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Old 13-02-2022, 00:43   #1
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Can anyone explain the katabatic fall winds on the leeward side of the Greek Islands

I am trying to work out why some places with steep cliff type shores have these deadly winds colliding with the water sending gusts in all directions and other similar shores do not and instead have a dead wind zone several miles from the shore.
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Old 13-02-2022, 02:53   #2
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Re: Can anyone explain the katabatic fall winds on the leeward side of the Greek Isla

A katabatic wind is any wind blowing down an incline; the opposite of anabatic wind [flowing up slope].

If the wind is warm, it is called a foehn or chinook; if cold, it may be a fall wind [bora], or a gravity wind [mountain wind, ‘Santa Anna’];
Katabatic winds generally occur at night, when the highlands radiate heat and are cooled. The air, in contact with these highlands, is thus also cooled, and it becomes denser than the air at the same elevation, but away from the slope; it therefore begins to flow downhill [due to gravity]. This process is most pronounced in calm air, because winds mix the air, and prevent cold pockets from forming.
https://www.weatheronline.co.uk/repo...atic-winds.htm
https://www.yachtingmonthly.com/sail...wind-not-62826
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Old 13-02-2022, 03:35   #3
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Re: Can anyone explain the katabatic fall winds on the leeward side of the Greek Isla

A simplistic sailorman view.
The form you describe is same-same a standard Patagonian 'Racha'.

An example - the western end of Estrecho de Magallanes is flanked to the south west by Isla Desolacion which is a substantial island rising to 300 or so metres and a hundred miles or so long.

When a hard south westerly is blowing in the Southern Ocean a mass of cold dense air finds itself up against Isla Desolacion - it can't go left - can't go right - must go over.
Having gone over the top of the island which is steep to on the lee side - it finds itself in a Coyote-v- Roadrunner situation. Pockets of heavy dense air find themselves some distance off shore and at altitude before they realise they are now in a less dense air mass - and so they drop - like a bomb - and explode in all directions when they reach sea level.
Nasty buggers.
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Old 13-02-2022, 10:17   #4
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Re: Can anyone explain the katabatic fall winds on the leeward side of the Greek Isla

Thanks for these explanations, Gord and El Pinquino. However I imagine that the Greek Islands Meltimi does not have the cold element, it just has the sustained speed and direction. So I always wonder why it reacts so inconsistently on the leeward sides…sometimes the leeward sides are steep and cliff like, resulting in an almost instant vertical downwards wind that smashes into the water meters from the land in dangerous gusts or, if the slope is more gentle, results in a wind speed at least 5 kts more than the windward side….this i guess I could accept as the downward slope could give the wind an acceleration I guess. Or it simply results in a leeward side wind shadow for some distance off the coast.
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Old 13-02-2022, 10:46   #5
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Re: Can anyone explain the katabatic fall winds on the leeward side of the Greek Isla

So it does have the cold element in the Greece and the Med. In October 2019 I dragged anchor in Vathy on Ithaki in a 40 knot wind that was cold at night and I had a nice black bora this past summer in the northern Adriatic with hail, 45 knot wind and definite cold during the mid day. It may not always have noticeable cold but as once explained to me it is cold air that falls and causes the wind.

I may have this totally wrong but as I understand it. Clouds over mountains create shade that cools the air. The cold air falls down the mountains and out over the water. Warm air over the water rises as the cold air falls and fills in where the warm air was. Croatians talk about the difference between a white bora and a black bora. The white bora has no rain and is great for drying meat. The black bora has rain that is not good for that.

This may help
https://www.sailingissues.com/meltemi.html
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Old 13-02-2022, 17:33   #6
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Re: Can anyone explain the katabatic fall winds on the leeward side of the Greek Isla

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Originally Posted by Fuss View Post
Thanks for these explanations, Gord and El Pinquino. However I imagine that the Greek Islands Meltimi does not have the cold element, it just has the sustained speed and direction. So I always wonder why it reacts so inconsistently on the leeward sides…sometimes the leeward sides are steep and cliff like, resulting in an almost instant vertical downwards wind that smashes into the water meters from the land in dangerous gusts or, if the slope is more gentle, results in a wind speed at least 5 kts more than the windward side….this i guess I could accept as the downward slope could give the wind an acceleration I guess. Or it simply results in a leeward side wind shadow for some distance off the coast.
Its important to remember when discussing weather caused by temperature differentials that "cold" is only relative to the air around it. So in Greece in the summer 85 degrees F could be "cold" compare to the 95 degree air it is displacing.
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Old 14-02-2022, 04:03   #7
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Re: Can anyone explain the katabatic fall winds on the leeward side of the Greek Isla

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Originally Posted by redneckrob View Post
Its important to remember when discussing weather caused by temperature differentials that "cold" is only relative to the air around it. So in Greece in the summer 85 degrees F could be "cold" compare to the 95 degree air it is displacing.

Yes, this is probably true that there is a temperature differential and this is causing the down gusts.

But what I don't understand is that with the Meltemi coming from the north and the leeward side on the south of the islands, why is the Meltemi not warmed as it passes over the land making the temperature differential in favour of long dead wind zones on the leeward side and not vertical downward gusts.
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