How do tides work, HW and LW?

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cliffdale

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Is there a good site to explain how tides work?

I understand the sun / moon pull, 2 humps of water and spring / neaps.

What I don't understand is when it is HW Falmouth, 200 miles up the coast, Dover, it is low water. What happens to the 2 humps of water that get pulled around the planet as Earth rotates?
 
cliffdale said:
circumference of Earth is around 25 000 miles, so there is a hw hump every 12 500 miles. In between the 2 humps is a low water, around 6000 miles away.

Dover is 200 miles up the channel.
Click to expand...

Yes, if the Earth was a perfect sphere with uniform density and no obstruction in the form of land masses or underwater features.

Of course you could just try this site...
http://lmgtfy.com/?q=How+do+tides+work

:)
 
As i understand it is to do with the fact that as the tide flows with a component with N/South in it then it starts to experience correollis forces that create a rotation. This generates amphidromes around the coast (such as near Poole) where the rotation has a centre. On either side of the centre of rotation tides will be roughly in antiphase. At the amphidrome there will be low tidal amplitude. There was a good article about 6 month ago in YM on this subject.
 
The two hump description is too simple.

Initially I imagined two predominant humps and some local land and seabed effects.
The reality is that the Sun, moon and all that spinning has been going on unabated for millions of years with the tides being channelled for the whole of that time. Localised patterns have developed and they are more prominent than the two humps.

Around the UK the localised land effects are very great. On the flood, the English Channel is a funnel between the Atlantic and the Dover straights. It takes time to squeeze all that water through and into the North Sea which is also being filled from the north. The "difference" between Falmouth and Dover is the result.

Take a look at this animation. It's a model of the tides building up from an imaginary start point. Towards the end of the animation you can still discern the effect of the 2 humps around the UK but the localised patterns are more prominent.

 
For the channel, think of the tide as a see saw with the fulcrum somewhere near the south Dorset coast. As the end of the see saw goes up at Falmouth, it goes down at Dover. The further you get from the fulcrum, the bigger the height between high and low.

For the Irish Sea, the tide is more like a rubber band, fixed at the north and south ends. The biggest range is in the middle, and gets smaller as you go north or south.

Does that help?
 
On the East Coast, high tide in a river more or less coincides with the current outside turning north.

When cruising the West country, I kept being caught out by leaving a river at HW and expecting the Channel to start running West. Often it was several hours before that happened
 
davidej said:
On the East Coast, high tide in a river more or less coincides with the current outside turning north.

When cruising the West country, I kept being caught out by leaving a river at HW and expecting the Channel to start running West. Often it was several hours before that happened
Click to expand...

And on the Scottish west coast there are many local anomalies, for example:
- HW Crinan and LW Ardrishaig being the same and vice versa (give or take a few minutes) despite being only about 9 miles apart.
- flow reversing direction part way through the flood/ebb on the east side of places like Jura (e.g. the tide is continuing to rise but over a few minutes the current is reversed)
 
cliffdale said:
Is there a good site to explain how tides work?

I understand the sun / moon pull, 2 humps of water and spring / neaps.

What I don't understand is when it is HW Falmouth, 200 miles up the coast, Dover, it is low water. What happens to the 2 humps of water that get pulled around the planet as Earth rotates?
Click to expand...

It's a good idea to think of tide tables rather like their close relative the train timetables, which I must say are similarly confusing, with all the trains rushing INTO London for example in the morning rush hour, and then hiding (where? how?) until the evening when there's another load of trains coming from apparently nowhere and going OUT of London every 10-20 minutes. The trains manage to pull of the same trick every day usually closely matching the timetables, and so does the sea in the form of tides, obeying the tide tables each day, with the only main difference being that Sundays are the same tide-wise, and not so with trains.

So anyway, the main thing is you need the tide tables, (or train timetables) and everything starts from there. HOWEVER ... I have always found that by far the best way to find out the trains is to ask some unlikely slightly-scruffy official who knows all the train times to the minute off the top of his head and all the platform numbers too, much easier than looking up the stuff in the very small writing. And so it is with the tides, where instead of looking up the stuff - it's miles easier to ring up the relevant port or marina and they know the times of high/low high tides to the minute just like the other guys know all the trains.

I think this much pretty much covers everything you need to know?
 
tcm said:
It's a good idea to think of tide tables rather like their close relative the train timetables, which I must say are similarly confusing, with all the trains rushing INTO London for example in the morning rush hour, and then hiding (where? how?) until the evening when there's another load of trains coming from apparently nowhere and going OUT of London every 10-20 minutes. The trains manage to pull of the same trick every day usually closely matching the timetables, and so does the sea in the form of tides, obeying the tide tables each day, with the only main difference being that Sundays are the same tide-wise, and not so with trains.

So anyway, the main thing is you need the tide tables, (or train timetables) and everything starts from there. HOWEVER ... I have always found that by far the best way to find out the trains is to ask some unlikely slightly-scruffy official who knows all the train times to the minute off the top of his head and all the platform numbers too, much easier than looking up the stuff in the very small writing. And so it is with the tides, where instead of looking up the stuff - it's miles easier to ring up the relevant port or marina and they know the times of high/low high tides to the minute just like the other guys know all the trains.

..
Click to expand...

..which reminds me, I've created the usual for God's own country: http://agurney.com/oban-tides
 
JohnGC said:
The two hump description is too simple.

Initially I imagined two predominant humps and some local land and seabed effects.
The reality is that the Sun, moon and all that spinning has been going on unabated for millions of years with the tides being channelled for the whole of that time. Localised patterns have developed and they are more prominent than the two humps.
Click to expand...

An interesting idea that fluid motion lasts for millions of years! No wonder it creates myths and legends...

Mike.
 
ronsurf said:
For the channel, think of the tide as a see saw with the fulcrum somewhere near the south Dorset coast. As the end of the see saw goes up at Falmouth, it goes down at Dover. The further you get from the fulcrum, the bigger the height between high and low.
Click to expand...

Something like this perhaps.

https://fbcdn-video-i-a.akamaihd.net/hvideo-ak-xpf1/v/t42.1790-2/10999705_683460184473_1865944309_n.mp4?efg=eyJxZSI6InBsZWFzYW50dmlsbGVfYmFzaWNfZXhwZXJpbWVudF9pdGVyYXRpb24yLHBsZWFzYW50dmlsbGUifQ%3D%3D&oh=f0be715a66a7586fd94956ff05a89f7d&oe=554283D0&__gda__=1430425945_c4c18e20779d305ecf30f2ccace5f994
 
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