Wintry Bay Area Mountainscapes
BY JAN NULL
Special to the Mercury News
Recent rains have all but washed away the last vestiges of our
mid-February snow from Bay Area peaks. Even though the snowy storms left
the region's hills looking more like Colorado than California, we're now
back to our regular winter reality -- it rarely gets cold enough here to
snow in many places.
For those who still hope to see some Bay Area snow this winter, the most likely place is Mount Hamilton. The approximately two feet of snow that fell there two weeks ago was the most recorded on the mountain since February 1989, but some snow is measured almost every year on the 4,213-foot summit. Annually there is an average of 17 inches, with a record of nearly 35 inches back in 1950.
What we're not likely to see again this year is any snow accumulating below about 2,000 feet; that happens only about once every 10 years. What is truly rare is to see widespread snow fall and ``stick'' near sea level around the bay. The last time that happened was Feb. 6, 1976, when much of the region awoke to a winter wonderland.
If you didn't make it to Mount Hamilton or any of the other snow-covered peaks this winter, some of this month's snowy splendor has been captured in online photos from Mount Hamilton taken by astronomers Elinor Gates and Tony Misch, who work at Lick Observatory near the mountain's summit (http://mh-www.ucolick.org/public/snowpics/snowpics.html).
Q What is the relation between the
measured amount of rain vs. snow? Sometimes the paper shows the same
amount of precipitation for Los Gatos and South Lake Tahoe. Elsewhere in
the paper it says that there was a foot of snow at lake level. J.C.
Vanderlaan - Los Gatos
A What you have noticed in the paper are different ways that
snow is measured. Sometimes snow is measured by depth, other times by its
water equivalent. The latter is the amount of water there would be if the
snow were melted. This number is important to water officials, who want to
know how much runoff there will be. On the other hand, skiers and
snowboarders want to know how deep and powdery the snow is.
In California there can be a wide variation in the water content of an inch of snow depth. Because most of our storms come across the relatively warm moist Pacific Ocean, they tend to have high water content, with about 8 inches of snow depth for 1 inch of water content. This wet, soggy mess is what Sierra skiers not too affectionately refer to as ``Sierra cement.''
But several times a year, the mountains of California get just the right combination of cold continental air and Pacific moisture for the dry, fluffy powder that is more common in the Rockies and has about 16 inches of snow depth for each inch of water content.
Q I have noticed occasionally that passenger jet airplanes flying over San Jose leave distinctive and visible vapor trails, some of which appear to be extremely long and other times comparatively short. They also seem to be more prevalent in early morning or evening. Can any type of weather interpretation or prediction be made of this phenomenon? Jim Cabeceiras - San Jose
A What you are seeing is the condensation of water vapor being exhausted from an aircraft's jet engines, just like the steamy cloud from your car's exhaust that you will sometimes see on cool, moist mornings around the Bay Area.
These condensation trails, often referred to as contrails, usually occur above 30,000 feet, where air temperatures are 20 to 40 degrees below zero, and the water condenses directly into ice crystals in a process called deposition.
These ice crystals are the same as those that make up the thin, wispy cirrus clouds that we see almost daily high in the sky. The extent of a contrail depends on the amount of moisture in the atmosphere, the amount being exhausted by the jet, the temperature and the amount of wind present.
When winds at these altitudes are light, the clouds spread out and dissipate slowly. With strong winds they are disrupted almost immediately. With all of these variables, the presence of contrails is of little aid in interpreting the weather.
It is interesting to note that contrails are a relatively new phenomenon that occurred as aircraft started flying above 25,000 feet or so in World War II. It was a challenge for pilots and meteorologists of the era to choose an altitude for their aircraft that would not leave a telltale streak across the sky.
Q On rare occasions, once or twice a year it seems, planes heading for San Francisco International Airport land to the northeast. This makes for the sometimes spectacular viewing of large aircraft flying low along I-280 before they dip off to the northeast over the Peninsula heading for the shorter cross-bay runways. Obviously, this is in response to the winds coming from the northeast, but what causes that apparently rare occurrence? Gary Cooper - Portola Valley
A These northeast winds are what are also called ``offshore'' winds and are responsible for warm, dry weather in the Bay Area. While these winds are often strong and gusty in the East Bay Hills, they are generally light near the airport. But unless they exceed about 12 mph or are gusty, which happens only a few times a year, aircraft can use the usual northwest-facing runways.
Jan Null, founder of Golden Gate Weather Services and Director of M eteorology for
Planetweather.com, 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 telephone and fax questions at (510) 657-2246 or e-mail
them
to weathercorner@ggweather.com. weathercorner@ggweather.com.