WEATHER CORNER

BY JAN NULL
Special to the Mercury News

Q I know from the newspaper the high-tide or low-tide time and magnitude in San Francisco. Knowing that, how can I determine tides at another location, Monterey or Fort Bragg, for example? Doug Ryan - Santa Clara

AYes, there are specific relationships between key tide stations, such as San Francisco, and secondary stations such as Fort Bragg and Monterey.

To use these, you need to know the predicted high and low tides at the key or ``reference'' station. You then make an adjustment for the time and height at the secondary site. The height adjustment is either a plus/minus difference in feet or a ratio where the reference station height is multiplied by constant. For example, the high tide this morning at the Golden Gate Bridge is 4.7 feet at 10:42 a.m. The adjustment for Monterey is minus .5 feet and minus 1 hour 8 minutes. That translates to 4.2 feet at 9:34 a.m.

Printed tide tables are usually available at marinas, harbors or any place fishing equipment is sold. And of course the data is available on the Internet. For the San Francisco reference data, go to www.opsd.nos.noaa.gov/tides/westSF.html and for the local
adjustments, www.opsd.nos.noaa.gov/tab2wc1a.html#130 .

A really neat online tide calculator for anywhere in the United States is available at http://tbone.biol.sc.edu/tide/sitesel.html .

Q Why do some areas have extreme temperatures (really hot in summer and really cold in winter) but other areas' year-round temperatures barely vary? Christine Lindell - Menlo Park

A By far the most significant factor in the Bay Area is proximity to the ocean. Along the central California coast, the temperature of the water varies little through the course of the year.

For example, the Farallon Islands range only from 52 to 55 degrees. This is why San Francisco, surrounded on three sides by water, has such a temperate climate, with only about three days a year as warm as 90 degrees and only about one day every 10 years with a reading below freezing.

Contrast this with San Jose, which on average reaches at least 90 degrees 18 days a year and is 32 degrees or below six times a year.

Q What is the speed of a lightning bolt? Clark Skidmore - Menlo Park

A Measurements indicate a stroke of lightning travels about one-third the speed of light or about 60,000 miles per second.

In case you're wondering, it's not the speed of the lightning that creates thunder, but the super-heating of the air. A lightning stroke can heat the air to 50,000 degrees Fahrenheit, causing the air to expand explosively and initiate a booming sound wave similar to a sonic boom.

For more information, check out an excellent article on the subject in the July 1993 issue of National Geographic.

Static from home

This question is one I frequently get asked around the house: Why do we always get shocked by static electricity when it's windy and dry?

First, static electricity is the accumulation of an electrical charge in an object. When objects are rubbed together, some electrons (charged components of atoms) jump from one object to the other. The object that loses the electrons becomes positively charged, while the object the electrons jump to becomes negatively charged.

A charged object can attract small particles, such as the dust that can get stuck to the front of the television. When the charge on an object builds up enough to jump to a nearby object, we see a small flash as it discharges.

This is more of a problem when humidity is low. When the humidity is high, most surfaces are coated with a thin film of water, which prevents electrons from jumping between objects.

Jan Null, founder of Golden Gate Weather Services, is a retired lead forecaster with the National Weather Service. Send questions to Weather Corner, San Jose Mercury News, 750 Ridder Park Drive, San Jose, Calif. 95190. You also can telephone and fax them at (510) 657-2246 or e-mail them weathercorner@ggweather.com.