BS: Tides

Subject: BS: Tides
From: Raggytash
Date: 27 Sep 11 - 03:19 AM

Can anyone explain how high tide in Whitby at the bridge can be 40 minutes or more before high tide at the Bell Buoy located about 3/4 of a mile off shore, I can't find anyone in town who can explain the difference. I can understand a difference as you move further North or South but when it's on the same latitude it does not compute in my little brain

Subject: RE: BS: Tides
From: Keith A of Hertford
Date: 27 Sep 11 - 03:28 AM

In open water, tides move slowly.
In partly enclosed bodies of water you can get oscillations of a period governed by the size of the basin.
Southampton water has an extra mini tide superimposed on the usual pattern.

Subject: RE: BS: Tides
From: Penny S.
Date: 27 Sep 11 - 04:49 AM

From my OU oceanography course, I recall that tides in the North Sea rotate round nodes, rather than just going up and down. Here's an explanation - for some reason with no diagrams. The link will open a document.

North Sea tidal patterns

I'm not sure how helpful it will be.

Penny

Subject: RE: BS: Tides
From: John MacKenzie
Date: 27 Sep 11 - 04:51 AM

When the tide's in, it's our turn to have it
When the tide's out, it's their turn to have it.

Subject: RE: BS: Tides
From: Black belt caterpillar wrestler
Date: 27 Sep 11 - 07:52 AM

The local shape of the sea bottom and surrounding shore-line will influence the effects of the different harmonic constants that comprise the total height of the tide at a particulat place.

The actual observed tides are analysed in order to model them for tidal prediction. There are over 60 different mathematical components that are usually considered in order to do this. Each one is a sine wave with its own amplitude and phase lag (think of it as a sine wave moved left or right against time). Each one of these will have a different amplitude for a different location.
They al have names by the way, like M2, S4, or something much more complicated, etc. which refer to the Moon or Sun for the two quoted and the number of times per day that they occur.

In a particular location it may well be that the sum of all the components from the relevant harmonic constants give a larger value (and hence a higher tide) some time after another nearby location.
It could be the effect of a sandbank emphasising particular harmonic constants etc.

Tides are constantly being monitored and fresh readings taken as change is always taking place. As a result you may have two locations that are close together where the published tidal predictions are based on readings taken in different time periods, perhap one set is twenty years older that the other. This could give you a more accurate prediiction for the one with the newer analysis.

I'm not mathematicaly qualified to any excetional degree but I was involved with publishing the Admiralty Tidal Predicitons for some time several years ago!

Subject: RE: BS: Tides
From: GUEST,Raggytash
Date: 27 Sep 11 - 09:02 AM

Like I said I'm a bear of very little brain, could someone explain it in words of one syllable.

Penny, thanks but unfortunately the link states that the web page can't be found.

Subject: RE: BS: Tides
From: Keith A of Hertford
Date: 27 Sep 11 - 09:14 AM

High Tide moves West across the world at about 940 N MPH at equator.
About half that at Latitude of Whitby.

If the world were all ocean, that would be it.
In seas like North Sea, the water will oscillate, (slosh backwards and forwards) at a rate determined by sea bed and coastline features.

E.g. In Southampton water it does not follow a single sine curve but is the sum of at least two of different frequencies.

Subject: RE: BS: Tides
From: Jim Dixon
Date: 27 Sep 11 - 07:14 PM

I think the tide everywhere is the sum of 2 sine waves: one for the sun and one for the moon.

I have lived all my life in the Midwest USA, hundreds of miles from any sea, and so have had very little experience with tides. Once when staying in a seaside resort town, I saw something incredibly cool—a tide clock. It had only one hand. When the hand is straight up, it's high tide. When it's straight down, it's low tide. I believe the 4 quadrants of the dial were labeled "hours before high tide," "hours after high tide," etc. According to this Wikipedia article, the hand rotates once every 12 hours and 25 minutes.

To this day, I have only seen 2 tide clocks. If I lived near the sea I would certainly want one.

I wonder if there is a computer widget that can show you tides?

