Clearing Away the Fog

It’s gloomy June in Southern California again, rarely an opportune time for out-of-state guests or natives seeking the sun. A friend of ours who has lived here long enough to observe that June fogs are even more faithful than spectacular weather for the Rose Bowl finally asked why.

Dr. Jerome Namias of the Scripps Institute of Oceanography in La Jolla explained graciously. In the colder times of the year, he said, Southern California gets the “good rains, the storms that bang into the coast.” These winter storm fronts move through quickly and frequently and clear out any buildup of moist air. But when spring comes, the storm fronts are weaker in Southern California and the centers of the fronts migrate north, as Seattle residents well know.

Meantime, the ocean is starting to warm up, but very slowly. The air masses moving eastward and northward over the ocean toward the coast are warmer. That creates condensation, because the moist air is being chilled from below. This fog, a low cloud that is only a few hundred feet above ground, is called a stratus regime, and there isn’t the vigorous air movement to clear it away that there is in winter.

High temperatures in the desert contribute to fog conditions on the coast, Namias added. When the temperatures soar in Palm Springs, the hot air rises. That encourages the cooler marine air to move inland.

By July and August the water is warmer, so this stratus regime doesn’t occur as frequently--except occasionally after a Santa Ana condition. The winds then carry hot air from the mountains westward out over the ocean. A few days later it often comes back as fog, Namias said. Still, it is the transition time between winter, when the ocean water is cold, and mid-summer, when it isn’t, that produces so much June fog.

There are those who attribute the fog to what’s known as the Catalina eddy--a condition in which the wind spins counterclockwise in a limited area, perhaps after contact with the coastal mountain range. This does have an effect, Namias said, but he added that it’s not large enough to create the fogs that dominate the early mornings all along the coast in June.

And why doesn’t the East Coast have the same condition in June? Simple. The air moves from west to east generally, and it’s hard to pick up much moisture over the plains. It’s all too easy over the Pacific.