UKH

White lines in the sea?

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Hoping someone can put me out of my misery here as I’m getting nowhere with google. Sometimes you see long, thin white lines snaking across the surface of the sea. Often across the mouth of a bay or similar. Anyone know what causes them? My best guess was that they maybe form between areas of water that are different temperatures or depths, but that really is a stab in the dark. 

In reply to Stuart Williams:

I think you are correct that it is where two bodies of water meet - it only occurs when it is really calm, so no mixing, caused by the wind, is going on. They could differ by temperature or salinity which both affect the water density,

Chris

 mjc1010 02 May 2021
In reply to Stuart Williams:

Couple of possible causes if they are occuring parallel to the wind direction would be due to Langmuir circulation caused by vortex's set up in the surface water by the wind alternatively due to fronts developing where there are converging surface flows causing material to gather where they meet.

 Tringa 02 May 2021
In reply to Stuart Williams:

I have also heard they can be caused by water of different salinity, either from a river or stream entering a bay, or just sometimes run off of rainwater from the land.

Dave

 Greenbanks 02 May 2021
In reply to Stuart Williams:

Jetskis. 
😆

In reply to Stuart Williams:

They're called wind-lanes.
Its where the foam and debris seems to concentrate and stretch down wind.
You can frequently see them on inland reservoirs and lakes.
In fly fishing they are important as in amongst the debris are insects that have become trapped.  The wind lanes provide a rich feeding ground for trout.
Consequently it's a good place to try and catch a fish.

 MikeR 02 May 2021
In reply to Stuart Williams:

I think they can be caused by a number of things. Tristan Gooleys book, 'How to read water' talks about them and is an interesting read.

From what I remember he was saying that they can be caused by wind, currents, or even leftover wakes from boats which can persist for a surprisingly long time under the right conditions.

In reply to Stuart Williams:

In the sea they'll be foam created by waves washing up against headlands which then gets dragged out to sea by wind and/or tidal currents.

In reply to MikeR:

Ah, I feel a fool. I’ve got a copy of that and should have thought of looking in there! Cheers

In reply to Stuart Williams:

Thanks all! So wind, currents, foam and crud  are the likely culprits. Sounds like I was overthinking it slightly. 

 Roadrunner6 02 May 2021
In reply to Chris Craggs:

This. Often the meeting of two water masses, a front in the water.

 mutt 03 May 2021
In reply to Stuart Williams:

The phenomenon is rightly called langmuir circulation. It is  caused by overturning 19 degrees from the wind direction. It's basically the same as the wind rows seen in clouds sometimes. 

 El Greyo 04 May 2021
In reply to Stuart Williams:

Are you are talking about a line of foam running approximately parallel to the coast maybe a few hundred metres or so offshore? If so then I've seen this too and have been puzzled by the cause. This is different to Langmuir circulation which is caused by wind and is a series of lines parallel to the wind direction.

The source of the foam is, I expect, from breaking waves, and then the foam likely coalesces into a line by some convergence of surface waters. But I haven't worked out what is causing that convergence, which, given I'm an oceanographer, I really feel I should know. As someone said above, it tends to form in relatively calm conditions.

 El Greyo 04 May 2021

The foam line that I have in mind (which may or may not be what Stuart is thinking about) can be seen in the video in the Longevity and Stability section of this page: https://en.wikipedia.org/wiki/Sea_foam . At the top of the image is fairly broad snaking line of foam running approximately parallel to the coast. It must be from some kind of convergence - I wonder if it is from interference of incident and reflected waves (at an antinode of long-period, low amplitude swell perhaps). As I say I haven't worked it out yet and haven't found an explanation elsewhere.

In reply to El Greyo:

Yes, that looks like what I was trying to explain. I think you are thinking of exactly the same thing. As you say, parallel to the coast, calm weather, couple of hundred metres out, long snaking line.

 Cobra_Head 04 May 2021
 El Greyo 04 May 2021
 Sherlock 04 May 2021
In reply to Stuart Williams:

> Hoping someone can put me out of my misery here as I’m getting nowhere with google. Sometimes you see long, thin white lines snaking across the surface of the sea. Often across the mouth of a bay or similar. Anyone know what causes them? My best guess was that they maybe form between areas of water that are different temperatures or depths, but that really is a stab in the dark. 

They're our new sea borders, now we've taken back control of, er, our fish.

 El Greyo 09:22 Wed
In reply to Stuart Williams:

Right, the collective wisdom of a bunch of oceanographers and coastal scientist is that no one really knows.

The source of the foam is clear - waves impacting on the cliff aerates the water creating foam. The foam drifts away from the cliff, pushed along by reflected waves and associated small, localised currents.

We then have two hypotheses as to why a snaking line of foam is formed:

The line is the convergence between the foam floating away from the cliff and Stokes drift from incoming waves (Stokes drift is a surface current caused by waves).

The line is where the foam drifting away from cliff meets a longshore current (probably tidal in the UK). The current drags the foam along in a streak - this can be seen in foamy rivers where a streak of foam is formed between the main stream and an eddy.

So how do we test these hypotheses? If the mechanism is the latter, then it won't be seen in places with no significant longshore current. Sea cliffs in the Mediterranean would fit the bill such as Majorca, Costa Blanca, Sardinia, Kalymnos etc. If anyone felt the urge, they could trawl through UKC photos. We would need examples where foam is clearly generated by small wave action on a cliff or rocky coastline and then we can see whether or not a line is formed.

There may, of course, be several factors that contribute to the formation of the line so I doubt we'll get a definitive answer.

In reply to El Greyo:

The other way to check tidal influence is to see if it's downtide of the wave action location or not.

 El Greyo 13:26 Wed
In reply to Toerag:

Yes, and to see how it behaves during the turning of the tide.

In reply to El Greyo:

Fantastic! Thank you. I love a good “nobody knows”!

 El Greyo 09:36 Thu
In reply to Stuart Williams:

I should say, no one in my department knows. Someone might know somewhere.

I'm currently favouring the longshore current hypothesis. Next time I'm hanging off a route in Pembroke, I'll make sure I have a good look.

In reply to El Greyo:

> There may, of course, be several factors that contribute to the formation of the line so I doubt we'll get a definitive answer.

The stability of the foam will depend on the concentration of organic molecules like seaweed alginates.  I don't think they'd form at all in perfectly pure water.

 Cobra_Head 16:06 Thu
In reply to Stuart Williams:

Wind and and current surely, the foam is made by the churning of the alginates, common sight especially in rough seas.

The foam will then be "controlled" by the sea currents but mainly the wind, pushing it in towards the cliff, while the waves push it out from the back wash from the cliffs.

 Greenbanks 07:40 Sun
In reply to Stuart Williams:

‘How to Read Water’ by Tristan Gooley might be just the book for those with interest in these matters...


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