Subject: RE: BS: Tides
From: Paul Burke
Date: 28 Sep 11 - 02:10 AM

I'd expect the inshore high tide to be later, not earler, but that probably just shows how much I know about it. But the effect isn't as dramatic as it sounds. Assume the shape of the tides to be approximately sinusoidal, high (and low) tide is around the turning point, where it is changing slowest. 40 minutes difference in about 24 hours 25 minutes, that's 0.027 cycles, or 10 degrees. Cosine of 10 degrees is 0.98, so the difference in tide height is 0.02. The tidal range at Whitby might be 10 metres, so the difference in water level between shore and buoy is 200mm in 3/4 mile, a gradient of 1 in 6000. That's less than the gradient of a canal, so there's no great current caused.

Subject: RE: BS: Tides
From: MartinRyan
Date: 28 Sep 11 - 04:46 AM

One of my favourite tidal curiousities occurs at Lough Hyne in West Cork, Ireland. The "narrows" (which you can't quite see in the photo) acts as a sill with the tide pouring over it into the lake. The tide drops faster than the lake can empty so it's very assymetric - about 7-8 hrs ebb to 4-5 of flood, IIRC. On a quiet day, as you sit by the rapid at high water, you can actually hear the tide change - the last trickle of flood being replaced by the first trickle of the ebb. As a bonus, the narrow is choked with huge seaweed plants. These lie pointing into the lake until the ebb is strong enough to lift them bodily above the water before they flop dramatically into the opposite direction!

Regards

Subject: RE: BS: Tides
From: MartinRyan
Date: 28 Sep 11 - 04:59 AM

p.s.

Jim Dixon

There's a tide clock sitting in my porch as I type. We live about 20 mins walk from the sea, without line-of-sight and it's handy to know the state of the tide before going for a walk - especially if you plan on a swim!

Regards

Subject: RE: BS: Tides
From: Keith A of Hertford
Date: 28 Sep 11 - 06:49 AM

I would love to see that lough Martin.
On my list for my next visit.

Subject: RE: BS: Tides
From: MartinRyan
Date: 28 Sep 11 - 07:05 AM

Yes, Keith - it's a fascinating place. Through the '80's and early '90's I used to teach sailing in Baltimore, a few miles away, for a few weeks each summer. I remember during an instructor training course one year, taking four dayboats into the lough on the last of the flood, sailing around for an hour or so, then heading back out as the ebb started. Magic.

Regards

p.s. Another local curiosity is the village of Catletownshend , home of the Somerville of Somerville and Ross! I've been known to refer to it as "a cultural Lough Hyne"! ;>)>

Regards

Subject: RE: BS: Tides
From: Black belt caterpillar wrestler
Date: 28 Sep 11 - 07:36 AM

The reason that a violin does not sound like a flute is that they have different amounts of harmonic vibrations going on as well as the actual note. So there will be some amount of a note that is twice the frequency, some of three times the frequency etc. These amounts will vary for a particular instument and all these added together give the distinctive sound of that instrument.

The same thing goes for tides at a particular place, hence all the amounts of harmonic constants that are analysed to get a basis for predictions. it is usual not to bother with including any constant in the calculation that would give a maximum difference of a couple of millimeters.

If I can find my lst of all the constants I'll post them on this thread but I'm not sure where I last saw it! Quite a few of them have Greek letters as their names as they are from traditional mathematical nomenclature.

The highest range tides (spring tides)occur with about 3 days lag behind having the moon at full or new, sort of depends on friction with the bodies of water having to catch up. At this time the moon and sun are in line so pulling in the same direction. You get the least tidal movement (neap tides) when the moon and sun are at right-angles.

Subject: RE: BS: Tides
From: Keith A of Hertford
Date: 28 Sep 11 - 07:52 AM

The distance earth-sun and earth-moon vary during the orbits.
Does that make a significant difference?

Also, are spring tides significantly higher when sun and moon are closely aligned, as during an eclipse?

Subject: RE: BS: Tides
From: Wolfhound person
Date: 28 Sep 11 - 08:27 AM

I did my undergraduate field ecology course at Lough Hyne in 1970 - fascinating place. We camped by a boathouse. I remember measuring the height of the tide every 10 mins for 12 hours one day, and rowing round the lough at midnight, singing.

One year just after that a group went for a walk one evening and ran into a ...um...."irregular military training exercise", and had to beat a hasty retreat.

It looks nice and peaceful now.

