Published Tuesday, October 21, 2003 in the San Jose Mercury News

Weather Corner

Fall weather pushes along color display

Special to the Mercury News

The weather is beginning to change from the long, warm days of summer to the longer, cooler nights of fall and winter. It's also time for the leaves on many trees to change colors and eventually fall. This process is already well under way in New England, the Rockies and the High Sierra, and any time now we will begin to see these changes at lower elevations of California.

The leaves on a tree are where most of the food production necessary for a tree's growth in the spring and summer months takes place. This occurs in leaf cells that contain chlorophyll, which, when combined with energy from sunlight, causes carbon dioxide and water to be converted into carbohydrates such as starch and sugar that a tree uses for fuel. The chlorophyll is what gives leaves their green color. Leaves also have yellow and orange pigments, but during most of the year these colors are masked by the greater amounts of green pigment.

In the autumn with fewer hours of sunlight and changes in the temperatures, leaves cease their food-making processes. This causes the chlorophyll to break down, resulting in the green colors fading while the yellows and oranges become prominent for the colorful look of fall.

The temperature, amount of light and water supply all affect the intensity and length of the fall colors. If minimum temperatures remain above freezing, the formation of the chemical anthocyanin, which produces brighter reds in maples, is favored. An early frost will diminish the red's intensity. Rainy or overcast days tend to increase the intensity of fall colors.

Other changes are taking place to trees in the fall. Near the end of the leaf stem, a layer of cells form and slowly break the tissues that support the leaf, which eventually drops to the ground. This process is sped along by colder weather and windy weather or storms.

Visit to see the status of this year's fall foliage in California.

Q Why are the temperature differentials between day and night so extreme this time of year? Is fall the time of year with the largest differential? David Abbott - Santa Rosa

A Late summer has the largest differences in the Bay Area, when the days are hot but there is cooling from the seabreeze at night. For example, in Santa Rosa the difference between the average maximum and average minimum is 29 degrees for July, August and September. Similarly, July has the largest difference for San Jose at 27 degrees, with June and August close behind at 26 degrees. These lessen into the winter with both Santa Rosa and San Jose having an average difference of just 18 degrees in January.

Q What's the current status of the PDO (Pacific Decadal Oscillation)? I read your article some months ago in which you reported that the PDO is shifting to the negative phase, which might mean the climate here in California will become cooler. Richard deSousa - Fremont

A First off, here's a brief refresher on the PDO. This describes long-lived El Niņo/La Niņa-type warm and cool periods in the North Pacific that last for 20 to 30 years. PDO comes in two flavors, a positive phase and a negative phase. During a positive PDO, the waters in the central North Pacific are cool, and the waters along the west coast of North America are warm. The converse is true with the negative phase. During the past century, PDO was in its negative phase from 1890 to 1924, 1947 to 1976 and 1998 to the present. Positive phases ran from 1925 to 1946 and again from 1977 to 1997.

Right now, the phasing of PDO appears to be still in transition with several positive phase months after a period of neutral to negative months. What I had written was that in the Pacific Northwest, there are typically cooler-than-average temperatures and above-average rainfall during the cool phase. Farther away from the Pacific Northwest, the effects are less distinct. Consequently the climatic signal from PDO is not strong enough to have a discernible impact on California's weather.

Q I noticed at the time of the most recent full moon that there were different dates posted for that on two different calendars we have. One, sent by the National Parks Conservation Association, gave the date as Oct. 10. The other, from the National Wildlife Federation, gave the date as Oct. 8. I'm curious as to why the difference. Mary Peterson -  San Jose

A Obviously there can only be one right answer, and the events such as a full moon or equinox are calculated down to the second. The problem with date inconsistencies occurs when the event is on one side of midnight in one time zone and the other side in another time zone, and is further compounded by daylight-saving time. And most of the tables used are calculated in Greenwich Mean Time. So there are lots of ways a calendar-maker can get confused, but the shift should never be more than one day and, in this case, the NPCA calendar was correct.

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, or fill out a form online at Please indicate in your e-mail what city you live in.