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Old 03-03-2021, 01:35 AM   #1
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Tidal differences

I have just bought a new tide clock to replace the old unit that has given up the ghost.

Reading the instructions I was bemused to read the following,"certain coasts around the world, such as the west coast of North & South America are not semi-diurnal, so the tide part of your clock will be of no use to you".

I never knew that.
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Old 03-03-2021, 02:31 AM   #2
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The high tides here today on Canada's west coast at the head of Douglas Channel are 16.7' and 19.4' and
low tides are 3.3' and 4.9'

Not sure if there is proper terminology about this stuff...we just call them the 'low high tide' or the 'high high tide' etc.

To make it even more complex, the difference between the high tides and low tides change all the time as well.
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Old 03-03-2021, 07:08 AM   #3
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Old 03-03-2021, 07:35 AM   #4
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According to NOAA the west coast of North America tides are semidiurnal.

https://oceanservice.noaa.gov/educat...p_tide07a.html
Quote:
Originally Posted by Andy G View Post
I have just bought a new tide clock to replace the old unit that has given up the ghost.

Reading the instructions I was bemused to read the following,"certain coasts around the world, such as the west coast of North & South America are not semi-diurnal, so the tide part of your clock will be of no use to you".

I never knew that.
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Old 03-03-2021, 11:07 AM   #5
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Semi diurnal is two highs and two lows of almost equal height in a 24 hour period. Diurnal is one high and one low in a 24 hour period. Then to add to the confusion there are areas of mixed tides. The old navigators who figured all this out, BEFORE COMPUTERS, were true geniuses.
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Old 03-03-2021, 11:18 AM   #6
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Semi diurnal is two highs and two lows of almost equal height in a 24 hour period. Diurnal is one high and one low in a 24 hour period. Then to add to the confusion there are areas of mixed tides. The old navigators who figured all this out, BEFORE COMPUTERS, were true geniuses.

Thank you!! I had no idea what Semi Diunal was!!!! Thank again for sharing.
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Old 03-03-2021, 11:20 AM   #7
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We can get two highs and two lows per day.
Sometimes the highs and lows are not much different.
Sometimes the high and lows are very different.
If you don't pay attention to the tables one week you could anchor safely in a spot and the next week be aground big time. Same can be said for rocks and shoal areas that may not be a problem one week but are the next week.


Tides and the tables are an important factor to take into account here.

They also affect the currents in the passes. Big tide change, big currents. Again the current tables.
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Old 03-03-2021, 11:21 AM   #8
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Greetings,
Diurnal??? Who knew?


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Old 03-03-2021, 11:24 AM   #9
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Greetings,
Diurnal??? Who knew?


LOL You forgot the Hanoi Jane stickers
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Old 03-03-2021, 02:18 PM   #10
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Here in the Panama City area we get only one high and one low daily - diurnal, and it stays that way until you get to Apalachicola, 60 miles east or to the west side of the Mississippi River delta a couple hundred miles west of here where the tides are semi-diurnal, two of each hi and low daily.
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Old 03-03-2021, 03:26 PM   #11
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Here on the west coast of FL we get some semi-diurnal then 1 week of diurnal. Its like one high tide is driven by the sun and the other driven by the moon. One week a month they are nearly in sync. Tide clocks don't work here.
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Old 03-03-2021, 03:46 PM   #12
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I have no idea where Pittwater is so I am assuming not the PNW, definitely not BC. So our tides here (PNW and higher) are mixed, not semi-dirunal, we get the occasional 1 1/2 cycle instead of two.

Semi diurnal is two highs and two lows of almost equal height in a 24 hour period. Diurnal is one high and one low in a 24 hour period. Then to add to the confusion there are areas of mixed tides. The old navigators who figured all this out, BEFORE COMPUTERS, were true geniuses.

There was a book thread not too long ago and I recommended a book called - Tides, the science and spirit of the ocean by Jonathan White. Now I can understand how this book sounds really boring, at least that is what I thought, but based on a review that was positive, I took a chance and bought it. This book delivers, much more interesting than you'd think. It does go into how tides were figured out, remember back in the time (excluding stupid people today) the earth was thought to be flat. So think about discovering how tides worked during past non-scientific times, and the spirituality of the whole thing.