Paws

Subject: RE: BS: Tides
From: JohnInKansas
Date: 28 Sep 11 - 10:17 AM

In the third post:

Subject: RE: BS: Tides
From: Penny S. - PM
Date: 27 Sep 11 - 04:49 AM

The link fails in the manner common to those who use the mudcat "make a link" which often adds an http:\\mudcat.org to the correct web address (http://mudcat.org/astro.ic.ac.uk/~mrr/starsntides/stars4.doc)

The document opens (for me at least) at http://astro.ic.ac.uk/~mrr/starsntides/stars4.doc

The link balks, objects, argues, and then opens in Word, but should open in whatever program you have for opening .doc files.

John

Subject: RE: BS: Tides
From: Ross Campbell
Date: 28 Sep 11 - 12:46 PM

Easytide on-line tidal predictions from UKHO
Hit the "Predict" button for a graphic representation of (Fleetwood, Lancs)tidal heights and times for the next week. Location and start date can be varied.

!0.5 metres (high water tonight @ 00.36) is about as high as it gets round here (south end of Morecambe Bay)

Ross

Subject: RE: BS: Tides
From: Penny S.
Date: 28 Sep 11 - 01:10 PM

If you look up the tables for the Severn Bore and the Trent Aegir, you will see that the heights vary on a sequence longer than the half months between springs, and this is indeed due to the changes in the distances between Sun, Moon, and Earth, also to the time of year as the Sun apparently moves between the Tropics, and the position of the Moon along its inclined orbit.

Thanks for fixing that link.

There's a place in Scotland that interests me - I have an app on my Palm Pilot with tide tables for various places, one of which is Port Ellen. This has very shallow tides, but not a simple sequence of two highs and two lows a day. There are hiccups at different times of month so that it changes direction more often. I have not been able to find a diagram like that on my PDA, but there's one on here Port Ellen tides that gives a bit of an idea.

Penny

Subject: RE: BS: Tides
From: gnu
Date: 28 Sep 11 - 02:07 PM

Nain, Labrador. Approx 13 miles from the actual "coast". Many islands bteween them restrict the flow of tide. When the tide does rise at Nain, it comes in fast enough to trap a young lad who has worked himself out on a boulder field by jumping from one to another to fish for arctic char.

Not much of a story... yet.

I had four large char and five huge cod. I noticed I was ankle deep in water and only noticed because the flat oily ca'm had been disturbed. It was disturbed by whatever was sporting a 6m dorsal fin about 100m away. I threw all the fish on my back and turned toward shore... FUCK ME I CAN'T SEE THE BOULDERS!!!! They were under water and the sun was reflecting off the water.

I felt around with my fishing rod where I thought I had jumped from. I found several that way but not the last one. It was only a few metres so I jumped in and made it to shore WITH my fish.

I lay on shore, thoroughly panic sticken. I looked back and saw nothing but calm water. Whatever (orca?) it was hadn't bothered with me.

Good thing I waded/swam the last bit or I would have had to go back in the water at least up to my nuts... think about it... >;-)

Subject: RE: BS: Tides
From: Paul Burke
Date: 28 Sep 11 - 05:39 PM

Of course, the tide comes TWICE a day, so the height difference should be more like 400mm. Still not huge over 3/4 mile.

Subject: RE: BS: Tides
From: Q (Frank Staplin)
Date: 28 Sep 11 - 06:09 PM

Got a National Geographic catalogue yesterday. It has a tide clock and a tide watch.

Subject: RE: BS: Tides
From: Ross Campbell
Date: 28 Sep 11 - 10:55 PM

eBay has tide clocks from about £15, or design your own with movements from £12.

Subject: RE: BS: Tides
From: Keith A of Hertford
Date: 28 Sep 11 - 11:55 PM

Penny, you mentioned time of year effects.
The sun and the moon moving between the tropics.
I think that would make a difference between the two tides in a day.
One would be the summer tide, and one the winter tide.
Is that right?

Subject: RE: BS: Tides
From: Black belt caterpillar wrestler
Date: 29 Sep 11 - 07:28 AM

There are several long period variations in the effects of the variation of the sun and moon distances from the earth. This means an effective length of about 29 years between repeats of circumstances.

Spring tides are so named because at the equinoxes you get more tidal effect as the equator is better lined up with the sun and moon.

In a way this is almost a musical thread as the resonances of the tides is the same principal as the resonances of musical instruments.

To give an idea of the force that the moon exerts on the earth, a 100,000 ton tanker will lose the weight equivalent to a bag of sugar when the moon is overhead.

