Published Tuesday, July 16,  2002 in the San Jose Mercury News

No drought yet despite records for low rainfall

Special to the Mercury News

It didn't rain much in the past year, but are parts of California really in a drought?

The 2001-2002 rainfall season just ended, and most of Southern California had less than a third of normal rainfall. A number of cities went through their driest season on record.

Between July 1, 2001, and June 30 this year, Los Angeles had 4.42 inches of rain -- only 29 percent of normal and the driest season since records began in 1877. San Diego got just 3.02 inches or 34 percent of normal, the driest since the city's records began in 1850. Long Beach was even drier with 1.89 inches, a meager 15 percent of normal. Palm Springs was bone-dry with 0.41 inches for the entire season. That's just 8 percent of normal.

Even with such minimal rainfall, there is no widespread drought in Southern California. How come? This is the perfect case to show that it takes more than lack of rainfall to make a drought.

The American Meteorological Society's ``Glossary of Meteorology'' defines drought as a ``period of abnormally dry weather sufficiently long enough to cause a serious hydrological imbalance.'' Consequently, if there is little impact on the water supply to the general populace, industry or agriculture, low rainfall doesn't necessarily equate to drought.

This is certainly the case in Southern California, because much of the water used in the region's generally arid climate is imported; it does not fall from the sky. Typically 60 to 80 percent of Southern California's water is imported from the Colorado River and from Northern California. The rest comes from groundwater below much of the coastal plain. Some mountain areas of Southern California are ``fractured rock,'' which doesn't hold groundwater, so all water for those regions has to be imported.

However, while most of the region isn't affected by what is defined as a drought, there are some people who are severely affected by the lack of water. Among them are grassland ranchers who rely entirely on rainfall, not irrigation, and firefighters who are facing tinder-dry wildlands just waiting for a spark.

Q When a long-range forecast predicts above-average precipitation, does it mean an above-average number of storms with varying amounts of precipitation, or above-average amounts of total rainfall? Dan Carlentine  - San Jose

A Would you believe, none of the above. The long-range 30- and 90-day forecasts from the National Weather Service show the probability of above- or below-average temperatures or rain. They begin with the assumption that there are equal chances -- 33 percent -- of normal, above-normal and below-normal conditions.

For example, if the 90-day temperature forecast map shows an ``A'' surrounded by a contour line marked ``20,'' the chance of above-normal temperatures is 20 percent greater than 33 percent, or a 53 percent chance. This methodology does not address the amount above normal, or in the case of rain, the number of rainy days. For 30- and 90-day forecasts, see

Q The recent forecast was for high pressure with hot, dry weather. Can you explain the association between high pressure and hot, dry weather? Hot, dry air is lighter, so it seems this weather should accompany low pressure readings. Curtis Panasuk -San Carlos

A There seems to be a great deal of misunderstanding about the relationship between high or low pressure and the resulting weather. The biggest source of this confusion is television weathercasters who don't tell you whether they are referring to high pressure near the surface of Earth or high pressure in the upper atmosphere.

The forecast you are referring to was made when there was a large area of high pressure in the upper atmosphere covering much of the western United States. High pressure aloft is associated with sinking air that warms by compression as it gets closer to the ground. This sinking air also results in mostly clear skies.

Conversely, if there is low pressure in the upper atmosphere, there is generally rising air that can lead to clouds and sometimes precipitation.

High pressure at the surface is usually associated with cooler air, while a nearby area with warm rising air results in lower barometric pressure. This causes our usual sea breeze. The waters of the Pacific cool the air mass directly above, and the resulting heavier air means higher pressures. As the land warms up during our long summer days, the air directly above warms and rises, creating lower pressure. Since nature abhors anything resembling a vacuum, the cool air from the high pressure offshore rushes inland toward the lower pressures, and we have a sea breeze.

Q What causes the gaps in ``stacked lenticular'' clouds? I understand how lenticular clouds form at the top of a standing wave. But what causes the clear zones between them? Why not just one big lenticular cloud? I guess this may be a bit esoteric for the Weather Corner, but I've wondered for years and never found anyone who knows.  Art Botterell - Fairfield

A Nothing is too esoteric for the Weather Corner. First, the basics: Lenticular clouds form over or in the lee of a mountain as the wind lifts moist air high enough for condensation to occur. The cloud that forms is shaped like a lens with a fairly flat bottom and a smooth rounded top that follows the contour of the wind as it flows over the mountain in the form of a wave.

There is sometimes stratified moisture in the air that can cause several lenticular clouds to form, one on top of another in a stack. There is very little if any gap between the layers, and the result is what some describe as a pagoda. Check out

Q How many gallons of water falls from 0.01 inches of rain in a 1-square-kilometer area?  Ted Schlaepfer - Oakland

A There are about 1.55 billion square inches in a square kilometer. A layer of water one-hundredth of an inch deep over that area would equal approximately 15.5 million cubic inches. Because one cubic inch is about 0.004 gallons, the total for one-hundredth of an inch of water over a square kilometer would be just slightly more than 67,000 gallons.

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 Please indicate in your e-mail what city you live in.