Received: from hplms26.hpl.hp.com by jr.hpl.hp.com with ESMTP (1.37.109.18/15.5+ECS 3.3+HPL1.1) id AA126824807; Tue, 15 Jul 1997 15:13:28 -0700 Return-Path: <Zeev_Gur@peoplesoft.com-DeleteThis> Received: from waltz.worldcom.com (waltz.interliant.com) by hplms26.hpl.hp.com with ESMTP (1.37.109.16/15.5+ECS 3.3+HPL1.1S) id AA149824806; Tue, 15 Jul 1997 15:13:26 -0700 Received: (from smtp@localhost) by waltz.worldcom.com (8.8.5/Interliant) id RAA04614 for <wind_talk@jr.hpl.hp.com-DeleteThis>; Tue, 15 Jul 1997 17:08:00 -0500 (CDT) Received: from internet-504.interliant.com(198.64.193.30) by waltz.worldcom.com via smap (V1.3) id sma004588; Tue Jul 15 17:07:58 1997 Received: by Internet-504.interliant.com(Lotus SMTP MTA v1.1 (385.6 5-6-1997)) id 862564D5.007AC200 ; Tue, 15 Jul 1997 17:20:50 -0500 X-Lotus-Fromdomain: PEOPLESOFT@PSEXTERNAL @ WORLDCOM@INTERLIANT @ OUTBOUND From: "Zeev Gur"<Zeev_Gur@peoplesoft.com-DeleteThis> To: wind_talk@jr.hpl.hp.com-DeleteThis Message-Id: <862564D5.007ABE0D.00@Internet-504.interliant.com-DeleteThis> Date: Tue, 15 Jul 1997 15:04:16 -0700 Subject: Why sherman blows am/pm Content-Type: text/plain; charset=us-ascii
Here is what my water logged brain recalls:
In an ideal setup, high pressure off the coast, hot inland,
big ass fog bank up and down the coast.....As the temperatures
go up in the valley (during the day), this creates a themal low
pressure in the central valley. The pressure gradient from the
ocean's high pressure and the thermal starts the sucking, not
blowing, although both are fun if you are the recepient....
Air always moves from high pressure to low. So we have
all this suction from the valley. The cool/fog laden air out
in the ocean has several gaps in the coastal hills to go through,
which effectively creates wind tunnels.
If you look at a topographic map of the bay area, there is a cool
path from the GGate to the delta created by Mt. Diablo and the
small hills on the other side of the river at the delta. This explains
the why it blows there and not say in Walnut Creek.
Why the pm/am effect? Here is my theory.....as the day heats
the pressure gradient increases until the suction gets to be too
much and the cool air starts to penetrate. This typically starts
around 4-5pm. The best wind is always
at the front of the cool moist air that is reaching for the low pressure.
All night long the cool air is reaching for the valley in search
of making the pressure be the same as on the coast. This process,
suction and making the pressure the same, takes all night and a big
part of the am. The wind dies off as the pressure gets to be about
the same in the valley as it is on the coast.
The reason we have all day blows sometimes, and who does not like
an all day blow ;-)?, is that by the time the pressure starts to get
equalized by the cool air comming from the coast to the valley, the
heating has already created another pressure gradient.
That is enough of my 'food-coma' induced rambling. It is really quite
simple: big fog bank on the coast/over city/bay, hot inland, 30 degree
difference from coast to valley typically means killer delta days (from
thermals.......If a low pressure comes in when you have a lot of fog, this
also lets the fog get over the coastal range and rush inland, where it
gets squeezed by the land mass and creates killer delta conditions.)
The latter being more common in the Sept/Oct. time frame......
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