Published Tuesday, April 25,2000, in the San Jose Mercury News


Special to the Mercury News

Q When are tornadoes most likely here in the Bay Area? I recall one in Sunnyvale not too long ago. Fred Friedlander - Palo Alto

A.  The Midwestern monster tornadoes are most likely in late spring when humid air from the Gulf of Mexico collides with cold, arctic air that has spilled down into the Great Plains. In the Bay Area, the few we get most often are triggered by cold, unstable air that moves through the region behind cold fronts. Consequently, we have no particular tornado season. There have been tornadoes in the Bay Area in every month but July, August and September.

Q.  Your explanation about the percentage chance of rain in your last column was clear enough. But it demonstrates the problem of highly technical people who come up with ``additional information to make a risk-benefit decision.'' Huh? If you have any influence with these folks, you might tell them that the nuances of rain vs. showers are confusing enough. To then factor in ``sometimes it's area and sometimes it's measurable precipitation'' as far as percentage chance of rain means exactly zilch.  I want to know what the chances are of my getting wet. Do I need to bring full rain gear to work or will I be able to get by with just a hat? Can I wear my Birkenstocks or will I need enclosed shoes? Martha Johnson - San Jose

A.  Guilty as charged for getting caught up in the jargon and assuming I was making sense. Are you sure you haven't been talking to my wife? She keeps saying, ``Just tell me if it's going to rain or not!'' For the vast majority (dare I say probably 90 percent) of the population, just knowing whether it will rain is enough. Unfortunately, forecasting skills are still not good enough to make a categorical statement one way or the other. Thus we use terms like ``chance of rain.''

The remaining 10 percent of the forecast users need to know not only whether it will rain but also how much. This is where the ``risk-benefit'' thing comes in. They need to ``risk'' the probability of rain vs. the potential beneficial (or detrimental) impact of rain occurring. For example, if you are a building contractor and it rains, you have to shut your job site for the day. If you know ahead of time, you can save your workers their unnecessary commute to the job, and you save having to pay them for showing up. You can also cancel shipments of materials and supplies, or you can cover parts of the job site so materials are not damaged.

Businesses can get rain insurance to cover potential losses. I recently consulted for a large firm planning a huge outdoor activity with a budget of more than $2 million. The company bought rain insurance to cover the cost if the event were canceled. But because I forecast that rain would most likely be light, the company got a limited policy with a significantly reduced premium.

Unfortunately, the terminology of the National Weather Service may be getting more complex. There is a proposal not only to keep the probabilities but also to issue probabilities for a specific amount of rain. Consequently, you might see a 60 percent chance of a tenth of an inch of rain and a 20 percent chance for a half-inch.

Q.  My daughter, who is in the seventh grade, has expressed an interest in meteorology and is considering it as her college major. I would like to give her more information on the educational requirements at the high school and college levels for becoming a meteorologist. Amy Nardini - Fremont

A.  Like many of the sciences, it is never too early to begin the educational groundwork to succeed. In junior high and high school, she should take as much science and math as they offer. This should include at a minimum pre-calculus, physics and any available earth science courses. (I find it regrettable that meteorology, the one science that affects all of us daily, is barely an afterthought in the majority of science curricula. This must be why there is a Weather Channel and not a Physics Channel or Biology Channel on television.)

I also recommend reading about and observing the weather around you every day.  In college, the emphasis during the first two years is strengthening this foundation with calculus and physics, very similar to the lower-division courses of engineering students.
The principles of these classes are translated into meteorology in atmospheric dynamics and thermodynamics. These physical
atmospheric principles are then applied through course work in numerical weather prediction and forecasting. It's a lot of work
getting there, but it can certainly be a satisfying career. Good luck to your daughter and others who take up the challenge.

San Jose Extremes: The longest cold spell in San Jose, with temperatures of 32 degrees or less, was 13 days. This occurred from Dec. 20, 1990, through Jan. 1, 1991. Daily minimum temperatures for this period were 32, 29, 19, 19, 23, 25, 29, 30, 31, 28, 27, 28 and 30 at the official San Jose Weather Station at Civic Center.

Jan Null, founder of Golden Gate Weather Services and Director of Meteorology for, 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 e-mail questions to or telephone and fax them at (510) 657-2246.