Published Tuesday, March 30 1999, in the San Jose Mercury News


For the weather elsewhere, take a trip to your computer or library

Special to the Mercury News

BY FAR, the most common question received at the Weather Corner is this one:  I'm going on vacation or moving to a particular location. What's the weather like there?

It is impossible to answer every request in any detail, but numerous excellent resources are available. Two great online ones are World Climate ( and the Washington Post Historical Weather Database ( At the library look for the USA Today Weather Almanac.

Q I know that cloud-seeding is used to cause precipitation. Is it possible to prevent precipitation through this same process?
Ryan Leary - Menlo Park

A First, here are a couple of general clarifications about cloud-seeding. The seeding of clouds does not really ``cause'' precipitation. Rather, it enhances the processes already occurring in a cloud by the introduction of tiny particles -- or nuclei -- upon which rain or snow can condense. Originally, dry ice (i.e., frozen carbon dioxide) was used, but the chemical of choice is now silver dioxide.

Under ideal conditions it is estimated that a maximum increase in precipitation of 10 percent can be achieved, though seldom are ideal conditions present, and the actual yields are considerably less. As you would expect, of course, it is extremely difficult to determine how much precipitation might occur naturally, which makes it impossible to tell the precise effects of cloud-seeding.

The answer to your question about preventing precipitation is a qualified yes. There has been some seeding of thunderstorm clouds in the Midwest to limit the size of damaging hail. The injection of condensation nuclei produces lots of smaller hailstones at the expense of the large ones that can produce enormous damage to property and crops.

Cloud-seeding is also used to help diminish thick fog near airports in very cold regions by having the moisture in the air condense onto the condensation nuclei as snow and fall to the ground.

Q What makes the wind blow, especially in the Bay Area? Patrick McGlennon - Menlo Park

A In general, wind is caused by differences in air pressure and the atmosphere's attempt to equalize these differences. To reach a state of equilibrium, air moves from areas of high pressure toward lower pressure. This movement of the air is wind.

These pressure differences are generally the result of differences in temperatures, with warmer areas having lower pressures than cold areas. Here in the Bay Area this is most pronounced during the summer months with our usual afternoon sea breeze.

During the long summer afternoons, the interior portions of the state heat up and the barometric pressure becomes lower. At the same time, the air over the Pacific remains cool with relatively higher pressure. To equalize these differences, cool air moves inland from the ocean, resulting in our afternoon sea breeze.

The converse is true at night as inland areas cool more than the coastal and bay waters. This reversal creates the wind that often comes down the Santa Clara and Salinas valleys right before sunrise.

Q We enjoy Jim Vanderzwaan, who reports for KSBW-TV (Ch. 8) in Salinas. We'd appreciate any credential information on him. Helen and Joe Pessillo - San Jose, Carol Stelling Arnoldy - San Jose,  K. Baird - Gilroy

A Leaving Vanderzwaan off the list of prime time weather forecasters was my oversight, as I don't get KSBW on my cable system. I have known and respected him for more than a decade, and he certainly deserved to be included.

Vanderzwaan got into the weather business after a background in broadcasting and from aviation flight training. He was a television reporter in Reno and Medford, Ore., before doing the weather in Boise, Idaho. He has been doing weather at KSBW for 16 years.

Information about the plethora of morning and weekend weather forecasters can usually be found on their respective station's Web sites.

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