WEATHER CORNER

BY JAN NULL
Special to the Mercury News

Santa Ana Winds Fan Dry SoCal Fires

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
hours when there is little seabreeze. They are most common from October
through February. Other similar dry downslope winds are the Diablo winds we
experience in the Bay Area, Chinook winds in the Rockies and foehn winds of
the Alps.

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.

Q.  Even though we've had a bit of rain the past few days, we are quite far behind our average for the year and finished last year substantially behind average, too.  Am I the only one who thinks we are having a drought?  Should I be concerned? Jean Myer - Mountain View

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
of Northern and North Central California cities are at or above normal. And more importantly, the Sierra Nevada and mountains of Northern California had abundant early season rain and snow. The accumulated rainfall in the northern mountains is currently at 111% of normal, the Sierra snowpack is at 94% of normal and the six largest reservoirs at right at 100% of normal.  The bottom line is that even with minimal rainfall for the rest of the season most of the state should be in pretty good shape.

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
8 inches in diameter. However it is my experience that anything bigger than 4 inches will give fairly accurate reading under most conditions.  Many of these
gauges do have a funnel whose purpose is to focus the rain into a calibrated cylinder. These cylinders make it easier to read the amount of rain instead of using a ruler, but you can also use a coffee can or anything with a straight side.  Some rain gauges have an internal tipping mechanism which consists of two tiny buckets that alternately fill and then tip allowing the other bucket to fill.  Each tip is measured electronically whenever .01 inches fills one of these ""tipping buckets.''

Q.  I have noticed that the temperatures in my backyard near Barron Creek in
Palo Alto have been consistently colder than those reported on television. Can you explain?  Joan McDonnell - Palo Alto

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 weathercorner@ggweather.com.  Please indicate in your e-mail what city you live in.