|Published Tuesday, September 12, 2000, in the San Jose
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
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 -
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 -
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 email@example.com.