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TLD



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Topic # 230600 4-Mar-2018 10:19
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The occasional mention of foehn winds on the weather segment of the six o'clock news has always fascinated me, and I am still not entirely sure how they work.  I understand that the air cools as it is lifted over high mountains and gives up its moisture as rain, but it's the warming effect of coming down the other side of the mountain I have not got a handle on.  This article on the BBC Science News talks about foehn winds in Antarctica and how they creating large ponds of melted ice.  This is the first time I have heard about this, although the link was branched from a more current story about a failed trip to study the waters under the huge 'A68' iceberg that broke away from the Antarctic ice shelf July last year (goodness, was it really that long ago?!)

 

http://www.bbc.com/news/science-environment-39759329

 

 

Listen to the radio discussion if you have ten minutes.  It's fascinating stuff, and helped me get a bit closer to understanding how foehn winds work.

 

 





Trevor Dennis
Rapaura (near Blenheim)

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  Reply # 1967804 4-Mar-2018 10:53
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Interesting stuff. We get them here in ChCh, the norwester. The release of rain at high altitudes when it reaches he mountains, then allows that air to be warm as its now drier. On the way down, air pressure increases which warms it up, and in our case here, the air is heated up further by travelling across the Canterbury Plains which are now cloud free. Not a pleasant wind, its like being in a fan heater. But the towels you just hung out will be dry in half an hour. The winds are strong too, which gives us the reverse of the chill factor.  


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  Reply # 1968373 5-Mar-2018 18:20
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Most likely related to the latent heat of water. Heat is released when water vapor condenses into liquid water. Approx 0.6KW/Hr for each litre of water.





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  Reply # 1968379 5-Mar-2018 18:34
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Aredwood: Most likely related to the latent heat of water. Heat is released when water vapor condenses into liquid water. Approx 0.6KW/Hr for each litre of water.

 

Yup.

 

It's moisture laden air on the windward side of the mountains dropping moisture which condenses as rain as the pressure drops as the air goes up, then temperature increasing as the air drops after going over the summit, so it's lost water and the temperature has risen, relative humidity is now much lower.

 

If there was no condensation, then the air temp would drop as it went over the mountain, then only heat back up to the original temperature as it dropped back down the other side.

 

In Chch, as well as the heat, these Nor'Westers come in with an approaching low from the Tasman, usually with fairly strong winds.  That tends to create rolling waves as it goes over the top of the hills.  Glider pilots love the updrafts etc, but it can be a hell of an uncomfortable descent into Chch on an airliner when there's a howling Nor'Wester.  OTOH commercial pilots I know have said they really enjoy it (as well as flying to Wellington) because of the fun factor.  As a passenger, I don't always enjoy their enthusiasm.


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  Reply # 1968385 5-Mar-2018 19:17
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Aredwood: Most likely related to the latent heat of water. Heat is released when water vapor condenses into liquid water. Approx 0.6KW/Hr for each litre of water.

 

Yes, water is key. So much of it, a degree makes a difference. Unlike land, that heat can be stored. Like my solar HW lol, but in fact it is serious. Gita could never come here but it did due to water energy. Arctic and Antarctic are being pressured with warmth. The issue is the Earth has severe cold, and cold, and temperate and sub tropical and tropical. Many regions. If that became two regions of  cool and warmer, thats an issue. 


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  Reply # 1968481 5-Mar-2018 20:36
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Over the Southern Alps, in the west the wet air cools at the wet adiabatic lapse rate as it's lifted over the alps, but warms at the dry rate on the eastern side as it descends, gaining 10deg or so.


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