Received: from hpmsd3.sj.hp.com by opus.hpl.hp.com with SMTP (1.37.109.18/15.5+ECS 3.3+HPL1.1) id AA018370272; Thu, 17 Jul 1997 07:37:52 -0700 Return-Path: <kirk@hpmsd3.sj.hp.com-DeleteThis> Received: from hpoclrf.sj.hp.com by hpmsd3.sj.hp.com with SMTP (1.38.193.4/15.5+IOS 3.22) id AA13347; Thu, 17 Jul 1997 07:37:48 -0700 Sender: kirk@hpmsd3.sj.hp.com-DeleteThis Message-Id: <33CE2E3C.425C@hpmsd3.sj.hp.com-DeleteThis> Date: Thu, 17 Jul 1997 07:37:48 -0700 From: Kirk Lindstrom <kirk@hpmsd3.sj.hp.com-DeleteThis> Organization: Sending from inside Hewlett Packard X-Mailer: Mozilla 3.0 (X11; I; HP-UX A.09.07 9000/735) To: wind_talk@opus.hpl.hp.com-DeleteThis Subject: Re: Why it blows at Sherman (second take) References: <v02130507aff2f309fade@[199.170.106.91]> Content-Type: text/plain; charset=us-ascii Content-Transfer-Encoding: 7bit
Interesting Pierre.
Comments (from my understanding):
1) Observation: Wind at Sherman and San Luis seems to only blow all
night when the marine layer is thick and on the hills.
Your model says we should get night wind whenever the sun goes down and
the air over the mountains cool. I don't observe this.
2) Physics....no such thing as suction (when explaining forces). Wind
is due to air in high pressure areas rushing into low pressure areas.
The force is the high pressure. I have a mental flaw that doesn't allow
me to give proper credance to otherwise excellent work when there are
vital flaws in the physics.
3) Guesswork: I believe the marine layer provides something like a
constant source that "pumps" or maintains the high allowing a constant
flow (or fairly constant flow as we observe compared to gusty
conditions) from the high to the very large low in the Central Valley.
4) More ???...The Penninsula is so narrow that I don't think it can
store that much energy since simple observation shows clouds traveling
about 20 knots or more West to East which would sweep most hot air over
the Bay in under an hour.
Also, shouldn't it get REALL windy in Los Gatos and other areas near HWY
17 since the mtn range is much wider here than at 3rd so much more
energy can be stored? I believe the winds at Lexington are only mild.
Finially) Great Discussion! Wind "sucked" yesterday at Coyote. Speed
and direction all over the place until it completely died. Sailed
6.5/285 for about an hr. Tough in lulls but way too bumpy to take
bigger 9'6" Course Board.
Kirk (some sucks are good and others are not) out
230#
>
> After good suggestions from Zeev and Brad Frazee <bradf@itsa.ucsf.edu-DeleteThis> I
> decided to rework the essay taking their ideas into account. I think the
> new model is more accurate, albeit a bit more complicated.
>
> Why Rio cranks at night and 3rd Av. dies at 5
>
> Let us start in the morning.
>
> 1 -- The central valley heats up, creating an area of low pressure (since
> hot air rises up). By early summer, a quasi stationary area of low
> pressure exists in the central valley because it is so large and surrounded
> by mountains. This is called a thermal low in the weather synopsis. The
> Santa Clara valley also heats up and creates its own low pressure area in
> the afternoon, but this dissipates at night.
>
> 2 - The Bay area also heats up, and is surrounded by mountains. There is
> also a large area of high pressure that sits offshore. two air flow
> currents start: the first one sucks air from the ocean and bay to the
> Central Valley and Santa Clara Valley - As the day progresses this wind
> pattern gives us the familiar wind at the Peninsula sites. The other flow
> pattern is less obvious to windsurfers: As the air around the Bay heats
> up, it also flows up all those mountains that surround us (to the delight
> of hang gliders). There is room for a lot of air along those slopes, as a
> simple inspection of the surrounding topography reveals.
>
> 3 - By mid to late afternoon (around 5 pm at 3rd av) the central and Santa
> Clara valleys starts to cool and the wind backs off somewhat in the
> Peninsula because of the reduced pressure gradient. Tea time.
>
> 4 - But all of that air that has been flowing up the hills also cools, and
> as it cools and gets heavier it slushes down the mountain side. This is
> called the catabatic flow (also called cooling wind). It can be quite
> strong in the mountain canyons.
>
> 5 - All that air flowing down the mountains makes it to the Bay
> eventually. Where can it go? Certainly not towards the ocean because of
> the high pressure. So it flows across the two only exit points, the Santa
> Clara Valley and Rio Vista. This is what gives us the "workingman's shift"
> at 3rd between 5:30 and 7 pm. It is most pronounced at Candlestick, where
> the air flow down the San Bruno Mountain often gives excellent late day
> sailing. This catabatic flow is very laminar, which is why the late day
> wind is usually more constant (and cooler). This is the late afternoon
> wind at Rio.
>
> 6 - In the Peninsula the catabatic flow usually stops by sunset, because the
> mountain area along the southern part of the bay is relatively small.
> Since the thermal low in the south Santa Clara valley also dissipates at
> night, the wind usually dies by early evening.
>
> The situation is quite different in the North Bay, where the Napa and
> Calistoga valleys can store large amounts of air i.e. potential energy .
> Because of this topography, and the narrow passageway for the wind exiting
> the Carquinez Strait, the catabatic wind flows all night at Rio. The
> persistence of the low in the central valley contributes as well to the
> sustained night winds.
>
> 7 - After sunrise the catabatic flow stops because the mountains heat up
> again. The North Bay also acts as a kind of antechamber to the Central
> Valley and gets itself quite hot, probably diminishing the pressure
> gradient over the Carquinez Strait by mid-day (this was suggested by Brad).
> The central valley also heats up, but this is apparently not enough to
> overcome the previous two effects. The wind thus backs off by mid-morning
> at Rio
>
> 8 - go to 3
>
> So three factors seem to determine the pattern of wind at Rio:
>
> 1 -solar energy that is stored as gravitational potential energy in the
> mountains surrounding the north Bay, Napa, and Callistoga during the day.
> At night this energy gets released and gives the evening wind.
>
> 2 - The low pressure through in the Central Valley is quasi permanent in
> the summer, thus a pressure gradient exists even at night.
>
> 3 - The North Bay might heat up enough during the day to diminish the
> gradient across the Carquinez Strait from late morning to late afternoon.
>
> A similar pattern happens at Lago the Garda in the mountains of northern
> Italy. There you also have morning and evening winds.
>
> Pierre St Hilaire
> Interval Research Corp.
--"There is more to life than increasing its speed." - Gandhi +----------------------------------------------------------------------+ | Kirk Lindstrom - CSSD Product R&D | Hewlett-Packard Co. M/S: 90UA | | Engineer/Scientist, Hardware | Communication Semiconductor | |------------------------------------| Solutions Division | | kirk_lindstrom@sj.hp.com-DeleteThis | | | Kirk Lindstrom / HP0100/UX | 370 W. Trimble Rd. | | ph 408 435 6404 | fax 408 435 6286 | San Jose, CA 95131-1096 | +----------------------------------------------------------------------+
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