|Published Tuesday, February
26, 2002 in the San Jose
The evening news recently showed wintertime images from a Southern California of a wildfire fanned by Santa Ana Winds. What causes this common weather phenomenon? This year it's a case of a strong ""offshore'' wind event and a dry winter that has seen less than 50% of the area's normal rainfall.
The term ""Santa Ana winds'' dates back to at least 1880 when it was used in the Los Angeles Express newspaper, and appears to be derived from strong northeast winds that blow ferociously down the nearby Santa Ana Canyon and river valley. The terrain funnels the dry warm winds down the canyon, sometimes in excess of 50 miles per hour. Through time the usage has become more general to include any offshore wind event in Southern California.
Frequently, the strongest Santa Ana winds occur
during the night and morning
Santa Ana winds occur when high pressure settles into the Great Basin and air spills down toward lower pressure off the southern California coast. As this air descends to sea level across the deserts of Southern California it warms by as much as 20 degrees and becomes very dry with relative humidities under 20 percent. When this is combined with strong gusty winds, below normal rainfall and an inadvertent spark, the unfortunate result can be a disastrous wildland fires.
Setting the record straight. My recent answer about temperatures and frost said that temperature readings are typically taken at 5 inches above the ground. As several readers pointed out, this should have read 5 feet above the ground.
A. Indeed, the San Jose area continues to lag
behind normal rainfall through the end of February with only about 82
percent of normal. But most of the remainder
Q. Two of us were talking recently about rain gauges and size was mentioned. Is there a specific size for a collection funnel and glass? Could a larger funnel be used and a smaller gauge glass for more precise measurements? How would a person calibrate the gauge? Mike Drouin - Campbell
A. The ""standard size'' rain gauge that the
National Weather Service uses is
Q. I have noticed that the temperatures in my
backyard near Barron Creek in
A. There could be several things that would account for your cooler temperatures. First, most of the locations that are reported on television are from Bay Area airports which tend to be near the bay and do not cool as much as most similar areas a bit inland. Additionally a location along a creek will be more vegetated and a little cooler than an area that is more developed.
Jan Null, founder of Golden Gate Weather Services, 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 questions at (510) 657-2246Š, FAX them to (510) 315-3015Š or e-mail them to firstname.lastname@example.org. Please indicate in your e-mail what city you live in.