88 ALLAN HANCOCK PACIFIC EXPEDITIONS VOL. 13 



offer a fascinating invitation to the student of plant evolution and 

 geography and their solution would contribute to the great latent story 

 of the California Gulf Region. 



Climate 



The California Gulf Region in spite of its large water surface, is 

 extremely arid around its upper portion. In so far as the continental air 

 circulation dominates, its climate is continental in type. Theoretically, 

 the prevailing air currents are those of (1) the western Pacific anti- 

 cyclone, the maritime character of which is eliminated by its passage 

 over the west coast land surface northwest of the gulf region; (2) the 

 continental anticyclone, dry, and with marked annual and daily tem- 

 perature extremes; (3) the mild Pacific westerlies prevailing through 

 the spring as on-shore breezes are consistently dry because of the warmer 

 land surface. The conspicuous fog desert along the outer coast of the 

 peninsula is lacking in the gulf region proper. Near the mouth of the 

 gulf epiphytes occur locally on the mainland, as at Cerro Tecomate. 

 (4) A maritime air current appears to swing in from the near south 

 Pacific as a western limb of the Caribbean anticyclone. In the fall, 

 storms develop along this track (Schiaffino, 1939) and may reach into 

 the gulf. 



However, unstable humid air masses are of two sources and two 

 seasons; the winter rains from the northwest Pacific storms which are 

 uncertain and do not fall every year; the summer rains of the tropical 

 convections, which occur regularly in the southern portion of the region, 

 but lighten materially in the interior of the gulf. Averages of yearly 

 rainfalls, as far as regional records are available and vegetative growth 

 indicate, range from 20 to 30 inches for the mountains on either side 

 of the mouth of the gulf to about 3 inches for the lowlands in the upper 

 portion of the gulf. Data are carried in the accompanying table (Table 

 5). Average winter rainfall is similar for all stations except the high of 

 Alamos in southern Sonora, which is explained by the proximity of that 

 station to Sierra de Alamos (height 1800 m) and its obvious precipitative 

 effect on humid air masses. The same phenomenon is operative through- 

 out the region on other mountains having comparable mass and height. 



Average sum.mer rainfall generally decreases from south to north. 

 The average for the five southern stations (Muleje, La Paz, Guaymas, 

 Alamos, Topolobampo) is 9.5 inches, while for the northern stations of 

 Brawley, Lechuguilla, Tule Tank, Cirio Point, and Libertad, it is only 

 1.6 inches. This greater summer rainfall in the southern part of the 



