Published Tuesday, April 22, 2003 in the San Jose Mercury News

Weather Corner

Region's sea breezes kick in as warm weather approaches

Special to the Mercury News

As the days get longer and warmer across California, it's also the time that Mother Nature
clicks on the San Francisco Bay Area's natural air-conditioning ... our springtime and summer
seabreeze and fog cycle.

   It is this nearly daily phenomenon, most prevalent from late April and into August, that
keeps coastal communities chilly and provides a cool late afternoon or evening breeze in even
the warmest Bay Area towns. Here's how it works.

   The weather pattern responsible for providing this daily relief is unique to
Mediterranean-type climates in the middle latitudes along the west coast of major continents.
And for it to occur requires several meteorological elements to fall into place.

   The first is strengthening of the quasi-permanent position of the ""Pacific High,'' a zone
of high atmospheric pressure to our northwest over the cool waters of the Pacific Ocean. At
the same time land and air over the interior of California begins to warm, with the resulting
rising air creating lower atmospheric pressure there. It is not uncommon for there to be a 40
degree difference between the coast and an inland valley 40 miles away.

   This marked difference in temperatures and pressures results in the persistent northwest
winds that blow from the high pressure toward the low pressure. The winds, blowing parallel to
the coastline, allow cool water to upwell along a strip of water about 60 miles wide, with
temperatures that are typically less than 56 degrees.

   As these cool breezes cross the cold upwelled water, the air will often cool it enough for
condensation to occur and for either low stratus clouds or fog to occur. These clouds are then
carried inland through gaps in the coastal mountains like the Golden Gate, San Bruno Gap and
Petaluma gap. If the fog or cloud layer is deep enough, it can even spread into the southern
Santa Clara valley from Monterey Bay.

   The seabreeze is strongest during the afternoon and early afternoon when inland
temperatures are warmest. Even when the seabreeze is initially cloud-free, it can cool enough
for stratus to form as it is lifted over the coastal hills.

   The cool air and clouds slowly penetrate into the Bay and coastal valleys during the
afternoon and evening hours. When the seabreeze slackens after dark, the cool air remains in
place until the next day's sun begins to evaporate the clouds and warm the land. And the cycle
begins all over again.


Q.  Does a baseball travel further in hot or cold weather? David Kaiser - Colonia, NJ

A.  In general it should fly farther in warmer weather for a couple of reasons. First,
assuming the humidity is constant, warmer air is less dense than cold air and offers less
resistance to the flight of the ball. Additionally the temperature affects the elasticity of a
baseball's materials. When it's colder, these materials absorb more energy and don't fly as

Q.  I have heard of double rainbows before, but can there ever be a triple or quadruple
rainbow? Emily Chang - Petaluma

 A.  There are indeed third order, or tertiary, rainbows but no quadruples. But it is not
an arc above the first and second arcs. And while the typical rainbow is located opposite the
sun a tertiary bow is a full circle that is located at an angular distance of about 40 degrees
from the sun. They are also very dim, due to three internal reflections of sunlight within the
raindrops that help create rainbows, and because they are overshadowed by the brightness of
the sun.

Q.  What causes ""microclimates'' in areas such as Sonoma County? David Abbott - Santa Rosa

A.  The Bay Area's varied climates are a result of its varied geography. There are very
few other places that see elevation changes from sea level to above 4000 feet within these
short distances and are right next to a large body of water. All of these factors combine to
cause create sharp differences in temperature regimes, rainfall amounts and wind patterns.

Q.  I regularly catch the weather forecast so I can have some idea about the coming
weather. However, I have always thought it would be more fun to know how I can be my own
weather forecaster. Got any tips or ideas? Angelina Martinez - Richmond

A.  The Internet certainly makes this possible. Personally, I like to look at what the
current conditions are first. This would include the present weather at stations around the
Bay Area plus the latest satellite and radar. Then I would look at the latest computer models
of the atmosphere for the next few days, comparing the differences between the models. And I
would also look at the forecasts from a variety of other sources to see what others were
thinking as meteorology is such an interpretive science that it is good to look at all
possible solutions.

To facilitate this process, I have put together a quick and easy Do-It-Yourself Weather
Forecast Guide at  It should serve as a starting point to
start looking at the weather and making your own forecasts.

Jan Null, founder of Golden Gate Weather Services, is a retired lead forecaster with the National Weather Service. Send questions to him c/o Weather Corner, San Jose Mercury News, 750 Ridder Park Drive, San Jose, Calif. 95190. You also can telephone questions at (510) 657-2246, fax them to (510) 315-3015 or e-mail them to or fill out a form online at