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windfind
Joined: 18 Mar 1997
Posts: 1877
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 Posted: Wed Jul 14, 2021 6:25 pm Post subject: Blog: Why have the Bay Area winds become gustier? |
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Hi Gang,
So mac, one of long-time customers, sent me the message below about the winds becoming gustier at many Bay Area launch sites.
The answer is complex but this blog shows how the increasingly prevalent eddies help make the wind gusty.
https://blog.weatherflow.com/west-coast-wind-blog-why-have-the-bay-area-sw-winds-become-gusty/
Mike Godsey
iwindsurf.com, ikitesurf.com
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mac
Joined: 07 Mar 1999
Posts: 17609
Location: Berkeley, California
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| Posted: Wed Jul 14, 2021 7:39 pm Post subject: |
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| Thanks Mike. Ironically, today was a bit steadier. Every day is different.
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WNDZRFR
Joined: 28 Mar 2000
Posts: 124
Location: Greater East Bay Area
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| Posted: Thu Jul 15, 2021 1:12 am Post subject: |
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Nice write up Mike.
So can we chalk the NNW direction to climate change? Because it sure has made us East Bay sailors happy this season!
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Kiteburner
Joined: 20 Jul 2013
Posts: 16
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| Posted: Fri Sep 17, 2021 11:32 pm Post subject: |
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| What truth is there to this..... This year, wind is being pushed in via eddy, vs more normal pulled in via pressure and temperature gradient variance.... ???
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windfind
Joined: 18 Mar 1997
Posts: 1877
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| Posted: Sat Sep 18, 2021 12:00 pm Post subject: |
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| Kiteburner wrote: | | What truth is there to this..... This year, wind is being pushed in via eddy, vs more normal pulled in via pressure and temperature gradient variance.... ??? |
Hi Kiteburner,
The concept is pretty solid but I greatly simplified things in that forum post:
Back when eddies were rare the waters off the Bay Area had mostly weak NW winds in the summer. These NW winds slowly pushed the marine layer clouds southward down the coast.*
Back then the marine layer was mostly very stable dense air since the continual NW winds created upwelling bringing very chilly water from the depths cooling the marine layer even more.
Each day when the Central Valley heated up, creating low-pressure, this marine air was "pulled" by the pressure gradient towards the Central Valley. But since it was very heavy stable air it was forced to mostly flow through gaps in the coast range especially the Golden Gate. And since it was mostly flowing over the smooth cool surface of the water it did not become very turbulent hence the winds were not very gusty.
In the last decade, the summer winds from the North Pacific High are more frequently NNW not NW out at the Farallon Islands so there is often an eddy creating southerly wind on the coast through mid-day like in the graphic below.
Then in the afternoon as the Central Valley heats and sucks this air inland there is also SW eddy winds "pushing" the marine layer clouds. This causes the clouds to stack up and flow over terrain as well as flowing through gaps in the coast range. (Somedays you can see this as more pronounced clouds building up on the Marin Headlands and all around the when Hwy. 92 gap.)
Why does this matter?
1. Marine air going over terrain mechanically creates turbulence
2. The air is less stable due to less upwelling so it is more gust prone.
3. Air going over warmer terrain heats and becomes less stable meaning more gusts.
Lastly, the mean marine layer is becoming shallower everywhere on the west coast which means less stable air and more gusts.
So the eddy is not the only culprit in the gustier winds. And of course, there are many days when there is no eddy or the eddies coast winds are more SSW to SSE which does not promote "stacking" which means we see a return to the steadier winds of the past.
There is actually a lot more to all this but I am out of time.
Mike Godsey
iw/ik
*The main exception to the NW pattern years ago was the occasional Marine Surge of strong southerly winds that brought an abrupt end to a heat wave as fog poured through the Golden Gate like this:
https://windnotes.smugmug.com/Videos-WindsurfKite/1-Gorge-Videos/i-w8Gt9Lv/A
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