Change in the Weather

Ask a Southern Californian, “How’s the weather?” and you’re likely to get a variety of responses. It might be blustery in the San Fernando Valley, sunny and warm at the coast, raining in the Santa Monica Mountains and snowing in the San Gabriels. All at the same time.

The reason for this is the existence of microclimates within the state’s larger climatic zones. Physical features such as mountains and oceans cause the barely perceptible changes in temperature and precipitation that characterize microclimates.

Weather forecasters are constantly challenged when it comes to predicting Southern California weather. “You can’t apply a broad-brush forecast to the entire state of California,” says WeatherData Inc. forecaster Jon Erdman, who calls Southern California a perfect example of a microclimate. “Because of the differing terrain, you see different climates on a much smaller scale than you normally see,” he said.

Valleyites are well aware that it’s usually hotter and smoggier here than the rest of the L.A. Basin. But what about the weather variations within the Valley? According to Woodland Hills resident William Reid, a weather observer for the National Weather Service, the valley floor rises from 600 feet to 1,000 feet in elevation from east to west. This altitude change is one just factor contributing to weather variations, such as more extreme temperatures in the West than the East Valley.

Valley Climate Fluctuations

Summer Conditions

The Valley is warmer because it is almost completely surrounded by mountains. Cool marine air, which tends to hug low-lying areas rather than travel over mountains, has few passes to flow through. The bulk of the marine air enters the Valley through the Cahuenga and Sepulveda passes and the Burbank-Glendale gap, weakening by the time it hits the West Valley, so the East Valley tends to have cooler temperatures.

Winter Conditions

Heavier cold air from the desert enters the Valley ad pools on the Valley floor on clear, dry nights. Upper-level warm air traps the cool pooled air, at times making Valley temperatures cooler than other areas in the Los Angeles Basin.

* Santa Ana winds tend to be stronger and more frequent in the north Valley which is closest to the winds easiest entry point,the Newhall Pass. Browns Canyon in the Santa Susana Mountains also funnel strong Santa Anas into Chatsworth and Northridge.

Precipitation

The Valley tends to be drier than surrounding foothills, which trap moisture from Pacific storms before getting to the Valley. Storms are forced to rise when they hit mountains. Air at lower levels rises, cools and condenses, forming precipitation, so it’s rainiest where air rises the highest.

Southern California Climate Zones

The Southland has a number of distinct climate zones. Changes in temperature, humidity and sunshine are usually caused by altitude and air flow from the ocean or inland sources.

(sage) Interior valleys with very little marine influence. Basins collect cold air and altitude creates colder temperatures in elevated areas.

(pink) Very little marine influence in these thermal belts where slopes are warmr in winter because colder heavier air drain off them.

(purple) Cold-air basin affected by cool marine are as well as warm interior air.

(lime) Thermal belt influenced by marine air and dry interior air, such as Santa Ana winds.

(yellow) Coastal climate where temperatures drop significantly near canyon floors in winter.

(orange) Canyon walls which are slightly warmer than floor below.

(green) Climate completely dominated by ocean. Winters are mild, summers cool, the air seldom dry. Seaside cliffs form barrier, trapping fog and moisture.

*

Note: Numbers on map indicate average annual precipitation in inches.

Sources: Sunset Western Garden Book; WeatherData, Inc.; William Reid, National Weather Service observer; Researched by JULIE SHEER/Los Angeles Times