WEATHER CORNER

BY JAN NULL
Special to the Mercury News

Definition of fog is foggy

Q.  I hope you can resolve a family debate about summer's morning cloudiness. My mother-in-law insists it isn't fog, because fog must touch the ground. I disagree. I say fog can either touch the ground or linger just above it (1,000 to 3,000 feet, I would guess).

If you drive up to Mount Hamilton or to the Highway 17 summit, you can see the same fog bank hugging the ground. So technically her definition of fog is correct, even though it's not on the ground here on the valley floor. Can you this clear up?  Gioni Pasquinelli - San Jose

A.  Getting into the middle of a dispute with any mother-in-law sounds dangerous. If my answer doesn't suffice, try Dr. Laura.

Fortunately, you are both right. It's more a matter of semantics and colloquial usage. On a typical summer morning in the South Bay, we have a low overcast with a base at about 1,000 feet and a top about 2,000 feet. It's called fog where this, or any other cloud, comes in contact with the
ground.

Rather than being a particular cloud type, fog should be thought of as a restriction to visibility. The Glossary of Weather and Climate further defines it as ``responsible for reducing visibility to less than one kilometer (5/8 mile).''

The morning overcast is an extension of the marine layer, created when moisture condenses into low stratus clouds over the cool waters off the coast and is carried inland overnight by the sea breeze. It usually spills into the San Francisco Bay basin through gaps in the hills and is confined by higher terrain of the Coast and Diablo ranges. An inversion, in which air aloft is warmer than air at the surface, traps this cloud layer until the morning sun warms and evaporates it.

Q.  The TV weather types often blame some of this past summer's unusual weather on what they call ``cut-off lows'' or ``upper-level lows.'' I don't remember hearing of these before. Is this new terminology to describe a weather phenomenon, or have there been more of them than usual this
past summer? Gregg Peterson - Saratoga

A.  Cut-off lows and upper-level lows are nothing new. Also known as ``upper-level devils,'' they just seem to be getting more notice from the news media and acknowledgement that it's difficult to forecast their movements and impact. I don't know if there have been more than usual, but awareness may be up because they are most common in the fall and spring. If we compare the flow of air in the atmosphere to a river, cut-off lows would be the little swirls and eddies that occur
along the periphery. They can take on separate characteristics from the main flow and have proven to be very difficult for the computer models of the atmosphere to get exactly right. So, unlike a broad weather front that sweeps through California from the northwest to the southeast, a cutoff may sit along the coast, spreading showers or thundershowers over a relatively small area. Consequently, getting the exact position correct can make a huge difference in the accuracy of a forecast.

Q.  Why is San Jose's weather never listed on weather pages of major newspapers in Chicago, New York, Boston and so forth? Only San Francisco is listed. Considering that San Jose is the
11th largest city in the country and San Francisco's weather is so different from San Jose's, why do they not list San Jose's weather? Luis Arevalo - San Jose

A.  I can only surmise why San Jose is ignored outside California. We all know how very different San Jose and San Francisco are, but people in most other parts of the country have never heard of microclimates. Where they live, there's very little difference in weather between cities that are only
50 miles apart. Newspaper weather pages can list only so many locations, so they choose San Francisco for the Bay Area.

Weather Web Site of the Week: The first snow of the season has dusted the slopes of the Sierra Nevada, and it's time to start thinking about snow. Kenneth G. Libbrecht, a physics professor at the California Institute of Technology, has put together a wonderful Web site that will tell you everything you ever wanted to know about snow flakes and the physics of how they are formed. See: http://www.its.caltech.edu/~atomic/snowcrystals/ 

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 can telephone questions at (510) 657-2246, fax them to (510) 315-3015 or e-mail them to weathercorner@ggweather.com. Please indicate in your e-mail where you live.