Re: Why Are Tides Skewed Positive?

From: Ken Poulton (poulton@zonker.hpl.hp.com-DeleteThis)
Date: Wed Jun 07 1995 - 12:30:07 PDT


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Date: Wed, 7 Jun 1995 12:30:07 -0700
From: Ken Poulton <poulton@zonker.hpl.hp.com-DeleteThis>
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To: wind_talk@opus.hpl.hp.com-DeleteThis
Subject: Re:  Why Are Tides Skewed Positive?


> I was looking at my Tidelines calendar, and I'm surprised to see that
> there are no tides during the month of June for the Gate that are
> below -1.5. Anything below 0 is very rare. My question is why is the
> tide scaling system skewed to a positive reading? You would think
> that they would have made 0 at some arbitrary midpoint so that tides
> would be negative as often as they are positive.

Zero feet is the "mean lower low water". That means that if you find
the lower of the two low tides from each day and take the average over
many days, you will get zero. This is probably a more important
practical figure to mariners than mean water level.

As near as I can tell, this means that a tidal level of "0 feet" is kind
of a local thing. Where the tidal amplitude is N feet high to low, "0
feet" is ~N/2 feet below the mean water level. This implies that the
absolute elevation of "0 feet" (as measured by GPS, for instance) is
about 1 foot lower at Coyote than at the Gate.

Ken Poulton
poulton@opus.hpl.hp.com-DeleteThis

"Arithmetic is being able to count up to twenty without taking off your
shoes." -- Mickey Mouse



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