Published Tuesday, January 5, 1999, in the San Jose Mercury News


Special to the Mercury News

Q. What caused the really dense fog we've had recently? It seemed worse than usual.
Rick Reed
Santa Clara

A. It certainly was dense and more persistent than we usually see, more like you'd expect in the Central Valley. That it happened during one of the busiest travel periods exacerbated the effect.

Normal tule fog occurs on long winter nights when the ground is saturated, usually right after rain; then cooling occurs under clear skies and light winds. Dense fog forms from the ground up as condensation occurs. This happens frequently in California's interior valleys, less often near the bay.

But the recent fog formation was rather rare. The lower atmosphere and the ground were very cold from the previous week's Arctic outbreak -- initially a very cold, dry air mass with no fog or even dew.

Then low-level moisture was pushed inland from the coast. As this moisture encountered the cold, trapped air around the bay, it condensed to create fog. The fog persisted until temperatures slowly warmed and there was enough wind to clear it out.

. Until lately my weather band radio received both marine and local weather broadcasts for the Bay Area on the same channel. Now, all I get is marine weather, which is of only passing interest unless I'm going fishing out of Half Moon Bay. Any answers?
C.J. Wright
Los Gatos

A.  The NOAA Weather Radio operated by the National Weather Service 24 hours a day broadcasts up-to-date conditions and forecasts that can be received on scanners or inexpensive (less than $25) weather radios available from electronics dealers such as Radio Shack.

Three separate radio frequencies serve the South Bay. Two broadcast a mix of general weather plus marine weather, and the third is dedicated to marine conditions. It sounds like you may have inadvertently switched to the marine-only station.

The dual-purpose broadcasts are KEC-49, at frequency 162.55, transmitted from Mount Umunhum in the Santa Cruz Mountains, and KHB-49, at frequency 162.40 from Mount Pise in the mountains above Redwood City. The marine-only broadcast is on KEC-49 at frequency 162.45 from Mount Umunhum. For more information on the Internet, check out (

QYou recently said that weather forecasts are very accurate (over 90 percent?). If that is the case, why does everybody say you can't trust the weatherman? And how do you measure accuracy when the forecast is for a chance of rain? It seems like you can't be wrong when you say that.
Mark Parcher

A.  It's not so much a matter of not trusting the weatherman as it is taking us for granted. When we are wrong, you remember. People look at a weather forecast critically when it affects them the most. They might not notice the nine days they send Johnny off to school dressed for success, but they remember the one day it unexpectedly rains. A Weather Corner reader recently wrote about ``a little boy (who) came into my classroom soaking wet from an unforeseen rainstorm. He wailed, `the weatherman lied.' ''

There are dozens of statistical ways to measure the accuracy of a forecast. For rain, the most common method is to examine the number of times it rains when rain was in the forecast and the number of times it rains when precipitation was not expected.

If the chance of rain is less than 50 percent, it's considered a no-rain forecast. This method excludes the nearly 300 days a year in San Jose when rain was not forecast and did not occur.

In almost any other field, a 90 percent success rate would be considered exemplary. Heck, a .333 batting average in major league baseball will get you into the Hall of Fame (and get you millions of dollars in the meantime). But if your outdoor wedding gets rained on, that 10 percent is really important.

Q.  A recent weather page printed 51 degrees as the 30-year average temperature for San Jose. Our encyclopedia lists the all-time recorded spread of minus 55 and 100 above for Newfoundland -- an average of 77.5. Minneapolis has recorded minus 33 and 106 -- an average 69.5. Both places are, on the average, much warmer than San Jose. What is the value of this figure? Who would want it and what would they do with it?
Bert Smith
San Jose

A.  I agree that the mean temperature is relatively meaningless for everyday use. It is simply the average of the average high and average low. Consequently, a mean temperature of 50 degrees could represent an average high of 60 and an average low of 40; or an average high of 70 and an average low of 30. Quite a difference.

However, the temperatures cited for Minneapolis and Newfoundland are the average of their all-time high and low readings, not the average of their average daily high and low temperatures.

Q.  Can you explain why the Weather Channel is showing San Francisco as local weather here in San Jose? It used to give San Jose information but for the last few weeks, all it shows is San Francisco.
Carol Ryall
San Jose

A. This has stumped me, too, for I have had similar problems. My local cable provider says its information comes from the Weather Channel. But the Weather Channel tells me the local provider selected the data. All I can suggest is to keep after both of them to get their stories straight.

SAN JOSE WEATHER FACT: In January 1911 San Jose got 12.78 inches of rain, nearly 89 percent of the average seasonal total.

SAN JOSE JANUARY STATISTICS: Average high/low temperature: 58/41. Average rain days/amount: 10 days/3.03 inches.

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