Re: Sherman AM report

From: JMILUM.US.ORACLE.COM (JMILUM@us.oracle.com-DeleteThis)
Date: Tue Jul 15 1997 - 15:43:47 PDT

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Date: 15 Jul 97 15:43:47 -0700
From: "JMILUM.US.ORACLE.COM" <JMILUM@us.oracle.com-DeleteThis>
To: wind_talk@opus.hpl.hp.com-DeleteThis
Subject: Re: Sherman AM report
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Wow! I never heard that explanation before, but it sounds better than any
others I have heard.

Taking out the Pacific Ocean of the equation is probably good, since I do not
believe that it really heats up a significant amount during the day.

What is people's take on the LACK of wind on the peninsula over the last 4 or
5 days? It looks from my window that the fog line, and temperature are right
where they are on a lot of nice windy days.

Thanks.

Jeff Milum
Regional Manager DMD New England
415-506-0575
jmilum@us.oracle.com-DeleteThis

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Date: 15 Jul 97 14:23:32
From:"pierre@interval.com-DeleteThis (Pierre St. Hilaire)" <listserv@jr.hpl.hp.com-DeleteThis>
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Subject:Re: Sherman AM report
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O.K. Here's my take on it:

The short answer is catabatic flow. The long answer:

Let us start in the morning.

1 -- The central valley heats up, creating an area of low pressure (since
hot air rises up).

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 valley
starts to cool and the wind backs off somewhat 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 this flows usually stops by sunset, because the
mountain area along the southern part of the bay is relatively small and
the south bay is wide. 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.

7 - After sunrise the catabatic flow stops because the mountains heat up
again. The central valley also heats up, but this is apparently not enough
to counteract the loss of cooling wind. The wind thus backs up by
mid-morning at Rio.

8 - go to 3

So the wind at Rio is due to 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 give the
evening wind.

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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