Is half tide height always the same? (All other things being equal)

[Mark-1]

I'm nervous about asking this because I fear I'm going to be (rightly) crucified for failing to work out something that ought to be blatantly obvious but ..... Assuming you were in some hypothetical place with a consistent tide curve devoid of local effects and identical atmospheric pressure, is half tide exactly the same height every single tide of the year, regardless of springs/neaps etc.

I think it must be, but google hasn't helped me find out for sure.

Flame away.

 

[Roberto]

Generally, no. In theory, only if you had tides with exclusively semidiurnal components.

 

[Davy_S]

Have a play with this site, it will allow you to select your area, then you can place the curser on the chart and move the dot around.https://environment.data.gov.uk/flo...e/index.html#filter=30&station=E70139-anglian

 

[lustyd]

Generally, no. In theory, only if you had tides with exclusively semidiurnal components.

I think that's what Mark meant by consistent tide curve. Given the number of caveats in the question, it would seem to boil down to a yes to me, the middle of a sine curve is always in the middle regardless of amplitude. Roberto is right though, outside of the textbook it's more likely to not be the case than be the case. Just like the cocked hat conundrum where you can say with some certainty that statistically you're probably not inside it, even though you know you should be.

For all practical purposes though, you could probably use it as a rule of thumb for depth at half tide in a given location. When I was behind a cill the rule was 2 hours after low water to leave, and that's equally as questionable!

 

[Roberto]

If this can help to visualize it:
a) Imagine a place with only a perfect semidiurnal component (say 12 hours cycle to make it simple): starting from MSL, water will go up up up up during 6 hours, then down down down down for 6 hours until it crosses MSL downwards, then continue down down down for another 6 hours, before starting go up again. In this case, half tide will always be the same, MSL.
b) Imagine a place where there is only a monthly component, a 28 day cycle without any other component. Water will go up up up for 14 days, then down down down for the following 14 days, and so on. It will cross its "MSL" once every 14 days.
Now go in a place where you have the two, a) and b) combined: you will have the half tide of case a) which will move up up up for 14 days, then down for the following 14 days --> each half tide will not be the same, except once every 14 days.

If you are in a place where the semidiurnal component is relatively big compared to the monthly one, the 12 hour oscillation will be prevailing, you will have little deviations from half-tide, which is what happens for example near the European atlantic coast.
If OTOH you are in a place with a relatively smaller semidiurnal component with regards to a prevailing monthly one, your "half tide" will move a lot more from day to day.
Add all the other components with their various periods and it gets even more varied

 

[lustyd]

I think either you or I misunderstood the question Roberto. I took it to mean "will the tide height always be the same halfway between low water and high water" and therefore would the mid tide height be the same in the same spot every day?

 

[rich]

Half tide just reachers the entrance to our harbour almost. give or take a couple of feet . Spring or neaps .

 

[LittleSister]

Assuming you were in some hypothetical place with a consistent tide curve devoid of local effects and identical atmospheric pressure, is half tide exactly the same height every single tide of the year, regardless of springs/neaps etc.

No, it will not.

The high tides and low tides will often not be symmetrical about mean sea level - e.g. one higher than current average low tide may well be followed by a higher than current average high tide, and in that instance the mid point between the two will almost certainly be higher than mean sea level.

In most places with a sine wave-like tidal curve, the difference will likely be negligible for practical purposes.

Your hypothetical place devoid of local effects does not exist, though. Such effects will be tiny mid-ocean, but there's nowhere in the UK that applies.

 

[rogerthebodger]

Where we are the mid tide is very much half way between high time and low tide. This is because we have a very simple coast and tide flow pattern.

WXtide32 will give you an approximate tide heights for various location. The UK does have complex tides due to the tide flows around the coast of the UK. The med has very little tide as any water entrance is so small.

We have such a simple tide the we do not need to use a tide curve we con use the rule of 12th and get a very accurate tide height.

 

[TernVI]

Is 'half tide height' a properly defined term which means exactly the same to everyone?

The tide moves around the UK as a wave.

Is it fair or useful to say that the amount of water in the Atlantic doesn't change a great deal, so the mean height must be the same?
How big a variation from day to day is needed to say it's not the same?

The outcome of such deliberatons might be masked by the effects of weather.

 

[Roberto]

We may all be talking about different aspects of the same phenomenon, each of them can be more or less visible depending on specific local tides.
Myself I was referring to this type of discrepancies a (strongly, like in the figure below) semidiurnal tide can show, because of non-semidiurnal components. The same LW at 1.7 may have the following HW at 3.8 - 4.0 - 4.2m; or also what was indicated in another recent thread: increasing "springish" tides (long blue arrow) where two subsequent HW have a decreasing height (blue lines).
When one goes into really mixed tides then everything can happen

 

[Mark-1]

I think that's what Mark meant by consistent tide curve. Given the number of caveats in the question, it would seem to boil down to a yes to me, the middle of a sine curve is always in the middle regardless of amplitude.

That's exactly what I meant. (I thought it was reasonably clear what I was getting at in spite of the cack handed language.)

Thanks everyone, nice easy one, I think everyone agrees.

 

[Never Grumble]

We may all be talking about different aspects of the same phenomenon, each of them can be more or less visible depending on specific local tides.
Myself I was referring to this type of discrepancies a (strongly, like in the figure below) semidiurnal tide can show, because of non-semidiurnal components. The same LW at 1.7 may have the following HW at 3.8 - 4.0 - 4.2m; or also what was indicated in another recent thread: increasing "springish" tides (long blue arrow) where two subsequent HW have a decreasing height (blue lines).
When one goes into really mixed tides then everything can happen

View attachment 111819

I have that Boatie app on my iPad and until you posted this I didn't really think about it. But if that is based on these smooth curves then its unlikely to be very accurate because tides down work like that you just have to look at the tidal curve for Portsmouth to see that. And to see that the midpoint in time isn't half he rise and fall.

 

[lustyd]

I assume you've noticed that graph is skewed and the ends overlap? Looks to me like the point the neaps and springs cross is half tide at 3 1/4 hours either side of HW roughly

 

[oldmanofthehills]

Even in the Bristol Channel with its 14m tide the difference is fairly trivial. The bigger issue is that the timing of half tide can be confusing as the tides are not 6 1/4 hours apart

 

[Pye_End]

I'm nervous about asking this because I fear I'm going to be (rightly) crucified for failing to work out something that ought to be blatantly obvious but ..... Assuming you were in some hypothetical place with a consistent tide curve devoid of local effects and identical atmospheric pressure, is half tide exactly the same height every single tide of the year, regardless of springs/neaps etc.

I think it must be, but google hasn't helped me find out for sure.

Flame away.

Doubt it.

Just think about what causes the tides, and where the heavenly bodies will be - ie always in different places, different angles and distances.

 

[Alan S]

If by half tide you mean half way in height (not time) between successive high and low tides, then a quick check of tide tables shows there is a small variation, max 0.3 metres between springs and neaps in my area. Could of course be more taking into account meteorological conditions.

 

[LadyInBed]

Given two sinusoidally curves with two amplitudes but the same frequency, the mid point will be the same.
If tides had no other influences then the answer would be Yes.

 

[Never Grumble]

I assume you've noticed that graph is skewed and the ends overlap? Looks to me like the point the neaps and springs cross is half tide at 3 1/4 hours either side of HW roughly

Not quite sure what you are saying? The time frequency differs for flood and ebb.

 

[lustyd]

I mean look at the x axis which has 7 hours before and just over 5 hours after HW making the graph skew. If you look the midpoint is at 3 1/4 either side as expected and is half the tidal height.