Subject: RE: BS: Tides
From: Penny S.
Date: 29 Sep 11 - 03:49 PM

The seasonal variation is in the magnitude of the tides, with the Sun being more close to overhead in the summer. However, the greatest tides are closer to the equinoxes. Presumably this is to do with the relationship to the Moon's position. Check out the large bores in the Severn prediction page. These vary fom year to year, as the position of the Moon varies. The sun is much more regular.

Severn Bore 2011

I see we've just missed the really big ones for this year. The three star ones next month should be good, too.

You can compare with other years at the foot of the page.

Here's the other one. There's a considerable difference between the morning and evening waves with this one.

Trent Aegir

Our friend up above who was involved with tide predictions obviously wasn't using the machine method, which involves programming with all the possible oscillations due to orbits, perihelion, aphelion, perigee, apogee, nodes, and anything else I may have left out. Lots of interacting sine waves. I saw a programmme about the machine which was more like an offshoot of Babbage's contraption or something by Heath Robinson than a modern computer.

I used to be considerably confused by the two tides. Why, if the water is pulled by the Moon, isn't there only one? But the water the other side is being NOT pulled by the Moon, so is left sticking out.

Penny

Subject: RE: BS: Tides
From: Keith A of Hertford
Date: 29 Sep 11 - 05:00 PM

The second tide.
Say it was just the sun and Earth.
The centre of gravity of Earth is just the right distance from the sun.
The water on the sun side is closer. It should orbit faster but can't.
It falls toward sun.
The water on far side should orbit slower but can't.
It is flung out.
A bulge on both sides.
Although Moon orbits Earth, both really orbit a common centre of gravity within Earth.
The same effect on the waters.

Subject: RE: BS: Tides
From: Sandy Mc Lean
Date: 29 Sep 11 - 05:38 PM

The shape of the shoreline delays and enhances the tidal effect. Canada's Bay Of Fundy funnels incomming tides causing them to reach enormous height. What are vast mudflats at low tide will float large ocean going ships at high tide. As the water flows into Minas basin it is funnelled some more creating the world's highest tide at Burntcoat Head.
The Bay Of Fundy is on the list to be named one of the world's seven natural wonders.
Burntcoat Head

Subject: RE: BS: Tides
From: gnu
Date: 29 Sep 11 - 06:12 PM

One day, I had to ride out a storm on my buddy's fishing boat for 6 hours waiting to get back into Alma, NB. She's dry bottom at low. Two in the bunks sick. We were fine with lots of beer and lobster... poached lobster from earlier in the day... about a 25 pounder. And it was NOT tough. It was deeeeelicious.

Subject: RE: BS: Tides
From: Black belt caterpillar wrestler
Date: 30 Sep 11 - 04:28 AM

The Hydrographic Office at Taunton did have a machine designed to predict tides but it had been superceded by computer based calculation by the time I started there back in the 1980s. It used to be dragged out as a museum piece for open days and exhibitions.

Basically it had a load of dials and on each one you could set up the amplitude of a different harmonic constant and also adjust its maximum height time offset from the start of the day. The displacement from all these dials (think of them as cams) contributed to a total amount by which a pen moved to record the predicted height as the machine was set in motion.

It was much less time consuming to use the computer!

Subject: RE: BS: Tides
From: Penny S.
Date: 30 Sep 11 - 04:25 PM

So that was where it was - I saw a demo on an OU programme I think. It ought to go to the Science museum along with the difference engine, perhaps.

Subject: RE: BS: Tides
From: Charley Noble
Date: 30 Sep 11 - 09:43 PM

gnu-

Glad to hear you survived, but I knew that!

"Tide and time wait for no man."

Charley Noble

Subject: RE: BS: Tides
From: gnu
Date: 01 Oct 11 - 08:47 PM

Survived? We had a blast! Buddy, we were at the wheel drinkin beer and "finger-foodin lobster" and it was up and down big time. Well, fer me, anyway. 2.5m or so and the boat was about 8m. Rain so's ya couldn't see squat. But Al joined the navy at 14 years old so I never gave it a second thought. I just had fun.

He told me about the time they were under fire in... in... ahhh...damn... I forget. Anyway, it was quite a story. He ducked under a desk in the comm room when a shell hit... just after they ran aground because someone got the orders wrong. This guy was one of the heroes in my life, especially when he told me what he did under the desk... it was a "human moment".

I must give him a call this eve. Haven't spoken to him in a dog's age.