earlybird
Well-known member
From Hiscock, "Cruising Under Sail"; the Southern Ocean tidal wave is deflected northwards up the length of the Atlantic Ocean taking approx 2 days to reach the UK when it augments the normal tidal range.
When exactly is a spring tide?
- Thread starter Midnight Drifter
- Start date
MontyMariner
Well-known member
Some will never be fully satiated
but have a read here Types and Causes of Tidal Cycles - Tides and Water Levels: NOAA's National Ocean Service Education and at the associated tabs (on the right) then please report back.
Frank Holden
Well-known member
Frank Holden
Well-known member
From the above '
'Before tides for a particular location can be predicted, a series of observed hourly heights, preferably of twelve months duration, are mathematically analysed. This analysis separates the observed tide into its astronomically forced components, known as harmonic constituents or harmonic constants, and is represented by amplitudes and phases. Once this has been done, tides can be predicted for any time in the future or past. An analysis for this machine resolved up to 42 constituents. A modern analysis frequently resolves 120 or more constituents.'
They were using 'tide sticks' in the very early 1800's in Bass Strait - which is when Surprise Bay at the southern tip of King Island was chosen -
through until post WW 2 when the RAN hydrographic department was using 'Bathurst ' class minesweepers as survey ships. A friend was once deposited on a remote island somewhere as radio man for the 'stick readers' for an indeterminate time in the early '50s.
These days the tide sticks are fancier, this is one at Puerto Navarino, southern Chile, in 2009.
'Before tides for a particular location can be predicted, a series of observed hourly heights, preferably of twelve months duration, are mathematically analysed. This analysis separates the observed tide into its astronomically forced components, known as harmonic constituents or harmonic constants, and is represented by amplitudes and phases. Once this has been done, tides can be predicted for any time in the future or past. An analysis for this machine resolved up to 42 constituents. A modern analysis frequently resolves 120 or more constituents.'
They were using 'tide sticks' in the very early 1800's in Bass Strait - which is when Surprise Bay at the southern tip of King Island was chosen -
through until post WW 2 when the RAN hydrographic department was using 'Bathurst ' class minesweepers as survey ships. A friend was once deposited on a remote island somewhere as radio man for the 'stick readers' for an indeterminate time in the early '50s.
These days the tide sticks are fancier, this is one at Puerto Navarino, southern Chile, in 2009.
Stemar
Well-known member
AIUI, that's the starting point, but the gravitational influence is tiny. It's a bit like pushing someone on a swing, a tiny push at the right time adds to the movement, so it builds up, but most of it is just a wave going round the oceans. Then the land gets in the way and complicated things.
I'd guess that the biggest tides come a little after full moon because the pushes on the swing get stronger and stronger, so the swing goes higher, but it isn't as high as it can go for the amount of push, so the first couple of decreasing pushes continues to add momentum; it only gets lower once the pushes aren't strong enough to add more.
Midnight Drifter
Member
From my reading, I think that friction is part of the explanation, as well as inertia.
Midnight Drifter
Member
The amplitude of the tidal curve increases towards springs and decreases towards neaps. The envelope of the curve has a maximum at springs and a minimum at neaps. My question about the time of springs is therefore about the time of the maximum of the envelope.
The shape of the tidal curve at a particular place is well known - it is used to produce the tide tables with which we are familiar. The shape of the envelope must therefore be well known, and so it must be possible to produce a table which shows the date and time of springs and neaps at that place. Does anyone know a source of such tables?
Although the time of HW varies from place to place, the time of full/new moon is the same everywhere, and so I assume that the maximum of the envelope of the tidal curves is the same everywhere (at least in the same region, such as around the coast of the UK - the lag of springs after full/new moon may be different in different parts of the world). So I would assume not only that a spring tide day is a spring tide day everywhere, but the time of that spring tide (NOT the time of HW) is the same everywhere (at least in a region).
Roberto
Well-known member
Where would you draw the upper/lower envelope?
