State's rainfall season ends with little snowpack, runoff

Saturday marked the end of the 2000-2001 rainfall season across California, and it certainly was a season of contrasts. Though most of the major urban areas in the state had near-normal rainfall, the snowpack and runoff from the mountains were definitely on the light side.

But, first, why is June 30 the end of the rainfall season? Why not Dec. 31?

California's predominantly Mediterranean climate is unique compared with the rest of the nation. While other areas have significant amounts of rain during the summer months, a distinguishing characteristic of the Mediterranean climate is its natural summertime drought. Consequently, a logical break is June 30 -- unless you are a hydrologist; then it is Sept. 30, although the difference in rainfall after June 30 is usually minor.

In the Bay Area, San Francisco and San Jose got 95 percent of their normal amount of rain this past season. Sacramento and Fresno had 93 and 100 percent, respectively. In the south, Los Angeles had 121 percent of normal rainfall and San Diego had 87 percent for the season. All of these are within the normal range, which is roughly between 85 and 115 percent of the 30-year average.

About 65 percent of the water consumed by California's residences, businesses and agriculture comes out of the Sierra snowpack, and nearly 25 percent of the state's electricity is generated by hydroelectric plants. And so, it is necessary to examine the Sierra Nevada precipitation.

This year, it was much different from precipitation in the lowlands. Overall, the precipitation in the Sierra was just 65 percent of normal, and the runoff a dismal 50 percent of normal.

Q Why is high pressure associated with ``good'' weather and low pressure associated with ``bad'' weather? Why not the other way around? You might have already addressed this, but it is such a fundamental concept that it might be useful to explain it again. Gary Cooper - Portola Valley

A It is a fundamental and very important concept that certainly bears repeating. In the northern hemisphere, air flows clockwise and outward from an area of high pressure and counterclockwise and toward the center of a low-pressure area. As the air converges near a center of low pressure, it rises and cools. This causes the moisture in the air to condense into tiny droplets that become clouds and sometimes rain. Conversely, air flowing away from an area of high pressure sinks, becomes drier and cloud-free, and warms.

Q What is it about the Pacific Ocean that creates high pressure that dominates late spring, summer and fall weather here? I grew up in Chicago, and the climate is much different away from an ocean. David - San Jose

A We take for granted our cool afternoon sea breeze that provides natural air conditioning in the summer, courtesy of the Pacific High. Most of the nation is much slower than the Pacific coast to cool off after a warm summer day. The Pacific High is one of a number of semi-permanent pressure systems that dominates weather maps. It is centered in the southern Gulf of Alaska and covers much of the northeast Pacific during the summer months. In the Atlantic, the Bermuda High dominates, and the flow from the south around its west side brings hot, humid weather to the eastern half of the country in the summer.

These pressure centers are created by the juxtaposition of the oceans, continents, mountains and ice fields. They form over relatively uniform surfaces such as the ocean where there are homogeneous temperature and humidity conditions. These systems move north and south with the seasons as the sun's position shifts in the sky. This is why the Pacific High is so dominant in the summer but has little effect in the winter. The Aleutian and Icelandic Lows become dominant in winter. They are the breeding grounds for many of the storms that hit the United States and Europe.

Jan Null, founder of Golden Gate Weather Services, is a retired lead forecaster with the National Weather Service. Send questions to him c/o WeatherCorner, San Jose Mercury News, 750 Ridder Park Drive, San Jose, Calif. 95190. You also can telephone questions at (510) 657-2246, fax them to (510) 315-3015 or e-mail them to weathercorner @ggweather.com. Please indicate in your e-mail what city you live in.