the two high tides meet out of phase

English translation: wave theory

09:37 Jan 12, 2009
English language (monolingual) [PRO]
Science - Ships, Sailing, Maritime
English term or phrase: the two high tides meet out of phase
A man describing his passion for sailing in Scotland

He stresses the dangers to small boats sailing here, describing the extreme tidal rips and the amphidromic point where the two high tides meet out of phase.

The text isn't about sailing (which is not my field at all). I don't really understand what this part of the sentence means. I've looked for a definition of " amphidromic point" but didn't find anything that corresponds exactly to the text.

My question: is the definition given by the text an exact definition?

Thanks in advance
Stéphanie Soudais
France
Local time: 13:12
Selected answer:wave theory
Explanation:
Looking at this from the viewpoint ef a telecoms engineer better-acquainted with electronic wave theory (which is fundamentally equivalent to water waves/tides), then we can say that the amphidromic point in the sea is the equivalent of a 'node' in a standing-wave pattern affecting radio waves. That is to say that where two waves/tides intersect, they interact in a manner dependent on their relative amplitudes and their phase relationship, forming a series of stationary (standing) peaks and troughs according to how the waves add together or cancel each other out.

If the waves (tides in the source text) are of the same amplitude and arrive in phase, then they will add together with no net flow of energy between them (because they are rising/falling synchronously). But if one tide is delayed with respect to the other, or if one is of greater amplitude than the other then, when they interact, there *will* be a flow of energy between them, causing tubulence. (Picture the crest of the higher of the two tides as 'falling on top of' the lower one.)

So, is the term 'amphidromic point' being used correctly? I think probably not. Technically this is a point where tides of similar amplitude arrive in anti-phase, cancel each other out and give a near-zero rise and fall of the water. I'm thinking the writer/speaker here is trying to impress his audience with sophisticated terminology, using amphidromic to refer to 'any' point where two unsynchronized tides meet. Off the West Coast of Scotland, your author's 'pseudo-amphidromic' point will almost certainly rise and fall considerably...
Selected response from:

Jennifer Levey
Chile
Local time: 09:12
Grading comment
Thank you very much
4 KudoZ points were awarded for this answer



SUMMARY OF ALL EXPLANATIONS PROVIDED
4 +2wave theory
Jennifer Levey
4Yes
Andrew Mason
4...and hence the tidal range is zero
Tony M


Discussion entries: 2





  

Answers


11 mins   confidence: Answerer confidence 4/5Answerer confidence 4/5
Yes


Explanation:
See Wikepedia

Andrew Mason
France
Local time: 13:12
Native speaker of: Native in EnglishEnglish
Notes to answerer
Asker: Sorry, it doesn't help me...the text is too complicated for me

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26 mins   confidence: Answerer confidence 4/5Answerer confidence 4/5
...and hence the tidal range is zero


Explanation:
Here is the definition as given by NS OED:

amphi'dromic a.

designating a point in a system of tides where the tidal range is zero

Clearly the explanation in the text is talking about the same thing, although the dictionary definition emphasizes the outcome more than the cause.

--------------------------------------------------
Note added at 46 mins (2009-01-12 10:24:06 GMT)
--------------------------------------------------

Well, sort of...

The writer refers to the point where the tides from (say) the west side of the island meet the tides from (say) the east side of the island; if they occur at exactly opposite times, they will cancel each other out. Think of the tides not as a flat rising / falling of the level of the water, but rather as a large-scale 'bulge' or 'ripple' in the surface of the sea that travels along; imagine that the path it travels is longer in one direction than the other — one 'bulge' may be delayed relative to the other, so that it arrives when the other is 'dipping'.

Of course this can generate dangerous sea conditions, since a very large volume of water is involved...


--------------------------------------------------
Note added at 49 mins (2009-01-12 10:26:25 GMT)
--------------------------------------------------

Yes, as Ken says, the writer is sowing confusion by referring to 'two high tides' — I think it would have been better to say simply 'two tides, where the high-waters occur out of phase'

Tony M
France
Local time: 13:12
Specializes in field
Native speaker of: Native in EnglishEnglish
PRO pts in category: 28
Notes to answerer
Asker: So when the author says "two high tides", he actually means "the low tide and the high tide"?

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1 hr   confidence: Answerer confidence 4/5Answerer confidence 4/5 peer agreement (net): +2
wave theory


Explanation:
Looking at this from the viewpoint ef a telecoms engineer better-acquainted with electronic wave theory (which is fundamentally equivalent to water waves/tides), then we can say that the amphidromic point in the sea is the equivalent of a 'node' in a standing-wave pattern affecting radio waves. That is to say that where two waves/tides intersect, they interact in a manner dependent on their relative amplitudes and their phase relationship, forming a series of stationary (standing) peaks and troughs according to how the waves add together or cancel each other out.

If the waves (tides in the source text) are of the same amplitude and arrive in phase, then they will add together with no net flow of energy between them (because they are rising/falling synchronously). But if one tide is delayed with respect to the other, or if one is of greater amplitude than the other then, when they interact, there *will* be a flow of energy between them, causing tubulence. (Picture the crest of the higher of the two tides as 'falling on top of' the lower one.)

So, is the term 'amphidromic point' being used correctly? I think probably not. Technically this is a point where tides of similar amplitude arrive in anti-phase, cancel each other out and give a near-zero rise and fall of the water. I'm thinking the writer/speaker here is trying to impress his audience with sophisticated terminology, using amphidromic to refer to 'any' point where two unsynchronized tides meet. Off the West Coast of Scotland, your author's 'pseudo-amphidromic' point will almost certainly rise and fall considerably...

Jennifer Levey
Chile
Local time: 09:12
Native speaker of: Native in EnglishEnglish
PRO pts in category: 4
Grading comment
Thank you very much

Peer comments on this answer (and responses from the answerer)
agree  Ken Cox
45 mins

agree  Alexandra Taggart: I've looked through everything I could. The author used the term to sound "global". Rising and falling tides-"wave theory" applicable to Scotland
22 hrs
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