Outside the highest HWs and the lowest LWs?
Least square errors interpolation between all HWs and between all LWs?
If you remember some earlier threads about tidal coefficients
a higher HW (or lower LW) does not necessarily mean necessarily a higher coefficient (IOW it does not imply a higher amplitude), the HW level can increase while the amplitude decreases (tide becomes less "springy")--> the "all-outside" envelope maxima and minima will not necessarily correspond to actual spring or neaps times.
Midnight Drifter
Member
Thank you for your link to the NOAA website and your invitation to report back. It provides a very clear explanation of the basic features of the tides. However, as far as I could see, it does not mention the fact that springs and neaps do not occur at the same time as the phase of the moon that causes them, but afterwards, nor that this lag varies from month to month.Some will never be fully satiated
and at the associated tabs (on the right) then please report back.
Midnight Drifter
Member
That is for mathematicians to decide, and I assume that some must have done so. What I am interested in is where I can find data on the times of springs, based on such envelopes.
Roberto
Well-known member
Example
Sept 4 and 5 there are three tides with exactly the same ampltude: which would be your "official" spring?
Sept 4 and 5 there are three tides with exactly the same ampltude: which would be your "official" spring?
B27
Well-known member
Not sure what 'maximum of the envelope' actually means?
If you compare it with an electrical waveform, you could be talking about peak value, peak to peak value, area under the curve, RMS value etc etc.
I don't think it's valid to assume 'the time of spring tide' will be the same everywhere, even if that's a meaningful concept, because for instance the tide passes up the English Channel as a wave and Dover is inherently delayed relative to Plymouth.
Neither do I think that 'maximum envelope' will occur on the same day everywhere, because I reckon that the tidal minima, the 'Amphidromic points' probably move somewhat between springs and neaps.
I query how well known the shapes or tidal curves really are, they are different every day. Springs are different to neaps, and the published curves are an average of predictions over the year. Then the weather changes it all anyway.
The predictions are based on curve fitting to past data, it's all good enough for what most users want.
But watch the data from 'sea level networks' to see that real time data diverges from predictions.
No two years are the same, because the orbital periods of earth, moon, etc are not locked in simple ratios.
People who've raced small boats a lot on the Solent will tell you that a neap tide is not just a smaller version of a spring tide. When (relative to HW time), where and how the tide turns varies.
There are a lot of complicated non-linear effects going on, because the moving mass of water is interacting with shallows, banks and channels.
Some textbooks will tell you 'spring tide is 2 days after new and full moon' others will say 'new and full moon'. It's a convention of limited value other than to tell you tides are biggish around those times.
As with terms like 'twilight' different conventions exist from different backgrounds for different purposes, which are all roughly the same thing to the man on the Clapham Omnibus.
If you compare it with an electrical waveform, you could be talking about peak value, peak to peak value, area under the curve, RMS value etc etc.
I don't think it's valid to assume 'the time of spring tide' will be the same everywhere, even if that's a meaningful concept, because for instance the tide passes up the English Channel as a wave and Dover is inherently delayed relative to Plymouth.
Neither do I think that 'maximum envelope' will occur on the same day everywhere, because I reckon that the tidal minima, the 'Amphidromic points' probably move somewhat between springs and neaps.
I query how well known the shapes or tidal curves really are, they are different every day. Springs are different to neaps, and the published curves are an average of predictions over the year. Then the weather changes it all anyway.
The predictions are based on curve fitting to past data, it's all good enough for what most users want.
But watch the data from 'sea level networks' to see that real time data diverges from predictions.
No two years are the same, because the orbital periods of earth, moon, etc are not locked in simple ratios.
People who've raced small boats a lot on the Solent will tell you that a neap tide is not just a smaller version of a spring tide. When (relative to HW time), where and how the tide turns varies.
There are a lot of complicated non-linear effects going on, because the moving mass of water is interacting with shallows, banks and channels.
