Tidal Curiosity

[geehaw]

Sitting in my boat one night I was going through the the tide table. I noticed that through the whole year there were only like 6 negative tides. AS I recall the highest tide was +14' with the lowest around - 3' I always thought that mean sea level was about the average of high and low tides. Therefore shouldn't high and low tides be about equal averaged out through the year? Is this common where you all sail or is this a northern latitude phenomenon?

 

[wetass]

Its mean low water, not average of the high and low.

 

[wetass]

Its mean low water, not average of the high and low.

Nevermind, just re-read and realized what you were asking...

That is interesting

 

[Joe]

Sitting in my boat one night I was going through the the tide table. I noticed that through the whole year there were only like 6 negative tides. AS I recall the highest tide was +14' with the lowest around - 3' I always thought that mean sea level was about the average of high and low tides. Therefore shouldn't high and low tides be about equal averaged out through the year? Is this common where you all sail or is this a northern latitude phenomenon?

I think that you have defined mean tide level, not mean sea level.

Mean Tide Level (M.T.L.)

The arithmetic mean of mean high water and mean low water over a suitably long period (e.g. a month).

http://www.psmsl.org/train_and_info/training/manuals/glossary.html

But... then again...

Mean sea level (MSL) is a measure of the average height of the ocean's surface (such as the halfway point between the mean high tide and the mean low tide); used as a standard in reckoning land elevation.[1] MSL also plays an extremely important role in aviation, where standard sea level pressure is used as the measurement datum of altitude at flight levels.

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

Huh????

Perhaps this will help:

http://tidesandcurrents.noaa.gov/datum_options.html


I think the confusion might be in assuming that mean sea level is 0. Or that it is the same everywhere, all the time..... or maybe not.

Gotta be something better to think on, my brother.

 

[tommays]

It depends on wear you live on Long Island a prolonged NE wind always forces large amounts of ocean water into the sound making for a nice extra high tide

If you go a bit further south in the Delmarva peninsula area the same wind can push a lot of water out of the area and make a real low tide

 

[Stu Jackson]

That's not unreasonable. Our six times a year higher tide levels are usually about 6.0 to 6.5 ft, while the lowest tides are -1.8, sometimes almost as much as -2.0.

It's normal. If you read a book, like, say, Nigel Calder's How to Read a Chart, or Chapman's or Dutton's, it explain all the different water levels and how to understand them.

 

[Ross]

A careful study of the sun and moon positions during those extreme tides will lend an understanding of the whys. If you make marks for the height of each low tide for a month and average those and make marks for each high tide and average those heights. Then you can have the average or mean low water and the mean high water and get the mean sea level.

 

[John Nantz]

Highs & Lows, Valdez & Juneau

One thing that impressed me about the pilings in Juneau and Ketchikan was their height. While we've got some pretty tall pilings in Puget Sound as compared to, say, the San Francisco Bay Area, the ones in The Southeast Alaska were really impressive.

When I read what you had for the maximum tidal ranges I did a little checking and apparently while Valdez is a lot, it isn't quite as much as the Southeast. Hmmmm......

Then Adak Island has a very small tidal range. For example, for Oct 15 it was only about 3.5 feet and Tin City in the Bearing Strait had less than 1 foot range!

Valdez:
http://tidesandcurrents.noaa.gov/no...12&bday=24&byear=2011&edate=&timelength=daily

If the above URL is messed up, here is a shorter one: http://tidesandcurrents.noaa.gov/tide_predictions.shtml?gid=276
But then one has to go abut halfway down the web page in the Prince William Sound and select Valdez.

Confession: one of my To-Do wish trips is to go across the Gulf of Alaska to Prince William Sound.

 

[Tom J]

I think what's confusing is that the tide tables show the range of the tide, that is, the height of the tide above the normal low tide. Thus, a normal tide range for Boston might be 9.5', and a higher than normal tide of 10.5' might be referred to as +1.0' at high tide. Likewise, a lower than normal tide would be referred to as -1.0', or whatever the case might be. There is no reference to actual sea level. What we're interested in is the depth of the water at a certain time of day, say, for safe anchoring.
What's weird is that while the range between high and low at Boston might be 9.5', not far away, at Cape Cod, it might be 2.0'! And if you're wandering along the East Coast of the U.S., and you can't find your present location in the tide tables, no matter how hard you try, it might be because there is no tide there! Can't tell you how long it took me to figure that out.

