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Wind in troughs vs crests

Gargleblaster

Well-known member
Joined
16 Dec 2003
Messages
1,143
Location
Medway, Gillingham Reach
I noticed while double crossing the Atlantic this year and encountering many lows with winds off F8 or above [my logs shows 9], that the wind in the troughs was stronger than the wind on the crests of waves once the barometer had started to rise again and while the wind was over F8. It may have been the fact that the biggest waves are always on the back of a depression also.
I am interested if anyone is aware of any research that may prove or disprove this fact. The thing that I find interesting is trying to work out 'why is it so?' The only logical explanation is that the wind is being driven by the waves in the troughs whereas it is already starting to die on the crests.
 

Noddy

Member
Joined
22 Jun 2005
Messages
621
Location
Thames Estuary
Could it be that you were accelerating down into the troughs and deccelerating up the side of the next wave??
 

Gargleblaster

Well-known member
Joined
16 Dec 2003
Messages
1,143
Location
Medway, Gillingham Reach
Having dealt with waves on all point of sail, it's a good theory but my acceleration is not enough to effect the wind as much as the difference was between the troughs and crests. More often than not I was trying to make way into the waves which meant I was climbing an approaching wave which would slow me down with both steepness and increased wind and then as I hit the crest the wind would lessen.
I've been thinking more about this and wondering if it is not a function of the wind having a greater distance to travel down through the trough and has to speed up to catch up with the wind that is going straight across the top. A bit like the faster wind on the lee side of a sail. However you'd think that the curve on teh crest would have as much effect as the curve in the trough. Of course many of the crests are not curving when they are breaking.
 

Jacarah

New member
Joined
2 Aug 2007
Messages
9
Having broken autohelms and rattled booms to bits in similar circumstances I have chased up the following theory - which does make seem to make sense.
A trough between two waves is the same as an aircraft wind upside down. The wind in the trough moves faster to avoid that which natures abhors - a vacuum. The wind will always flow from high to low pressure as you are all too well aware. The faster moving wind in the trough creates a lower pressure - so the wind at the crest is a high pressure area and it moves more slowly, relatively speaking. The stronger the wind the more contrast between trough and crest. Likewise the height and distance between waves will also affect the wind speed in the trough..........allegedly.
 

andlauer

New member
Joined
15 Mar 2007
Messages
310
Location
Paris France
Bonjour
Under an aerodynamic point of vue, considering that the waves are motionless in regard to the wind speed :
- the wave may be considered as half a ventury tube or to be more prcise something more like a mountain collar.

- in a ventury tube the speed increase while the diameter decrease; So the wind speed should increase at the top of the wave.

-Under the layer limit point of view, considering the sea as a plane rugged (and rude) surface in regard to the air wind size, the wind speed should increase with the hight. The higher the top of the mast the more wind you have. Once again the wind should increase when the boat is on top of the wave.

for myself I haven't noticed the phenomena described by John, perhaps because i'm too much a scientist and I lack technical poetry.

How booring I may be!!!
Eric /forums/images/graemlins/ooo.gif
 
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