WEATHER CORNER

BY JAN NULL
Special to the Mercury News

MUCH of the San Francisco Bay Area's weather is the result of the region's unique topography, microclimates and wind
patterns. This week we will examine wind in general and here in particular.

Q.  What causes wind?

A.  The atmosphere is always trying to reach a state of equilibrium between areas of high and low pressure. These different pressure areas are created by differences in temperatures, locally and globally.

On the global scale, areas nearer the equator receive more energy from the sun than the polar regions. This means that the equatorial regions are warm and have generally low pressure, while the colder regions have higher pressures. To balance these temperature differences, air flows from areas of high barometric pressure to low pressure. This movement of air we call wind.

Q. How are winds measured?

A.  To measure winds, you have to consider the speed of the wind and its direction. Wind speed is measured with an anemometer. There are many types, but the most common consists of three small cups that catch the wind and spin. The faster they spin, the greater the wind speed.

Wind direction is determined using a wind vane. Because winds are generally named for the direction they come from (a westerly wind, for example, blows from west to east), wind vanes are designed to point toward the wind. Because of the variable nature of wind, the usual convention is to average wind speed and direction over two minutes.

Q. How does our usual afternoon sea breeze work?

A.  The winds that are felt many afternoons from late spring and into autumn result from the aforementioned temperature differential. In this case, the interior regions of California heat up during the afternoon, sometimes by as much as 40 degrees
warmer than the cool Pacific waters. This results in lower barometric pressure inland and an afternoon breeze that flows from the sea toward the land.

These winds, which start out as westerly winds blowing through the Golden Gate and San Bruno Gap, are channeled by the terrain in the South Bay and are primarily north-northwest as they move through Santa Clara County. The sea breeze also brings cooler marine air over the area, and as the inland areas cool off, the differences in temperatures and winds drop off.

Q.  On a lot of early mornings there is a wind that blows from the Coyote Valley into San Jose. What causes this?

A.  Combined factors cause these ``drainage'' winds. First, the areas of southern Santa Clara County cool off more because they are at a slightly higher elevation and are farther from the relatively warmer waters of San Francisco Bay, making the barometric pressures slightly higher. In turn, a light southerly wind develops toward the relatively lower pressures over the bay. These winds are compounded by gravity, which pulls the colder, heavier air downhill toward San Jose.

Q. Does the Bay Area get Santa Ana winds?

A.  No. Santa Ana winds are locally named Southern California events. However, the Bay Area does experience a similar
meteorological phenomenon with our so-called ``Diablo'' winds. These result when there is a strong area high pressure over
Nevada and lower pressures west of San Francisco and Monterey.

But the consequence is not just wind; there are also warm, dry conditions. As these winds blow from the high desert regions of Nevada down to sea level, they warm by compressional heating and lose much of their humidity. Sometimes this warming can be as much as 20 degrees. This gives the name Diablo a double meaning. First, these northeast winds tend to blow from the direction of Mount Diablo. And, the Spanish translation of the word diablo is devil, which seems particularly apt for these hot, dry winds. It was Diablo winds that resulted in the Oakland hills firestorm in October 1991.

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.