 

[Ross]

In the Boston area there are circular tide currents. In the Bay of Fundy the tidal range is 40 feet in Bangor on the Penobscot river it is about 12 feet here in Perryville it is three feet but 3 miles down river it is just over four feet. Think of water sloshing in a bath tub and you can get a sense of the ebb and flow of the tide. In Bermuda the tidal range is very small and in Gulfport Miss there is only one high tide in 24 hours. The tides are driven by the moon and influenced by the sun. When the sun and moon are in line you get the greatest tidal ranges and at the quarter phase of the moon you get the least range. When the moon is on your side of the ocean the higher of the high tides occurs and when it is oipposite the lower of the high tides occurs. check them out: http://www.google.com/search?q=tide...3kAeb30gGwzfyvCA&ved=0CFcQsAQ&biw=894&bih=495

 

[sinnettc]

If you want to go insane, grow up on the east coast, say in Bass Harbor, Maine, where the tides are (generally) regular, then take a look at the tide charts for the west coast & in particular the San Juan Islands. While they follow a pattern it's nothing like we had in Maine.

I have a nice Timex watch with a tide timer on it that's completely useless here in Oregon. It works fine if I want to keep track of the tides in Bass Harbor though.

 

[Rick486]

In the Boston area there are circular tide currents. In the Bay of Fundy the tidal range is 40 feet in Bangor on the Penobscot river it is about 12 feet here in Perryville it is three feet but 3 miles down river it is just over four feet. Think of water sloshing in a bath tub and you can get a sense of the ebb and flow of the tide. In Bermuda the tidal range is very small and in Gulfport Miss there is only one high tide in 24 hours. The tides are driven by the moon and influenced by the sun. When the sun and moon are in line you get the greatest tidal ranges and at the quarter phase of the moon you get the least range. When the moon is on your side of the ocean the higher of the high tides occurs and when it is oipposite the lower of the high tides occurs. check them out: http://www.google.com/search?q=tide...3kAeb30gGwzfyvCA&ved=0CFcQsAQ&biw=894&bih=495

40 ft tidal range in Bangor????? I don't think so. Bangor is a long way from the Bay of Fundy.

 

[Ross]

Rick, My punctuation leaves much to be desired add a comma after Fundy and it will make sense to you. I know my geography.

 

[RichH]

The orbit of the moon around the earth isnt circular but elliptical ... and the difference in earth to moon varying distances is one cause of the different tide height/magnitude values.
The earth seasonally tilts on its axis and with respect to the orbit of the moon is another variable.

The coastal geometry that influences the 'approach' of the tide (multiplied by all the other, including above, variables also adds additional variables ..... making EACH tide station very 'unique'.
So, a lot of tide data is based on actual 'history' of each location / tide station multiplied by the variables.

 

[Tom J]

The west coast of Florida had weird tides, too. Some days we had one high tide, some days we had two high tides. That tide watch would be useless there, too! Then there were the wind tides. A north wind would blow the water into the south side of the bays and suck it out of the north side. We found this out the hard way, when we tried to sail up the ICW from Marco Island to Naples. After we had grounded a half-dozen times, we realized the north wind from the passing cold front had drastically changed the depths along the ICW.
Another weird thing, the highest tides seemed higher and more often in 2010 than in 2009. Our dock would sometimes get covered by water during a "plus" high tide, and this was very noticeable in 2010. Climate change, maybe?

 

[All U Get]

I thought the OP was who gave the original 0 from which the tide changes are measured from. Greenwich England is the spot for time, there must be a spot like the equator for the tides. (No interest other than a local store has medium and large drinks and I always ask for a small).
All U Get

 

[Stu Jackson]

A little research: http://tidesandcurrents.noaa.gov/datum_options.html

Chapman's also covers this quite well.

Some places have two highs and two lows a day, others only one.

Magic, of course.

 

[All U Get]

Thanks Joe and Stu, I got it under the GEODETIC DATUMS section.
All U Get

 

[Bill Roosa]

So where does the energy come from that causes tides? I'm thinking the moon but there does not appear to be any direct wiring to conduct the energy there to the Earth's oceans. And certianly no pump big enough to handle the load.

 

[Stu Jackson]

Bill, it all comes from an ACR!