The Nature of Things

Weather is nature's

great equalizer

Cool wet winters, hot dry summers: It's a pattern of weather seen frequently in coastal Southern California. Although mild compared to many other climates in the world, this weather pattern has a profound effect on the character of the landscape. In fact, the weather is responsible for many of the adaptations and facets of the plant and animal life that give the coastal mountains their personality.

Much of California is characterized as a Mediterranean climate. Several integrated factors- latitude, a cold ocean and a large high pressure air mass- contribute to the hot dry summers and wet cool winters. Looking at a map, one can see that these factors all come together at 30 to 40 degrees latitude on either side of the equator. The Mediterranean basin, central Chile, the southern tip of Africa and some parts of Australia share this weather pattern. In total, the Mediterranean climate covers less than 3 percent of the earth's surface.

Photo Courtesy of David C. Needham

The large air mass that controls California's climate is called the Pacific High. This high pressure air mass extends from the central Pacific Ocean and covers California. Areas lying beneath this air mass are protected from storms, resulting in dry summers and falls. As the year progresses and the sun moves southward in the fall and winter, the Pacific High air mass moves with the sun and releases its hold. Storms that were deflected from the north and east are now free to move in and shed their water. The Pacific High exerts more influence on Southern California than Northern California, causing longer drier summers in the south and heavier rainy seasons in the north.

Even with this general weather pattern, rainfall and drought are unpredictable in Southern California. While it is foreseeable that rain will fall between the months of October and May, the amount of rainfall varies significantly year to year, ranging from under 5 inches one year and over 20 inches the following. Also unpredictable are the severity and frequency of the storms. One storm may drop a third of the total rainfall for the year while the following storm barely wets the ground.

Beyond the inconsistent rainfall, drought is also unpredictable. The severity and duration of summer drought is erratic from year to year. Looking at data collected over a 20year period, it's not uncommon to see many months of undetectable moisture, including one year with no precipitation at all. In addition to month-to-month dryness, several years of drought is not uncommon.

Wind and ocean currents originating in the equatorial Pacific Ocean play an enormous part in the weather patterns of the world, creating significant variability from year to year. While generalizations can be made about California's Mediterranean climate, it is difficult to characterize the weather as "normal" or not.

Also contributing to the character of our climate are the prevailing winds. Two main wind patterns are the westerlies and winds from the east, often called the Santa Anas. The westerlies, which occur during every season, carry moisture from over the ocean inland. This moisture condenses in the form of fog, dew or rain. In addition, the westerlies help create a temperate climate, moderating extreme temperatures.

Opposing the westerlies are the winds from the east. Rather than bringing in cool air and moisture from the ocean, the eastern winds blow in hot, dry air. As the Pacific High air mass moves southward in fall, a complex dance among several different pressure systems pushes the westerlies back over the Pacific Ocean, creating a sort of air pressure vacuum over California.

This vacuum allows air to flow and tumble down from the Great Basin Desert to the coast, compressing, heating and drying the air as it flows. As it moves over California through the mountains, the air is constricted and increases in velocity. These hot, dry blowing winds are the Santa Anas, their name derived from the mountains from which they flow.

Amazingly, converging with all of these weather patterns is a perfectly adapted plant community, characterized by the external forces of climate. To survive the extremes of the Mediterranean climate, the chaparral is dominated by evergreen, drought-resistant, sclerophyllous (hard-leaved) plants. Also in response to this unusual climate, many chaparral plants grow profusely from late winter through early summer during heaviest rainfall. This contrasts with other regions where summer is the season of greatest growth.

Seeing how outside forces significantly affect the character of the chaparral plants, I wonder what forces are working on my own character. I'll check the forecast.