Some textbooks will tell you 'spring tide is 2 days after new and full moon' others will say 'new and full moon'. It's a convention of limited value other than to tell you tides are biggish around those times.
As with terms like 'twilight' different conventions exist from different backgrounds for different purposes, which are all roughly the same thing to the man on the Clapham Omnibus.
B27
Well-known member
These days, a lot of observations are done by satellites, which can probably measure that framework moving up and down.
Midnight Drifter
Member
None of them. The time of 'springs' will be somewhere between them. At that time, the actual tide at that place could be anywhere between HW and LW.
Midnight Drifter
Member
As far as I can see, the almanacs show spring tide to be on the same day at every port around the UK. If the time of springs (which is NOT the same as the time of spring HW) varies from port to port, then sometimes springs would occur on different days at different ports.
earlybird
Well-known member
???????????
The original query is why the delay between spring tides and the full / new moon, which seems to have got somewhat lost.
MontyMariner
Well-known member
Have a look at
Why don't spring tides occur at full/new moon (age of tide)?.
and
Lunitidal interval - Wikipedia
and
age of phase inequality - NOAA Tides & Currents.
Midnight Drifter
Member
All those references are informative, but they do not answer my question of when exactly is a spring tide. In order to measure the age of the tide at a location (i.e. the interval between full/new moon and spring tide at that location), it is necessary to know the time of spring tide at that location. How is this defined? If it is defined as the time of the highest high tide, which tide is chosen when (as often happens) there is more than one successive high tide of the same height? This suggests that it is more likely to be defined as the maximum of the envelope of the tidal curve at that location, which is not necessarily at the same time as a particular high tide.
B27
Well-known member
There's a theory used in electronics about not being able to measure the timing of anything more accurately than twice the sample period.
So measuring or defining the timing of 'spring tide' more 'exactly' than the day or two with biggest tidal ranges could well be void anyway.
Useful' practical boat owners generally speak in terms of 'springs' being the 3 or 4 days around the biggest tides, 'neaps' being the 3 or 4 days of smallest tides and various words for the other two quarters of the fortnight.
I think anyone from Dover would call each of the 4 tides on the 26th and 27th 'a neap tide'.
Sailors I know don't generally go around arguing 'today is not spring tide , tomorrow is THE spring tide'.
So measuring or defining the timing of 'spring tide' more 'exactly' than the day or two with biggest tidal ranges could well be void anyway.
Useful' practical boat owners generally speak in terms of 'springs' being the 3 or 4 days around the biggest tides, 'neaps' being the 3 or 4 days of smallest tides and various words for the other two quarters of the fortnight.
I think anyone from Dover would call each of the 4 tides on the 26th and 27th 'a neap tide'.
Sailors I know don't generally go around arguing 'today is not spring tide , tomorrow is THE spring tide'.
Roberto
Well-known member
Spring/Neaps are related to amplitude, not levels of single HW or LW. A time series of amplitudes is not a continuous line, giving a local maximum/minimum which you could choose to get your istantaneous hh:mm "time" of springs, it's more like a bar chart with the bars spanning from every LW to HW to LW etc.
You cannot take the outside envelope of the tidal curve as that would be tangent to the local highest HW and the lowest LW -possibly a curved interpolation would yield even higher levels- giving much higher amplitudes than the actual ones. Wrong curve, wrong position, wrong "exact" time inferred.
You drive 500km from 0800h to 1400h, what is the istantaneous, hh:mm time of your 500km trip?
If there was only the moon (let's stick to purely semidiurnal tides)) one might get an astronomical hh:mm time determined by a specific astronomical position of the two, the problem is tidal curves use a lot more harmonics (some >100) many of which have no astronomical relevance, to get amplitudes one must use all these components to compute differences between subsequent max and min : if you find a local maximum or local minimum of this difference you will be in springs or neaps for the entire interval of time. If, as it sometimes happens, one finds the same amplitude during more than one cycle, they will all be springs/neaps, there cannot be an istantaneous hh:mm time somewhere in the middle.