Received: from hplms26.hpl.hp.com by opus.hpl.hp.com with ESMTP (1.37.109.18/15.5+ECS 3.3+HPL1.1) id AA256663203; Wed, 16 Jul 1997 15:46:43 -0700 Return-Path: <pierre@interval.com-DeleteThis> Received: from fred.interval.com by hplms26.hpl.hp.com with ESMTP (1.37.109.16/15.5+ECS 3.3+HPL1.1S) id AA106313204; Wed, 16 Jul 1997 15:46:44 -0700 Received: from interval.interval.com (interval.interval.com [199.170.107.10]) by fred.interval.com with ESMTP id PAA11940 for <wind_talk@opus.hpl.hp.com-DeleteThis>; Wed, 16 Jul 1997 15:42:57 -0700 (PDT) Received: by interval.interval.com id PAA05572; Wed, 16 Jul 1997 15:42:56 -0700 (PDT) Message-Id: <v02130507aff2f309fade@[199.170.106.91]> Content-Type: text/plain; charset="us-ascii" Date: Wed, 16 Jul 1997 15:50:16 -0700 To: wind_talk@opus.hpl.hp.com-DeleteThis From: pierre@interval.com-DeleteThis (Pierre St. Hilaire) Subject: Why it blows at Sherman (second take)
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.
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