"A wave tide is a flooding tide that travels upriver as a single wave" is called a "tidal bore." The most extreme tidal bore is in China on the Qiantang river - the Qiantang Bore. This Bore can be up to 25 feet in height and comes in over a very few minutes. There is a festival dedicated to it.



The Atlantic tides are guided more by the moon. The Pacific tides more by the sun.

The first tide table came out in China in 1025, two hundred years before any Western table.

The earths rotation is slowing down, each day is longer by 1/50,000 of a second. The length of a day400 million years ago was 21 hours. Having a 24 hour day happened only briefly. As the earth's rotation slows down so the moon, held in orbit by the earth's gravitational pull, slowly moves away from the earth at a rate of 1 1/2 inch per year. This measurement was confirmed by measuring instruments placed on the moon in the first landing. To the early humans, the moon would have appeared 20 % larger than it does today.

The Bay of Fundy was considered to have the largest tidal range at 54.6 feet but new evidence came to light placing Ungava Bay (in northern Quebec) at 55.84 feet. The residence of Fundy were not happy and challenged this new recording. Both Fundy's and Ungava are the greatest tides in the world. In Ungava Bay in the winter with thick ice, when the tide recedes some of the first nation's locals go into the caves to harvest food from the ocean beds. Occasionally these caves collapse killing everyone in them.

https://www.nytimes.com/video/world/...sels-cave.html
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Old 03-03-2021, 07:06 PM   #13
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I have no idea where Pittwater is so I am assuming not the PNW, definitely not BC. So our tides here (PNW and higher) are mixed, not semi-diurnal, we get the occasional 1 1/2 cycle instead of two.

l
Pittwater is a body of water opening off Broken Bay close to the opening of the Bay to the Pacific. It`s in northern Sydney.
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Old 03-07-2021, 12:23 PM   #14
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I found this map interesting.
Global Tidal Range (height difference between high and low tide)

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Old 03-07-2021, 12:36 PM   #15
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In the English channel where the Atlantic meets the North Sea the tidal ranges can also be semi diurnal which was a learning curve for me coming from cruising the Atlantic tidal range for many years.
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Old 03-07-2021, 01:23 PM   #16
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I found this map interesting.
Global Tidal Range (height difference between high and low tide)

That's cool. Thanks for sharing.
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Old 03-07-2021, 03:12 PM   #17
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Pacific NW tides are NOT semi-diurnal because there is a high high and a low high and a low low and a high low, and those effects are magnified by phase of the moon (neap tides and spring tides). If boating in these waters, buy a tide and current guide or you may be sorry!

I found a .edu webpage that explains it all:

https://faculty.washington.edu/pmacc...ackground.html
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Old 03-07-2021, 03:23 PM   #18
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While I know what you meant....I think even places that are semi diurnal have a higher high and a lower low. It's just they are nearly the same...but can still be an issue for some boating things.


"An area has a semidiurnal tidal cycle if it experiences two high and two low tides of approximately equal size every lunar day. Many areas on the eastern coast of North America experience these tidal cycles "(source NOAA)
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Old 03-07-2021, 03:56 PM   #19
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Pacific NW tides are NOT semi-diurnal because there is a high high and a low high and a low low and a high low, and those effects are magnified by phase of the moon (neap tides and spring tides).

The PNW doesn't always have two complete cycles, here is a link to Comox - Tides, Currents and ... whatever. Look at March, now you will see most days have four numbers, two highs and two lows. Now look at March the 5th, three numbers:

https://tides.gc.ca/eng/data/table/2021/wlev_sec/7965
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Old 03-07-2021, 04:56 PM   #20
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Quote:
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Pacific NW tides are NOT semi-diurnal because there is a high high and a low high and a low low and a high low, and those effects are magnified by phase of the moon (neap tides and spring tides). If boating in these waters, buy a tide and current guide or you may be sorry!

I found a .edu webpage that explains it all:

https://faculty.washington.edu/pmacc...ackground.html

"Mixed semi-diurnal"
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