Published Tuesday, December 22, 1998, in the San Jose Mercury News


Special to the Mercury News

Q  Why, when the local TV news has a live feed from the Sierra during a storm, is it usually from Blue Canyon? What is so great about Blue Canyon? I would think Donner Summit would be better.

Mike Murray

A  The answer here actually has more to do with accessibility than with meteorology. Blue Canyon, about 65 miles east of Sacramento at elevation 5,280 feet, is relatively easy for television crews to get to without being stuck by the heavier snows at higher sites. And according to meteorologist Joel Bartlett at KGO-TV (Ch. 7), they also know they can get a ``clean microwave signal'' back to the Bay Area from this point, which is difficult when it's raining or snowing.


Q  What is the difference between a rainstorm, rain and showers? Is heavy rain a storm? Is heavy showers rain? Is light rain a shower?

Wallace H. Byron

A  The various terms used in describing rain (or snow) events are some of the most misunderstood elements of meteorology. The most common usage of ``rain'' is to describe the steady, widespread precipitation accompanying a weather front. Conversely, ``showers'' are of short duration and localized over small areas. Typically these occur in cold, unstable air behind a cold front. There is no definition of a rainstorm, but general usage for storm is the combination of winds and rain.

The issue is further complicated by the use of percentages to describe the chance of rain. When a percentage is used with rain, it indicates the probability of rain. For example, if a forecaster says there is a 60 percent chance of rain, he's saying the forecast weather pattern has a 6-in-10 chance of producing rain at a given location. However, a 60 percent chance of showers means that showers are likely over 60 percent of the forecast area.


Q.  Is Doppler radar different from other radars? All the TV stations talk about ``their'' Doppler radars. I thought only the weather service had Doppler.

Tom Robinson
Redwood City

ADoppler is only one component of the weather radars used by the National Weather Service, and I have never seen that part of the display ever used by the media in the Bay Area.

That portion shows the horizontal movement of clouds and precipitation and is most commonly used to detect tornadoes and mesocyclones in other parts of the country. The green blobs displayed on local TV stations do not use Doppler but are merely the areas where rain is falling.

And, yes, the only weather radar in the Bay Area is the National Weather Service radar on Mount Umunhum in the Santa Cruz Mountains. The data from this site is available to anyone via the Internet -- -- and other sources, so it is disingenuous at best for anyone to claim this as ``exclusive'' radar.

QThe radar imagery linked to the NWS Web site is provided by private companies and is delayed five to 30 minutes. Why doesn't the NWS provide this imagery directly and closer to real time?

Bill Tomkovic
Los Gatos

AThe NWS is forbidden, by contractual agreement with a number of private companies, from placing any radar data directly onto the Internet. It can provide links only to commercial vendors, who have a sole-source deal to sell the data that the taxpayers have already paid for once. A couple of these companies are graciously putting a limited amount of their data onto the Web for free, but it is usally delayed by about a half-hour.

Jan Null, founder of Golden Gate Weather Services, is a retired lead forecaster with the National Weather Service. Send him questions c/o 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 to