Published Tuesday, January 15, 2002 in the San Jose Mercury News


Special to the Mercury News

Around the first of the year, every meteorologist knows the question that will be asked most often: ``I got a new barometer for Christmas. How do I set it? And why is it raining when the little needle-thingy is pointing to FAIR?''

First, what does a barometer tell us? It is a simple device that gives us the atmospheric pressure, which is essentially the weight of the column of air above us, and can indicate weather changes such as a pending rainstorm. When the column is heavier we have ``high pressure,'' and when it's lighter we call it ``low pressure.''

These differences are the result of more air being added or subtracted from the column, the temperature of the air in the column changing or a combination of both. The pressure also changes as you go up into the column because there is less air above you pressing downward. For example, at 5,000 feet the pressure is only 85 percent of sea level.

Barometric pressure in English units is measured in inches of mercury, or Hg, while in the metric system the units are in millibars, or mb. The average surface pressure is 1,013 millibars, 29.92 Hg, and the range here in the Bay Area is from about 980 to 1,037 millibars. To convert from inches to millibars, multiply by 33.865; and from millibars to inches, multiply by 0.02953. The atmospheric pressure can be obtained for many Bay Area locations from

The actual reading is whether the pressure is rising or falling. Called the ``trend,'' it is important in telling us if the pressure is going toward FAIR or RAIN -- but only as a general guide.

I'm wondering about the colorful effect in the clouds at dawn. The times the clouds have changed this past year have been unpredictable, and each time colors were different. Does this have to do with the way the sun is angled?  Darlynn Fong - San Francisco

With the high frequency of weather systems that we have seen in the Bay Area over the last few months, there have been a number of spectacular sunrises and sunsets. These usually occur when there is a layer of middle clouds -- such as altostratus or altocumulus -- about 5,000 and 20,000 feet in altitude that does not obscure the horizon and sun. When this happens, the red, orange and yellow colors at sunrise or sunset are reflected off the bases of the clouds, making a spectacular display.

My husband and I are considering retiring to Kernville in Kern County. The town is east of Bakersfield, at an elevation of approximately 3,000 feet. I found average temperatures and rainfall, but I'd like to know how windy it is. Deb Henz - Felton

Kernville has an annual average wind speed of 7.8 mph. Compare this to an annual average of 10.1 mph at San Francisco Airport. In Kern County, stronger winds are found in Tehachapi Pass, where there are many wind farms and the pass funnels the wind.

 I recently saw your list of notable weather events at Would you not include the storms and floods of 1997 and 1998? I remember when San Francisquito Creek overflowed into Palo Alto as well as the torrential rainfalls. Kevin Collins - Palo Alto

When I put together the list, I did consider the 1997-1998 winter rains during El Niņo. However, despite a record number days of rain, there was no significant flooding and actual property damage statewide was relatively limited. You might be interested in some research I have done about the relationship of El Niņo to California flooding 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 or e-mail them to Please indicate in your e-mail what city you live in.