WEATHER CORNER

BY JAN NULL
Special to the Mercury News

Q.  I have heard that, because of La Niņa, the ``May and June gloom'' will continue through the summer. Do you agree, and if so, why? Anna Greenberg   Fremont

A.  Despite the calendar's telling us that summer began Monday at 12:49 p.m., until this week's hot weather it certainly hasn't felt very seasonable.   June, like April and May, has been cooler than normal.

However, it's arguable whether La Niņa is the direct cause of the coolness thus far. There is a large pool of cooler than normal water along the Pacific Coast, but it appears to have more of an origin in the Gulf of Alaska rather than moving north from the equatorial Pacific.

The effects of cool water off the coast are threefold. First, it makes for a greater difference in temperatures between the coast and the warmer interior. This greater temperature difference also means a greater pressure difference between the coast and interior, and ultimately a stronger sea breeze. Second, the cooler than usual air from offshore is spread inland by the aforementioned sea breeze. And finally, the stronger sea breeze is responsible for a phenomenon along the coast known as upwelling.

In this process, the wind causes surface water along the coast to be pushed offshore and replaced by colder water from below. (And it's why your toes turn blue from the 55-degree water this time of year!) This upwelling further adds to the cooling effects of the enhanced sea breeze and already chilly water.

All this translates into coastal areas of California being cooler than usual thus far this season. For a more detailed look at the spring and summer weather patterns during La Niņa, I've put together some information at http://ggweather.com/spring99/.

Q.  What is the difference between a wave, a swell, surf and a ``sea''? I have heard each mentioned in weather forecasts. Lorna Norbeck  San Jose

A.   All four refer to the conditions of the surface of the water. The generic term ``wave'' refers to undulations of the surface of a body of water.

Waves are made up of sea and swell. Swell results from the wind's past action on the water and has a gentler rolling action. Surf is the interaction of the waves and the shoreline. It is influenced not only by the height of the waves but also by the underlying topography of the coast. Sea is the amount of the wave that is the result of the wind currently blowing across the surface of the water and tends to have distinct features like a crest. A beach with a gentle offshore slope will have a small surf; one that drops off sharply will have larger surf.

Q.  When the paper lists the sun and moon rise/set times, what is the frame of reference? Obviously the times are different depending on where you are standing. Is it a theoretical sea-level horizon or what? Pete Letchworth  Cupertino

A.  Your supposition is correct. A theoretical horizon is used. To use otherwise would require knowing both the height of the observer and that of any terrain obscuring the horizon. The U.S. Naval Observatory has a whole set of definitions for sunrise/sunset and twilight at http://aa.usno.navy.mil/AA/faq/docs/RST_defs.html.

To find sunrise and sunset times for San Jose (or anywhere), go to http://nws.mbay.net/sunset.html or check the Mercury News weather page.

WEATHER ON THE WEB: As summer replaces spring on the calendar, many more people are heading for the bay and ocean. A great resource for current conditions, forecasts and tides is the National Weather Service Marine Weather Page at http://nws.mbay.net/marine.html.

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.