It has been more foggy than not along the beaches for the past week or so. Blame the recurring Southern California weather phenomenon known as the Catalina eddy, shown here as a swirling, circular cloud pattern off the coast of San Diego. An explanation from NASA's Earth Observatory website.
The pattern is known to meteorologists as a Catalina eddy, or coastal eddy, and it forms as upper-level flows interact with the rugged coastline and islands off of Southern California. The interaction of high-pressure—bringing offshore winds blowing out of the north—and low-pressure—driving coastal winds blowing out of the south—combine with the topography to give the marine stratus clouds a cyclonic, counter-clockwise spin. The eddy is named for Santa Catalina Island, one of the Channel Islands offshore between Los Angeles and San Diego.
Catalina eddies can bring cooler weather, fog, and better air quality into Southern California as they push the marine boundary layer further inland. These mesoscale eddy patterns can stretch across 100 to 200 kilometers (60 to 120 miles) and can last from a few hours to a few days. They most often form between April and October, peaking in June.