The timing of High water, low water question.

[cliffdale]

Please will someone answer this in easy to understand terms.

I understand the Moon and Sun cycle and springs every 2 weeks, the rotation of the planet and high water is around every 12hrs and 25 mins etc... The books describe the bulge of water on opposite sides of the Earth due to gravitational pull.

I cant figure out how it is high water at Falmouth and at the same time low water is Dover. I guess the distance between the 2 ports is 300m or so.

Is this hump of water going up the Channel at a rate of 300miles per 6 hours? Does it mean a further 300 miles past Dover it will be low water again?

I cant get my thoughts around how many high tides there are around the coasts in relation to the 2 bulges of water caused by the gravitational pull.

Am I making sense here??

 

[guernseyman]

See the second map in this wikipedia entry to help you believe:


http://en.wikipedia.org/wiki/Amphidromic_point

As for the speed: just think of the speed at which the earth's surface is moving under the moon.
At our latitude of 50 deg., in round numbers, the speed is 24,000 miles x cos(50) /24 hours = approx 600 mph.

 

[[70521]]

The UK is also an island so the tide comes from many directions.

 

[VicS]

Please will someone answer this in easy to understand terms.

I understand the Moon and Sun cycle and springs every 2 weeks, the rotation of the planet and high water is around every 12hrs and 25 mins etc... The books describe the bulge of water on opposite sides of the Earth due to gravitational pull.

I cant figure out how it is high water at Falmouth and at the same time low water is Dover. I guess the distance between the 2 ports is 300m or so.

Is this hump of water going up the Channel at a rate of 300miles per 6 hours? Does it mean a further 300 miles past Dover it will be low water again?

I cant get my thoughts around how many high tides there are around the coasts in relation to the 2 bulges of water caused by the gravitational pull.

Am I making sense here??

Dont try to figure it out.

Read the tide tables and accept !

 

[macd]

A helpful way to visualise tides is not to imagine a bulge moving across the surface of the earth, but instead to imagine a tidal standing wave inside which the earth rotates (which is also more like what actually happens). Nor is it necessary to think of huge bodies of water rushing about at 300mph, which it clearly doesn't happen. The lateral displacement needed to produce a 10m hump is, except in shoal water, very small. Nonetheless, in narrows such as the Channel the water has to get to the place where it's 'due' for HW. It can't ignore the realities of hydrodynamics while doing so: instead there will be a lag on what you might call 'astronomical' HW. This is why much of the Irish Sea and Dover have similar tidal timings, despite a significant difference in Longitude.

Wiki, as guernseyman mentions, is quite helpful on tides. It shows, for instance, that at springs "the bulge of water on [the] opposite side(s) of the Earth" as not due to 'extra' gravity but due to the lack of it.

 

[chris.troutner]

While the direction of the gravitation pull of the sun is relatively strait forward, because it effectively rises and sets in the same location every day, the moon is not so strait forward. It also has much greater effect on tides. The moon rises and sets in different locations every day. That's why tidal predictions are so difficult.

 

[agurney]

Please will someone answer this in easy to understand terms.

I understand the Moon and Sun cycle and springs every 2 weeks, the rotation of the planet and high water is around every 12hrs and 25 mins etc... The books describe the bulge of water on opposite sides of the Earth due to gravitational pull.

I cant figure out how it is high water at Falmouth and at the same time low water is Dover. I guess the distance between the 2 ports is 300m or so.

Is this hump of water going up the Channel at a rate of 300miles per 6 hours? Does it mean a further 300 miles past Dover it will be low water again?

I cant get my thoughts around how many high tides there are around the coasts in relation to the 2 bulges of water caused by the gravitational pull.

Am I making sense here??

There are considerably more things at play here than following a bulge.

Take Kintyre for example; when it's HW on the Ardrishaig side it's LW on the Crinan side just a few miles west and vice versa .. and a few miles further south there's an amphodromic point with no tide.

 

[cliffdale]

Have I got this right? There are actually more than 2 bulges of water as I have been led to believe when reading books about tides. There are lots of bulges of water around the planet based on the locations of Amphidromic points.

 

[johnalison]

I think the problem is due to the land mass. Your bulge is actually travelling from east to west while the Channel tides go the other way.

 

[LadyInBed]

Another related fact that is worth a note is that Slack Water does not necessarily coincide with the time of High water or Low water.

An example of this is the slack water (changing from East going to West going) at Portland Bill and Cap de la Hague coincides with HW Dover (+ half an hour).
It is nowhere near the times of High water or Low water Portland, Weymouth, Cherbourg or Alderney.

