Because of the prevailing wind pattern in this area, the result of the 

 semi-permanent high over the Pacific, and of the thermal low over the 

 Sonoran Desert region, the deflecting force due to terrestrial rotation 

 north of about 25° lat. is augmented by the winds, so that many tropical 

 cyclones are deflected ashore here. 



Hurricanes, and similar disturbances, which travel directly up the 

 Gulf of California usually, but not invariably, vanish within the Sonoran 

 Low (Fig. 3), playing out in that area as a series of violent squalls, pro- 

 ducing excessive localized rainfall. 



In consequence of the wind and current pattern at sea, most hurri- 

 canes in this area either dissipate, or are driven inland, south of latitude 

 30° N., and none have travelled as far north as 35° during the last half- 

 century. This is in marked contrast to conditions in the eastern part 

 of the United States, where hurricanes not infrequently travel, with 

 strength practically unabated, at least as far north as Boston, in latitude 

 42° 22' N. On the eastern coast of North America the Gulf Stream 

 causes a northward extension of the environment favourable to huiTicanes 

 for a distance of more than 500 miles relative to that on the west coast. 



Hurricanes on Land 



Most hurricanes which run ashore from Pacific waters on to the 

 lands of North America either dissipate, or undergo extensive modi- 

 fications, within a distance of from 25 to 300 miles. 



A typical west-coast hurricane, on running ashore, produces violent 

 wind squalls, accompanied by intense rainfall, over a small area only. 

 Winds strong enough to carry away the instruments, and to twist radio 

 towers into pretzel-like shapes, are common under such conditions, and 

 rainfalls of as much as 5 ins. in two and a half hours have been recorded. 

 The area affected, however, is usually small, having a diameter of less 

 than 50 miles near the sea, and usually of less than 10 miles 50 miles 

 inland. Because of the aridity and barrenness of much of this area, 

 even a minor hurricane produces serious local floods (10). 



Near the head of the Gulf of California many small tropical cyclones 

 are preceded by desiccating fallwinds on the east side of the Peninsular 

 Ranges (Sierra de San Pedro Martyr, Sierra de la Giganta, &c.), in which 

 enormous dust-devils, or miniature tornados, are not uncommon. These 

 minor vortices, on passing from land to water, commonly change to 

 waterspouts. 



After travelling for some to many miles over land, tropical cyclones 

 normally diminish in size and intensity, so that they are not easily 

 recognized as hurricane residuals. In many instances they break up 

 into several vortical disturbances, which follow terrain-determined 

 trajectories, usually valleys for many miles. 



In some instances these hurricane residuals have been traced as small 

 cyclones or as families of squalls, for more than 1,000 miles. In 

 general, but not invariably, the limit of travel of these disturbances is 

 the boundary of the air mass, usually unmodified or slightty modified 

 Tropical Maritime air, first entered by the hurricane. When these 

 disturbances reach a front, they " ride " up the frontal slope, producing 

 local " cloudbursts." Some of the cloudbursts recorded from such 

 distant points as El Paso, Texas, and Salt Lake City, Utah, are apparently 

 caused by hurricane residuals. 



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