Re: Back here, at 40°C
- Subject: Re: Back here, at 40°C
- From: &* A* O*
- Date: Thu, 02 Aug 2001 16:38:01 -0700
At 08:32 AM 8/2/01 -0700, Jason D wrote:
>Here in the San Francisco Bay Area, we've had a string of at least three
>weeks of foggy nights, overcast mornings, and cool days - a July pattern
>that may be typical but always feels a bit gloomy. Of course, without it,
>our redwoods wouldn't survive until the next rainy season. We take
>whatever chance we can get to escape to warmer zones, some a half an hour
>drive inland, or up a mountain, above the chilly marine layer.
This caused me to recall when Allesandra came to San Francisco last year in
September to speak at our Gardening Under Mediterranean Skies II
Symposium. She was curious to see "this San Francisco Fog" and found she
was not disappointed, even on her first night!
Many people are perplexed about why we get this fog. It has as much to do
with the complex topography of the Bay Area as it does the Pacific Coast
waters to our west. The fog is created by the relatively cool waters of
the Pacific and the warmer summer air temperatures. It often 'hangs about'
off the coast for weeks (months?) at a time. The reason it invades the
land is due to the heating up of our very large central valley. See the
following satellite image:
http://tornado.sfsu.edu/geosciences/California_WebPages/PointReyesEddy.jpeg
This show fog lying off the coast as well as some topographical indication
of the central valley. Note that the only opening out of the central
valley is through our delta which exits our the Golden Gate.
When the interior valley heats up, that mass of air rises. The vacuum
caused by this rising air pulls air in from somewhere else - the only
available place is the Golden Gate. This pulls the fog onshore and into
the delta. Some of our local areas (San Francisco, Berkeley, Richmond,
etc.) lie right in the path of this pull and therefore tend to end up with
foggy days during this effect. If the fog invades the interior valley
enough to have a cooling effect, then this lessens the 'pull' and there
fore the fog effect. Ultimately the valley will warm again and start the
whole thing over.
If there is a warm inversion layer, the fog is kept low and to only the
openings into the interior. One of our favorite sightings is on a sunny
day when the fog is entering through the Golden Gate and maintaining a
distinct 'tunnel' across the bay and into the delta. You can see it for
miles and drive in and then out of it. If the inversion layer is less
successful, the fog can spill over the coastal mountains like some huge
slow-motion wave. All of this usually happens when there is warmer weather
so depending upon where you are that day, you could either want to go in
short sleeves or in your polar fleece!
A view of the fog coming into the Golden Gate (viewed from the Bay side):
http://www.aerialimages.com/fineart/scenic/Golden_Gate/Golden_Gate_3.jpg
This is what makes the Bay Area so livable - the mitigating effect of the
fog. Without it, we'd be much hotter and much drier. While all of the
above is happening, many of the interior area are suffering through intense
heat that drive people indoors to their air-conditioners!
Meanwhile, My garden is loving these moderate days (we get the cooling
effect but little of the actual fog itself), especially after numerous
record-setting heat waves we have this spring and early summer!
Seán O.
h o r t u l u s a p t u s - 'a garden suited to its purpose'
Seán A. O'Hara fax (707) 667-1173 sean@support.net
710 Jean Street, Oakland, CA 94610-1459, U.S.A.
