dependent on the amount and spacing of rainfall for the parti- 

 cular year. 



Chief among the meteorological factors influencing the 

 southern California climate is the prevalence in the vicinity 

 throughout most of the year of the eastern lobe of the Pacific 

 High. When it is strong and lies close to the shore, clear 

 skies and mild weather previal since it causes storms from 

 the North Pacific to be turned eastward before reaching this 

 latitude. As the strength of the high weakens, or if it 

 moves to the south or west, cold fronts may sweep in from the 

 northwest. During the summer when the high pressure belt has 

 moved northward, these rarely arrive. If they do, they are 

 quite weak. During the winter, however, the center of the 

 high is usually about a third of the distance between Hawaii 

 and the Aleutians. A col in the high pressure belt is 

 located over the eastern margin of the Pacific which results 

 in a strong northwesterly flow, and the southern parts of 

 cyclones are able to reach our coasts bringing fairly strong 

 winds and rain. 



This northwesterly flow is apt to be as strong in summer 

 as in winter, but in the summer the trajectory of an air 

 particle would take it back to a position nearer the center 

 of the high pressure cell. Even though northwesterly winds 

 prevail^jtherefore, they do not originate in the North Pacific 

 and accordingly do not bring the cold fronts to the coast. 

 In the summer, the intensity of the northwesterlies may be 

 increased by the development of a deep thermal low over the 

 arid regions of the southwestern United States. It is not 



