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Old 26-02-2018, 18:29   #1
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Wind shadow

Many of the eastern Caribbean islands have big wind shadows. With Guadeloupe the upper winds are often totally cut off such that you can have a sea breeze blowing in the opposite direction from the gradient wind.

How far out does the wind shadow go? Is there a rough calculator for it? I realise it will depend on topography.
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Old 26-02-2018, 19:15   #2
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Re: Wind shadow

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How far out does the wind shadow go? Is there a rough calculator for it? I realise it will depend on topography.
One rule of thumb - because the size and shapes of hills and mountains and the wind speed vary obviously - is that the island lee phenomenon diminishes around 9h to 10h (where h is the height of the geographical feature in question.

My sources are:

Bruce van Sant, Tricks of the Trades; and

Alan Watts, Wind and Sailing Boats: The structure and behaviour of the wind as it affects sailing craft.

Watts devotes considerable space to considering the local affects of cliffs, trees, and so on to waterways with an eye on local knowledge for racing.

Van Sant is my source for the term 'island lee'. Van Sant, who wrote several books on how best to sail a Thorny Path, suggested that when sailing to weather along a Thorny Path, a sweet spot often exists around 10h in the lee of a high island.
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Old 26-02-2018, 21:32   #3
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Re: Wind shadow

Good to have a specific distant like 9h, Alan. Thanks.
Before my first time sailing in the lee of those islands, I looked at the windtv.com model and PredictWind models, and watched the model as wind speed and direction changed. The shadows were further in the curvature greater than I would’ve thought, and when I actually sailed there I found the shadow to be out and about as far as the models predicted.

There’s also the possibility of picking up currents, either fowl or fair, in the lee of the Windward and the Leeward Islands. Check out the windtV.com maps and you’ll see what I’m talking about
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Old 26-02-2018, 23:42   #4
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Re: Wind shadow

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Originally Posted by Alan Mighty View Post
One rule of thumb - because the size and shapes of hills and mountains and the wind speed vary obviously - is that the island lee phenomenon diminishes around 9h to 10h (where h is the height of the geographical feature in question.

My sources are:

Bruce van Sant, Tricks of the Trades; and

Alan Watts, Wind and Sailing Boats: The structure and behaviour of the wind as it affects sailing craft.

Watts devotes considerable space to considering the local affects of cliffs, trees, and so on to waterways with an eye on local knowledge for racing.

Van Sant is my source for the term 'island lee'. Van Sant, who wrote several books on how best to sail a Thorny Path, suggested that when sailing to weather along a Thorny Path, a sweet spot often exists around 10h in the lee of a high island.
Interesting he puts a figure on it. The mountains in Guadeloupe are around 1000m high, so 10 times thar is 10km or roughly 5nm. I an not sure if that figure is from the mountain top or from the shore, but it isn’t far enough in my experience even from the shore.

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Good to have a specific distant like 9h, Alan. Thanks.
Before my first time sailing in the lee of those islands, I looked at the windtv.com model and PredictWind models, and watched the model as wind speed and direction changed. The shadows were further in the curvature greater than I would’ve thought, and when I actually sailed there I found the shadow to be out and about as far as the models predicted.

There’s also the possibility of picking up currents, either fowl or fair, in the lee of the Windward and the Leeward Islands. Check out the windtV.com maps and you’ll see what I’m talking about
That’s an interesting representation and closer to what I would expect. A long way, as I have not found the boundary yet.The website is windy.com not as you quoted, presumably a typo.
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Old 27-02-2018, 02:18   #5
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Re: Wind shadow

Similar data from a different application:

“... Regarding the height (h) of the dune the wind flow will be perturbed on a distance of four to ten times “h”...
... Same thing for buildings. After impact the air flow will be perturbed on a distance of seven times the height of the building...”

Overcoming wind shadow « Ras Al-Khaimah


I expect wind energy designers will have explored this phenomenon in depth.
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Old 27-02-2018, 04:24   #6
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Re: Wind shadow

Ha. Yes, thanks to autocorrect I see the website is misspelled and the sea current has gone to the birds.

We can also add Fastseas.com and www.PredictWind.com to the sites with models
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Old 27-02-2018, 06:16   #7
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Re: Wind shadow

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Originally Posted by GordMay View Post
Similar data from a different application:

“... Regarding the height (h) of the dune the wind flow will be perturbed on a distance of four to ten times “h”...
... Same thing for buildings. After impact the air flow will be perturbed on a distance of seven times the height of the building...”

Overcoming wind shadow « Ras Al-Khaimah


I expect wind energy designers will have explored this phenomenon in depth.
I can only think that buildings and sand dunes have a different dynamic to islands. The wind shadows of tall islands are much bigger than 10 times height.
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Old 28-02-2018, 12:52   #8
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Re: Wind shadow

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I can only think that buildings and sand dunes have a different dynamic to islands. The wind shadows of tall islands are much bigger than 10 times height.
Hmm ... methinks that:

1. you are using windy.com (which formerly was windyty.com and windytv.com) rather than direct observation to test the 9h - 10h rule of thumb. If that be the case, you need to be aware that windy.com uses numeric models, either GFS or the European model. Those models have granular resolution. When using the European model, the grain size or resolution is about 4.8 nautical miles. When using GFS, the resolution is around 13 nautical miles.

2. you have approximated the peak on Guadalope and the conversion from metres to nautical miles. Let's say the peak on G is about 1.4 km. Then 10h would be about 14 km or about 7.5 nautical miles.

3. 9h - 10h is a rule of thumb. The atmosphere is chaotic. But we see patterns in it and create rules of thumb.
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Old 28-02-2018, 16:41   #9
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Re: Wind shadow

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Hmm ... methinks that:

1. you are using windy.com (which formerly was windyty.com and windytv.com) rather than direct observation to test the 9h - 10h rule of thumb. If that be the case, you need to be aware that windy.com uses numeric models, either GFS or the European model. Those models have granular resolution. When using the European model, the grain size or resolution is about 4.8 nautical miles. When using GFS, the resolution is around 13 nautical miles.

2. you have approximated the peak on Guadalope and the conversion from metres to nautical miles. Let's say the peak on G is about 1.4 km. Then 10h would be about 14 km or about 7.5 nautical miles.

3. 9h - 10h is a rule of thumb. The atmosphere is chaotic. But we see patterns in it and create rules of thumb.
I have tested the theory in person as you will see me say in my post. Windy.com is I believe accurate. Not just from the distance, but the shape of the shadow. Bang on probably.

Secondly that 1400m is a tiny pointy peak. The bulk of the central island land is well below 1000m. 1000m is already a conservative figure to represent height.

I have been as much as 12 miles out from the shore, not the summit, so again conservative and can report from multiple trips, it is in the wind shadow centre island.
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