WEATHER CORNER

BY JAN NULL
Special to the Mercury News

Q. My question has little relevance to the Bay Area, but I've always wondered: What exactly is lake effect snow?  Christopher Flowers - San Jose

A. While ``lake effect snow'' does not occur here, its impact was certainly felt when it stranded many travelers trying to get to and from the Eastern United States during the holidays.

Lake effect snows occur on the south and east sides of the Great Lakes. When very cold, dry arctic air sweeps across them from Canada, it picks up moisture and heat from the lakes' surface. This in turn makes air unstable and ripe for the development of heavy snows.

The areas receiving the most snow from this effect are on the downwind side of the lakes, after the air has picked up the most moisture. This is why the annual snowfall can be two to three times greater than at other locations. For example, areas around Watertown, N.Y., on the eastern end of Lake Ontario receive more than 200 inches of snow, while Albany -- about 160 miles to the southeast -- gets only 63 inches (see map above).

Q. I live in the east foothills of San Jose. On very cold mornings, it seems to be much colder on the valley floor, in places like Willow Glen, as opposed to where I live, possibly 100 feet higher. I have always understood that for every 1,000 feet of elevation gain, the temperature drops three to five degrees. Why at our slightly higher elevation, does it seem warmer? Richard Tomasso -
San Jose

AGreat question, and you are absolutely right about it normally cooling three to five degrees Fahrenheit for every 1,000 feet of elevation.

But that normally refers to the standard atmosphere and doesn't factor in terrain. And when you include terrain -- an important element in the Bay Area's climate -- two factors conspire to negate this on many mornings. First, cold air is heavier and naturally settles at lower elevations. This is why you probably experience a wind down from the hills at night. Second, the ground cools more quickly than the air at night, and the ground in turn cools the layer of air adjacent to it. The net result of these two factors is cooler air near the surface and warmer air above. This is what we call an inversion. This is a very stable pattern and often traps smoke and haze near the ground on these long winter nights.

Q.  Is the story of Groundhog Day true? Jennifer Christian - St. Raymond's School, Menlo Park

A.  No, but its history is very interesting. Groundhog Day has its roots in an ancient Celtic celebration called Imbolog. This date is the midpoint between the winter solstice and the spring equinox. In an agrarian society that was very dependent on the weather, this was a time to celebrate having made it halfway through winter. The superstition arose that if the weather was fair on Imbolog, the second half of the winter would be cold and stormy, but if the weather was cold and overcast or stormy, the second half of the winter would be mild.

In Christian times, Feb. 2 was celebrated as Candlemas, but the earlier Imbolog superstition continued. In Scotland they said, ``If Candlemas be bright and clear, there'll be two winters in the year'' and in England, they said, ``If Candlemas be sunny and warm, ye may mend your mittens and look for a storm.''

The Romans learned of this tradition from the Celts, and eventually brought them to the area that became Germany. German immigrants brought these beliefs to Pennsylvania, and the tradition of predicting the weather became centered on the woodchuck, or groundhog.

In case you're curious, a Mercury News analysis in 1966 showed that Punxsutawney Phil -- the nation's leading rodent forecaster -- turned out to be correct about 39 percent of the time.

Q.  When we view weather maps, there are areas in white that I understand to be precipitation and/or clouds. However, I cannot understand how you weather forecasters can tell if the precipitation will fall as rain, snow or ice. How can you determine where a ``snow line'' will be and where sleet will fall, or icy rain or just rain?  Jonathan Streete - San Jose

AIt's not possible to tell what's falling out of the bottom of a cloud merely by looking at satellite images. We use numerous other tools to determine temperatures at various altitudes.

These tools include weather balloons, which are launched twice a day around the world. The nearest launch location is Oakland. Suspended from the balloon is a small instrument package that sends back the temperature, humidity and winds as it ascends. Data about the upper atmosphere is also retrieved from aircraft, radar or ground-based sounding equipment.

The meteorologist then uses this information to determine where the freezing level will be in the atmosphere. The snow line is about 1,500 feet lower than the freezing level.

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 weathercorner@ggweather.com.