When should you have a picnic?
Rarely will the summer disappoint you with rain, and if it does, the rain will
be minuscule. But in some neighborhoods and towns the fog may seem like rain.
The Pacific
Speeding across the
Pacific, the spring and summer winds pick up moisture and, approaching the
coast at an angle, strip the warm water from the surface and brings the frigid
to the top.
Cold water exposed to
warm wet air makes a wonderfully thick fog. In summer, the Sunset and Richmond
districts in San Francisco, Monterey, Half Moon Bay and Pacifica, among others,
often look like they are buried in mountains of cotton.
The Golden Gate and the
Mountains
This fog would love to
scoot inland to the Bay neighborhoods — Bayview, Potrero Hill, Mission,
Noe Valley and others — and Bay towns such as San Mateo, Redwood City,
Burlingame and Foster City.
But the coastal hills
and mountains stop or greatly impede its progress — except where there
are openings. Of the half dozen or so major gaps, the biggest is that marvelous
work of nature, the Golden Gate.
The fog shoots through
the Golden Gate in the spring and summer, visually delighting motorists on the
Bay Bridge, banging into the East Bay hills, and easing down toward San Jose,
where it takes the edge off the summer temperatures.
For most of San Mateo
County, the Santa Cruz Mountains hold back the fog — one major reason why
the weather delights. The fog takes the edge off the summer heat yet leaves the
Bay towns basking in the sun.
Where the mountains or
hills dip or flatten out, the fog penetrates. Golden Gate Park, flat,
ushers fog into Haight Ashbury and parts of the downtown. The Alemany Gap, near
Lake Merced, allows some fog into Hunters Point.
In San Mateo County, the
hills dip at Daly City and just north of Daly City. In July and August, it’s
not unusual for residents of Daly City, Colma and South San Francisco to bundle
up in sweaters and jackets.
The hills dip to a
lesser extent about the middle of Crystal Springs, near the City of San Mateo,
which will get some of the cooler ocean air. South San Francisco and Brisbane
are located side by side. Brisbane is tucked behind San Bruno Mountain. South
San Francisco is more exposed to ocean winds so it will get more fog.
The sun dances in here.
On many days it burns the fog away by 2 or 3 in the afternoon. But come night,
the cold and the fog often assert themselves.
By day, the radio tower
atop Mt. Sutro appeals visually to no one. At night, the hills for a few hours
will hold back the fog, and all of a sudden, surrender. Great clouds will billow
around the tower and down into the valleys, and the upper antennas, with their
guy wires, will seem like the masts of a sailing ship making its way across the
heavens.
The Central Valley
Also known as the San
Joaquin Valley. Located about 75 miles inland, the Central Valley is influenced
more by continental weather than coastal. In the summer, this means heat.
Hot air rises, pulling
in cold air like a vacuum. The Central Valley sucks in the coastal air through
the Golden Gate and openings in the East Bay hills, until the Valley cools.
Then the Valley says to the coast: no more cool air.
With the suction gone,
the inland pull on the ocean fog drops off, often breaking down the
fog-producing apparatus and clearing San Francisco and the coastline. Coast residents
enjoy days of sunshine. Meanwhile, lacking the cooling air, the Valley heats up
again, creating the vacuum that pulls in the fog.
This cha-cha between
coast and inland valley gave rise to the Bay Region’s boast of “natural air
conditioning.” In hot weather, nature works to bring in cool air; in cool
weather, she works to bring in heat.
The Mountains and
Pacific Again
In the winter, great
banks of tule fog often form in the Central Valley and chill the air. The
Pacific Ocean in the winter holds the heat better than the land and, when not
raining, often settles balmy weather along coast and Bay cities — another
major reason why San Francisco and San Mateo enjoy a mild climate.
Recall that cold moves
toward heat, much as if heat were a suction vacuum. The Central Valley fog
would like to move into the Bay but is blocked by the mountain range running up
the East Bay. Occasionally, however, Central Valley fog will penetrate through
its openings, foremost the Carquinez Strait near Vallejo, and work down into the
Bay — a perilous time for shipping.
Coastal fog often forms
well above the Pacific and, pushed by the wind, generally moves at a good clip.
In thick coastal fog, you will have to slow down but you can see the tail
lights of a car 50 to 75 yards ahead. In valley or tule fog, you sometimes can
barely make out your hood ornament. This winter fog blossoms at shoe level when
cold air pulls moisture from the earth. When you read of 50- and 75-car pileups
in the Central Valley, tule fog is to blame. Dust storms are another culprit in
the Valley.
Within the Bay, tule
fog, before radar, was often responsible for shipping accidents, including the
1901 sinking of the liner, Rio de Janeiro, 130 lives lost.
On rare days, tule fog
will settle over San Francisco Airport, making takeoffs and landings risky. On
the ocean side of the county, Half Moon Bay Airport will be basking in the sun.
In late 1995, a patch of fog settled over the runway at San Francisco Airport
and grounded flights; the control tower and terminals were fog free.
Mountains and Rain
Besides blocking the
fog, the hills also greatly decide how much rain falls in a particular
location. Many storms travel south to north, so a valley that opens to the
south (San Lorenzo-Santa Cruz) will receive more rain than one that opens to
the North (Santa Clara-San Jose.)
When storm clouds rise
to pass over a hill, they cool and drop much of their rain. Some towns in the
Bay Region will be deluged during a storm, while a few miles away another town
will escape with showers.
That basically is how
the weather works in the Bay Area but, unfortunately for regularity’s sake, the
actors often forget their lines or are upstaged by minor stars.
Weather Tidbits: The
’Stick
Why the Giants moved.
Candlestick Park lies at the junction of two wind streams that shoot through
the Alemany Gap in the hills. The result: eddies and vacuums that did circus
tricks with fly balls. The new stadium, sheltered better by the hills, does a
better job of keeping out the fog. As for the Forty Niners, they play in fall
and winter, by which time the summer fog and winds are long gone from
Candlestick Park, the sun back in fine form.
Swimming
September and October
are the best months to swim in the Pacific. The upwelling of the cold water has
stopped. Often the fog has departed. Sunshine glows upon the water and the
coast. Almost every year the summer ends with hot spells in September and
October. Be careful of the ocean coast; treacherous rip tides.
Sunshine
Like sunshine? You’re in
the right place. Records show that during daylight hours the sun shines in New
York City 60 percent of the time; in Boston, 57 percent; in Detroit 53 percent;
and in Seattle, 43 percent.
Atop Mt. Tamalpais (in
Marin County) the sun shines 73 percent of the year. San Jose averages 63
percent. San Mateo and San Francisco ... well, it depends on where you’re
located. But 60 percent plus is a realistic figure.
Humidity
When hot spells arrive,
the air usually has little moisture — dry heat. When the air is moist
(the fog), the temperatures drop. On rare days, you will notice the humidity but it is nothing like what's found on the East Coast.
Redwood Drizzle
If you are planning a
redwoods excursion to Big Basin or Muir Woods in the summer, bring a jacket and
an umbrella. Redwoods are creatures of the fog, need it to thrive. Where you
find a good redwood stand, you will, in summer, often find cold thick fog.
When fog passes through
a redwood grove, the trees strip the moisture right out of the air. In some
parts of the Bay Region redwood-fog drip has been measured at 10 inches
annually.
One of the editors
recalls attending a summer picnic in a redwood grove. We arrived about 11 a.m.
to find the air cold and the drip heavy enough to soak our clothes. Parents
bundled up children, plastic tablecloths were used as rain jackets. Two hours
later, we were playing softball under a hot sun.
