Published Tuesday, December 17, 2002 in the San Jose Mercury News

Weather Corner
El Niņo has a weather-making partner: Pacific decadal oscillation
By Jan Null
Special to the Mercury News


Yes, there is an El Niņo, and the waters of the tropical Pacific have warmed to the point that the current weather phenomenon is being labeled a ``moderate'' event. Although El Niņo has become a media darling, getting almost as much hype as a J. Lo sighting, there are many other things going on that are affecting our weather. One of these is the Pacific decadal oscillation.

The Pacific decadal oscillation (PDO) is a term first coined by a fisheries scientist, Steven Hare, at the University of Washington in 1996. While researching the relationship between the Alaskan salmon population and the Pacific climate, he discovered that there were long-lived El Niņo/La Niņa-type warm and cool periods in the north Pacific that last for 20 to 30 years. Typical El Niņos and La Niņas last from six to 18 months.

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.

The primary effects of the ocean temperature differences due to PDO and its effects upon the atmosphere are in the north Pacific and North America, while the primary effects of El Niņo are in the tropics, with secondary impacts elsewhere. The strongest PDO impacts are in the Pacific Northwest, where warm-phase PDO periods are associated with below-average rainfall and above-average temperatures.

During the cool phase of the PDO, there are typically cooler-than-average temperatures and above-average rainfall. Farther away from the Pacific Northwest, the impacts are less distinct.

But PDO and El Niņo are just two of the irregular ocean-atmosphere cycles that can influence any season's weather. For more information on PDO and other types of cycles, see

Q  Several months ago, you listed a Web site in your column that I loved and used daily. It showed the seven-day forecast for any city by typing in the ZIP code or city. It also had the current satellite image. Then I purchased a new computer and lost the Web address. Leon Avrech - San Jose

A The forecast site you are referring to is the IPS Meteostar site ( In addition to seven-day forecasts, the IPS site has a plethora of climate, satellite and international weather data available. There is a shortcut to these seven-day forecasts on my links-page at

Q  What's the explanation for ``white frost'' forming on things such as auto tops when the air temperature is still 3 or more degrees above freezing?Bob Hochmuth - San Jose

A There are a couple of reasons why frost forms when the air temperature is greater than 32 degrees. One is that most air temperatures are taken about five feet above the ground, and the temperature at ground level is usually 3 to 4 degrees cooler. Additionally, solid objects generally cool off faster than the air, and thus these objects can be below freezing while the air temperature is still above 32. Thus, when moisture in the air condenses on these objects, it is in its frozen form as frost.

Q  I wanted to know what the humidity was inside my house, so I purchased an electronic temperature/ humidity gadget. It also reads the temperature and humidity outside from a remote sensor. I was led to believe the humidity inside a typical house in winter (or what passes for winter in California) was 20-30 percent with the heater running off and on all day. I'm getting readings of 50-60 percent. Could these be correct? We have a forced-air furnace (without a humidifier) and double-paned windows. Also, I've been getting outside readings of 38-80 percent the last few days depending on where I place the sensor. Could the humidity vary that much? Also, 80 percent seems high. What is the average or normal humidity in San Jose (in the area near San Jose City College)? Are these readings published anywhere in print or on the Web? George Brown - San Jose

A  Fifty to 60 percent does seems pretty high for an inside humidity. I would expect readings closer to 25 or 35 percent. You may need to calibrate your humidity sensor. The best way to do this is in a steamy bathroom where the humidity would be 100 percent.  The outside humidity can easily range from 40 to 80 percent, or even 100 percent during the course of a cool damp winter day. You can get real-time hourly readings at:

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