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Question About The Tides

Tue Jun 14, 2011 3:16 pm

We go to the beach often during the summer. We went last week and, as I was watching the waves crash on the rocks at high tide, I wondered: Is it low tide in Japan?

If high tide comes at San Francisco at, let's say 3PM (That would be Pacific time or GMT -8), what is the tide status in Japan at that exact moment? What about the Atlantic? If high tide comes at Miami at 3PM Eastern, what would the tide status be in Portugal that exact moment?
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RE: Question About The Tides

Tue Jun 14, 2011 3:23 pm


But honestly, I think you're right, high tide at your location = low tide at Japan. But I'll let an expert happily correct me if I'm wrong.
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RE: Question About The Tides

Tue Jun 14, 2011 3:34 pm

Emo Pony in Japan wonders what happened to the tides.
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RE: Question About The Tides

Tue Jun 14, 2011 3:39 pm

Tides are determined by the gravitational pull of the Sun and Moon, and in certain cases low and high pressure weather systems (depending on strength).

If a site is directly below the moon or opposed to the moon on the earth, then it is high tide
if a site is 90 degrees opposed to the moon, then it is usually low tide.

When the sun and the moon are aligned, IE Full moon or new moon, you get Spring tides, which hare the highest, and when the moon is haflway through the process, you get the neap tides which are the lowest.

So to add to all that, if a strong low (Noreaster/Hurricane) hits during a Spring tide at high tide, you will get a very large storm surge, but , but a Spring tide at low tide, can be good news and reduce flooding.

Now to the question of Japan and SF and tides. since SF sits ar around 123 W, and Japan sits at 144 East, they are about 90 degrees opposed, so tides should be opposite appoxmimately. So high tide in SF should be low tide in Japan.
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RE: Question About The Tides

Tue Jun 14, 2011 3:49 pm

Quoting casinterest (Reply 3):

Good explanation   

As a weekend mariner, I have to take into account river (estuary) tides too, and they seem to follow their own 24-hour logic - talking about the Hudson and East Rivers, Long Island Sound and New York Harbor.
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RE: Question About The Tides

Tue Jun 14, 2011 3:50 pm

Quoting comorin (Reply 2):
Emo Pony in Japan wonders what happened to the tides.

   Excellent, just borrowed that for a lecture!

From wiki:
Most places in the ocean usually experience two high tides and two low tides each day (semidiurnal tide), but some locations experience only one high and one low tide each day (diurnal tide). The times and amplitude of the tides at the coast are influenced by the alignment of the Sun and Moon, by the pattern of tides in the deep ocean (see figure 4) and by the shape of the coastline and near-shore bathymetry (see figure 7).
Basically two bulges following the moon around. When the moon and sun are opposed, lower tidal range and when in alignment higher tidal ranges.
Nice diagram of that in

But over this pattern are oscillations of bodies of water. Near the nodes, you get low tidal range, and at the antinodes, high tidal range. In some complex areas (English channel is one) you get very complex tidal patterns.

Bodies of water will also seiche in response to winds and changes in atmospheric pressure. To understand a seiche, fill you coffee cup too full and try and walk downstairs! When it splashes on the floor you have a seiche. Changes of level due to seiching can be much more rapid than for normal tides. See North Sea flood in 1953. That did more damage in Walcheren in Holland than heavy bombing and demolitions in 1944 during the battle for access to Antwerp.

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