Published Tuesday, January 8, 2002 in the San Jose Mercury News


Special to the Mercury News

Normal Averages of Weather Data Climbing Higher

Every 10 years, meteorologists get out their calculators to figure what's ``normal.'' That's because the International Meteorological Conference at Warsaw in 1935 sought to standardize the way that meteorological parameters are averaged.

Normal temperatures, rainfall and other weather data are calculated for the previous 30-year period at the end of each decade. This was just done for the 1971-2000 period.

While the recalculation is usually a ho-hum bit of number-crunching that is paid little notice, there are some interesting changes from the 1961-1990 period in the Bay Area.

Annual normal rainfall changed the most, rising 4 percent to 9 percent. In San Jose, the average increased from 14.42 inches to 15.08 inches, boosted by the last 10 years in which it rained an average 16.88 inches.

San Francisco's normal rose from 20.52 inches to 22.28 inches, again helped by the last 10 very wet years that averaged 25.96 inches. In both cities, the averages were influenced most by wet Februarys -- up 33 percent in San Francisco and 31 percent in San Jose.

Evaluating changes in normal temperatures is trickier because they are more easily biased by changes in instrumentation and location.

For example, the normal annual high temperature increased 2.4 degrees in downtown San Francisco, while San Francisco International Airport's average temperature was relatively unchanged, dropping 0.1 degree.

Most likely the downtown temperature chanced significantly because the weather station was moved from Civic Center Plaza to Mission Dolores in 1983 and to Duboce Park in 1987.

This is a common problem in many urban areas where the proximity of buildings, concrete and asphalt can artificially inflate temperatures. It shows why great care must be taken when examining meteorological data in regards to issues such as global warming.

For a comparison of the old and new normals in seven key California cities, see

Q During the big thunderstorm in September, I noticed an unusual phenomenon several times. At the exact INSTANT of a lightning flash, I would hear what's best described as a sharp ``fzzzt'' sound, followed several seconds later by the regular peal of thunder. What is this ``fzzzt'' sound? Also, given that light travels much faster than sound, how is it possible for this sound and a lightning flash to occur simultaneously? Rick Helley - San Jose

A Because what you heard occurred simultaneously with the lightning, it must be in some way electrical. Without knowing specifics, it comes to mind that you might have heard static from a nearby radio. Lightning discharges produce radio waves called sferics. These waves are the basis for lightning detection networks that can pinpoint the location of a lightning strike by triangulation.

Q While the days of the winter and summer solstices are respectively the shortest and the longest of the year, they are not the days with either the latest or earliest sunrises or sunsets.

In the Bay Area, the earliest sunset is in early December, about two weeks before the winter solstice, while the latest sunrise is in early January, about two weeks after the winter solstice. This discrepancy seems to grow larger farther south, and is less as you go north. Why? Lars Ostberg -
Los Altos

A Two separate phenomena account for this discrepancy. One is called the equation of time, and the other is the sun's declination.

The equation of time mathematically describes variations in the times of sunrise and sunset at different locations within each time zone. The sun's declination is its height above the horizon at any given time.

During the weeks preceding the solstice, the two phenomena act in opposite ways on the time of sunset. The effects of declination make sunset earlier, while the offsetting equation of time makes it later. On Dec. 8, the equation of time becomes dominant, and the sunset becomes later each day.

At the same time, both phenomena are pushing the time of sunrise later. This situation is reversed after the solstice, when both push sunset later.

Sound complicated? Actually, that's the short answer. For a detailed explanation from the United States Naval Observatory, see

Q Many newspapers publish a composite weather map that shows highs, lows, fronts, isobars, precipitation and sometimes temperatures and other information at selected locations. Such a weather map also appears on the home page at However, I have not found composite weather maps at the National Weather Service Web sites. There are plenty of single-purpose maps for temperatures, barometric pressure, precipitation and so forth, but none that show everything together. Nor have I found a map that shows the fronts. Does the weather service publish such maps, or is this done only by commercial weather services? Ed Taft -Mountain View

There are some nice maps from the National Weather Service that show highs, lows, fronts and areas of precipitation. These can be found online at Numerous types of maps are available, including some with satellite images overlaid. Note that the times of the maps are in Z or Zulu time, the same as Greenwich Mean Time, or GMT. This is eight hours ahead of Pacific time. For example, 2100Z is the same as 9 p.m. GMT or 1 p.m. Pacific time.

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.