The typical northwest wind of summer is reinforced by the dynamics of the thermal low pressure area located over the Central Valley and the Southeastern Desert area.  In the San Francisco Bay area there is a marked diurnal pattern in the strength of the wind even though an onshore circulation tends to continue throughout the 24-hour period.  This helps to carry locally produced air pollution products away from the Bay area, but creates problems for the regions immediately south and east of the source area.

In the Los Angeles area, however, the Basin is almost completely enclosed by mountains on the north and east.  Coupled with this is a characteristic of the air along most of the coastal area of California.  The vertical temperature structure (inversion) tends to prevent vertical mixing of the air through more than a shallow layer (1,000 to 2,000 feet deep). The geographical configuration and the southerly location of the Basin permit a fairly regular daily reversal of wind direction—offshore at night and onshore during the day. With the concentrated population and industry, pollution products tend to accumulate and remain within this circulation pattern.

Another local characteristic of the northwest wind alongshore is the creation of a jet effect in the vicinity of some of the more prominent headlands.  The most outstanding of these currents of air is found off of and to the south of Pt. Arguello.  Here a strong jet of air is projected southward past San Miguel and San Nicholas Islands, driving a huge eddy as much as 200 miles in diameter.  The air swings eastward near San Diego then northward and westward along the coast to rejoin the southward flowing air at the west end of the Santa Barbara Channel.  Similar but smaller eddies form in the vicinity of the Golden Gate, just south of Pt. Reyes, and south of Monterey Bay around Pt. Sur.  Wind speeds in the immediate vicinity of these major headlands can be two or three times as great as the wind flow at nearby points.