WEATHER CORNER

BY JAN NULL
Special to the Mercury News

COME meet me at the San Jose Mercury News Weather Forum Sept. 20. It's at 7 p.m. at the Mercury News, 750 Ridder Park Drive. Staffers from Weather Data Inc. will also be on hand; they put together the Mercury News weather page.

Also note: The Summer Olympics begin Friday. Go to http://ggweather.com/sydney.htm to get the latest weather in
Sydney.

Q.  I was on a backpacking trip Aug. 20-25 in the Sierra Nevada south of Yosemite at 7,000 to 9,000 feet. I was disappointed
by the night skies. They were a silvery gray rather than black, and only the brighter stars were visible. I couldn't discern the
Milky Way. The daytime sky was practically cloudless, although as night fell, one could see some clouds and airplane contrails. Is this bright, hazy night sky normal and/or predictable? I want to watch the 2002 Perseid meteor shower  from a good place in California. I thought an altitude of 10,000 feet in the Sierra would be that place, but now I'm not sure. Allen Wood

A. At the time you were in the Sierra there were a couple of fires burning to the north. I think the reduced visibility and the silvery gray you observed were due to smoke in the atmosphere filtering out the starlight. Your plan to be at 10,000 feet for the 2002 Perseids sounds like a good one. Just bundle up!! The Perseids is one of the most consistent meteor showers, with a peak around Aug. 11 each year.

Q. Beyond the altitude and low humidity, is there any particular reason Truckee seems to be the temperature-swing capital of California? Lately the difference between the daytime high and nighttime low has been 40 to 50 degrees. Steve Misiewicz - Mountain View

A. The altitude is certainly the most important factor. Additionally, when there are clear skies at night, like the ones we have seen recently, there can be significant cooling. The fact that Truckee lies in a valley is also significant. Cold air from the surrounding mountains settles in the lowlands, resulting in cold temperatures. Less than 10 miles northeast of Truckee is the coldest spot in the state, at Boca Reservoir, which dipped to a brisk minus 45 degrees Jan. 20, 1937.

Q. Why don't we have hurricanes or cyclones on the West Coast? Joe Nardini - Fremont

A. The key ingredient for the formation of hurricanes and tropical cyclones is warm water. The water temperature needed to sustain a hurricane is at least 80 degrees. Along the East Coast, the Gulf Stream carries warm tropical waters northward, creating a pathway for hurricanes to travel into the Southeast and the Atlantic Seaboard. However, along the West Coast, the California Current travels north to south, bringing chilly water from the Gulf of Alaska. The water doesn't warm sufficiently to support a hurricane until it is well south of San Diego.

Q. I run along the Guadalupe River near San Jose International Airport several times a week. I noticed this summer that the planes seem to be taking off and landing to the south far more frequently than usual. I presume this is wind-related. Has there been a documented change in our wind patterns this summer? John Michael O'Connor - San Jose

A. There has not been a significant change in the winds at San Jose, so I suspect the takeoff and landing patterns are more the result of other air-traffic control issues. In the mornings, San Jose, especially the southern part of the city, does get a light southerly ``drainage'' wind flowing out of Coyote Valley. The valley air cools, becomes heavier and rolls downhill toward the bay. This is why you can sometimes smell garlic in the mornings.

Q. Is it possible to calculate the atmospheric pressure at 2,200 feet without the use of a barometer? I am trying to determine how much air is available at this altitude compared with 1,000 feet. Troy E. Gorbett - Atlanta

A. The pressure can be estimated using something called the Standard Atmosphere. This is an average of a variety of meteorological parameters in the atmosphere, including the temperature, pressure, air density and speed of sound relative to the altitude. In the example you cite, at 2,200 feet the air density is 1.15 kilograms per cubic meter, while at 1,000 feet, it's 1.19 kilograms per cubic meter. This means that at 2,200 feet, there's about 3.4 percent less air than at the lower altitude.

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.