Wednesday, January 02, 2013

Dumb Question

In the winter, we often see these white lines snaking across Mount Hope Bay.

I assume that they are lines of ice crystals but can anyone explain to me why they form in a line like this?

Some basic facts...

The photo was taken at 12:50 pm today looking a few degrees north of west.

The line is about 3/4 mile long.

High tide at Fall River was at 11:01 am.

Fall River (where the Taunton River meets Mount Hope Bay) is about 5 miles to the north (right of the photo.)

The tide is going out right to left and the curve in the line does correspond the to deeper water in the channel going into the Sakonnet River to our left. The line was moving right to left with the outgoing tide. I think I would be correct in saying that the curve in the line was caused by the faster current in the channel?

The Sakonnet River isn't really a river. It is a tidal strait about 14 miles long leading to the open ocean.

The wind was blowing from the west at about 10 knots.

The air temperature was 28 degrees F.

The water temperature at Fall River was 36 degrees F.

I can understand why ice might form on the surface of the water in conditions like these. If I were sailing I would expect to see ice forming on the deck and lines of my Laser.

But why does it form in a line like this?


Baydog said...

I've always been told that there's no such thing as a dumb question.

Baydog said...

And I have absolutely no idea why this line forms. It's just one of those funky foam lines.

Eric said...

Great question! I'll take a stab at it. How 'bout a line of ice forming along the shore during the cold night, lifted up and pushed out by the rising tide and current?

Litoralis said...

I think it's a tidal intrusion front formed when water of a higher density passes under water of lower density. The shear at the interface causes the line and the shape of the bottom causes the curve. Presumably winter water temperatures produce relative densities that are are more likely to cause this effect.

Baydog said...

That answer is the product of the Tillercouple's investment in their son's upbringing and education. Well done, whether or not that was the correct answer. It sounded damn good.

Pat said...

Is it perhaps related somehow to tide lines. Wikipedia says of them,

"A tideline refers to where two currents in the ocean converge. Driftwood, floating seaweed, foam, and other floating debris may accumulate, forming sinuous lines called tidelines (even though they generally have nothing to do with the tide.)

There are four mechanisms that can cause tidelines to form:

Where one body of water is sinking beneath or riding over top of the surface layer of another body of water (somewhat similar in mechanics to subduction of the earth plates at continental margins). These types of tidelines are often found where rivers enter the ocean.
Along the margins of back-eddies.
Convergence zones associated with internal gravity waves.
Along adjacent cells formed by wind currents."

Tillerman said...

I'm inclined to think that Pat and Litoralis are on the right track. Eric's theory is interesting but the shoreline is at 90 degrees to the line we can see, so I can't quite see why it would turn that way.

So it could be one body of water meeting another and one going under the other. I suspect it might have to do with fresh water coming down the Taunton River and meeting the saline waters of the bay. But that happens all day long, so is there some particular point in the tide cycle where this collision will cause this line?

Something to do with the high tide about 2 hours earlier?

Which will be the denser water? The (presumably) colder fresh water in the river? Or the warmer salt water in the bay?

And why does this cause the formation of the ice? Or is it ice? Or maybe some kind of foam?

And if it's foam, why do I only see this in winter?

I'm not sure I whether Litorais is saying that the shape of the bottom at the point where the two water masses meet forms the curve? Or is he agreeing with my suggestion that the line was originally straight and only formed a curve because of a faster surface current in the deeper channel?

Baydog's theory about Litoralis's education may also be valid. He did take some courses in Ocean Engineering in his freshman year in college. Or could it just be that he is even better than me at using the Google?

More study is clearly needed.

O Docker said...

I see tidelines like that in SF Bay at places where the tide is changing. The flow on the right (in this case) would have begun to ebb and would be meeting the end of the flood on the left. I'd expect the line to move downstream (to the left) and not remain at that location as the ebb gradually increases.

I'd think the 'S' shape is caused by the counter current close to shore working against the main current and shifting the line where opposing tides are meeting further 'upstream'.

And, since it's just a few days past a full moon, I'd expect any tidal effects like this to be more apparent now.

I'd also guess that's not ice, but foam, in briskly moving salt water near 32 degrees.

Baydog's right. There are no dumb questions, just dumb answers like this one.

JP said...

It does look like a tideline of foam to me too.

Anonymous said...

Water movement is a complex thing. But this indicates either different density water meeting, different temperature water or different speed waters meeting. The meeting place creates friction. Some water gets pushed down whilst some gets pushed aside and begins to creat a series of small edies. You will often see this line in terms of ice, foam, leaves or general flotsam. This is also a place seagulls love to fish as the turbulent layer pulls up fish that are feeding. This line will then be pushed along at various rates according to current speeds which of course are related to various depths and other geographic effects.

Tillerman said...

The whole line was moving to the left. I'm not sure there was a counter-current at the shore. The current was just faster to the left in the middle of the channel.

It could well be foam. I wonder how I can find out? Maybe if I see another one I can run down the hill to the beach and look at it from a much closer vantage point?

Tillerman said...

Excellent explanation. This is starting to make sense to me now.

John in PDX said...

I agree with the tideline theory. We see these in the NW all the time.

Question - do you see multiple lines at one time or only one?

Tillerman said...

Good question John. On the day I took the picture there was only the one line. But today there seem to be several faint lines oriented in a different direction to the one in the picture.

More study is needed...

Dallas Dude said...

I think this should be an entry in the Natural Navigator's contest. Really enjoyed the skiing post as my oldest daughter is due with my first grandchild today -- a boy.

Happy New Year!

Anonymous said...

The current shear line is probably there most of the time with the right tidal condition (so a couple of times a day); it is just much more visible when it collects the white foam as opposed to seaweed and various bits of flotsam and jetsam.

Tillerman said...

Glad you liked the skiing post and best wishes for the birth of your grandson. Becoming a grandparent will change your life.

torrid said...

That looks a lot like the tidelines in Charleston harbor. Only they usually aren't frozen.

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