Return of the Santa Anas
Santa Anas, Hot and Smoggy Days, Zodiacal Light
Late September signals the beginning of Santa Ana winds, which may continue intermittently through the winter. Early Santa Anas are often responsible for coastal San Diego's hottest and driest days of the year. Records show that 62 percent of the days at Lindbergh Field with 90-degree readings or higher have occurred during the months of September and October. The desert-like days are a consequence of dry air from a high-pressure area centered over Nevada or Utah moving across southern California's coastal mountains toward a low pressure area offshore in the Pacific Ocean. West of the mountains the air sinks as it rushes toward the coast, heating (by compression) about 5-1/2 degrees F for every 1000 feet of descent. During a full-blown Santa Ana, gale-force winds rake the foothills downwind of the mountain passes, and San Diego's coastal communities often experience temperatures that are among the nation's highest.
Escaping the heat of this season's first couple of Santa Anas will not be easy — if you can't be inside air-conditioned buildings at any rate. Along the coastline, where temperatures may reach the 90s, taking a dip in the surf solves the problem. Inland, the furnace-like heat is moderated only by increases in altitude. Head for the high places of the county-- the mile-plus heights of the Palomar and Laguna Mountains, for example. You could climb a couple of the county's loftiest peaks: Cuyamaca Peak (6512') in Cuyamaca Rancho State Park, or Boucher Hill (5438') in Palomar Mountain State Park. On peaks such as these during a Santa-Ana-wind episode, the thermometer registers about 25-30 degrees lower than in the lowlands. Since the early phase of a Santa Ana sweeps away atmospheric water vapor and air pollution, hundred-mile vistas in every direction may be yours to enjoy.
The smoggiest days of the year are most likely to occur during the next several weeks. Persistent temperature inversions (warmer air overlying cool marine air) are frequent this time of year. These inversions conspire with the mountainous topography of East County to trap locally generated air pollution under a low-lying lid. During the worst episodes, San Diego County's coastal area is the hapless recipient of additional smog sneaking down from the Los Angeles Basin. This often occurs when a Santa Ana wind condition begins to weaken: L.A. smog blown offshore by a Santa Ana may get pushed back our way when the normal sea breeze returns.
The zodiacal light, which can appear as a diffuse, pillar-shaped glow in the eastern sky for up to an hour before dawn, will be quite conspicuous at dark locations this weekend and through the next two weeks (September 27 through October 10). The subtle glow is caused by the sun's light reflecting off dust particles floating in space in the plane of our solar system. In the sky, this reflected light appears superimposed along the string of constellations we know as the zodiac, and especially those zodiacal constellations closest to the direction of the sun. An old name for this phenomenon is "false dawn" -- a good description of the effect under clear and very dark skies.