And yet another related fact that is also worth a note is that at the top of Springs
[Patronise]just after New / Full moon[/Patronise]
HW Dover is at mid-day and mid-night.
At the top of Neaps LW Dover is at mid-day and mid-night.

 

[john_morris_uk]

I haven't read the Wiki article, but if you really want to know, read "The Admiralty Manual of Tides". I spent a very happy period of my career looking after the ships of the UK Hydrographic Squadron and I remember asking the senior Hydrogropher on board one of the ships almost exactly the same question. I glazed over after the first twenty minutes, but the double tides we get are to do with the way the whole water in the Atlantic basin is affected by lots of things. The sun and the moon are only two if the harmonic constants that affect things and I seem to remember that the mathematical model that's used to produce the tide predictions we get in the almanacs uses more than twenty fivd tidal harmonic factors.

 

[Tomahawk]

These standing waves can be quite useful...

Along the S coast there is a node point between Royal Soveign and Dungeness..
If you leave the Solent at LW you take the rising tide East.. If you get to the node point in time you cross into the water that falls to the north and continue with the tide to N Forland... where you can take the rising tide into the Crouch...

 

[John Barry]

Have I got this right? There are actually more than 2 bulges of water as I have been led to believe when reading books about tides. There are lots of bulges of water around the planet based on the locations of Amphidromic points.

Do not confuse the equilibrium tide (the two bulges) which is a theoretical effect with the actual tides around our coast.

The equilibrium tide is relevant as it drives the actual tides, but if you want to know high and low times you need to study amphidromic systems. The actual high tide bulges revolve around the amphidrome (you can see the details of the UK systems on chart 5058). The nearer you are to the amphidrome the smaller the range but the quicker the change in high tide time over distance. Amphidromic systems (normally) rotate anticlockwise in the northern hemisphere.

2 minutes are now up so half of the readers will have dozed off.

At a particular place the tide is based on a series of harmonics - Dr Doodson identifies 388 harmonic frequencies in 1921 - which are based on multiples and combinations of 6 astronomical frequencies. For a particular time you can calculate the contribution of each harmonic element, and by addition calculate the theoretical height of the tide. Calculating for a range of times gives a curve, and you can take maxima and minima from that curve (actually you can calculate maxima and minima directly, but that needs a bit more maths).

You can do this yourself from harmonic data published in the ATT. This provides tidal data based on about 25 harmonics, which gives sufficient accuracy for most purposes.

As mentioned previously the Admiralty Manual of Tides gives an excellent background to tidal theory. Be cautious of internerd sources (and internet forums ), which often give misleading information about fundamental theory. Be particularly suspicious (and read the AMT) if anyone starts going on about barycentres and centrifugal force.

5 minute up, so at probably 80% are now asleep. Sorry - should have posted this late yesterday evening...

 

[NUTMEG]

Do not confuse the equilibrium tide (the two bulges) which is a theoretical effect with the actual tides around our coast.

The equilibrium tide is relevant as it drives the actual tides, but if you want to know high and low times you need to study amphidromic systems. The actual high tide bulges revolve around the amphidrome (you can see the details of the UK systems on chart 5058). The nearer you are to the amphidrome the smaller the range but the quicker the change in high tide time over distance. Amphidromic systems (normally) rotate anticlockwise in the northern hemisphere.

2 minutes are now up so half of the readers will have dozed off.

At a particular place the tide is based on a series of harmonics - Dr Doodson identifies 388 harmonic frequencies in 1921 - which are based on multiples and combinations of 6 astronomical frequencies. For a particular time you can calculate the contribution of each harmonic element, and by addition calculate the theoretical height of the tide. Calculating for a range of times gives a curve, and you can take maxima and minima from that curve (actually you can calculate maxima and minima directly, but that needs a bit more maths).

You can do this yourself from harmonic data published in the ATT. This provides tidal data based on about 25 harmonics, which gives sufficient accuracy for most purposes.

As mentioned previously the Admiralty Manual of Tides gives an excellent background to tidal theory. Be cautious of internerd sources (and internet forums ), which often give misleading information about fundamental theory. Be particularly suspicious (and read the AMT) if anyone starts going on about barycentres and centrifugal force.

5 minute up, so at probably 80% are now asleep. Sorry - should have posted this late yesterday evening...

Not at all. Very interesting! Thanks.

 

[castoffandgone]

Am I the only one getting a headache?