocky Mountains — The Rockies are high enough to catch the 

 westerlies and intercept a remnant of their moisture, and thus they 

 partake year by year to some degree in the winter variations which 

 come to the Pacific Coast. But in the warmer months the mechanism 

 just referred to as acting north from the Gulf of California produces 

 a similar effect north from the Gulf of Mexico, and the eastern Rockies 

 show a great summer maximum. 



THE THREE ZONES 

 Thus, in reference to climatic types, there are three zones lying in 

 north-and-south strips delineated by the mountain ranges. On the west 

 is the Pacific or Coast zone, where the precipitation is only in winter, 

 from the westerly winds coming in off the ocean. The arid interior 

 region forms the Arizona zone, whose higher points where the pines 



