Amphidromic Points

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DaveWalsh
Posts: 138
Joined: Sat Oct 26, 2013 9:37 am

Amphidromic Points

Post by DaveWalsh » Tue Dec 15, 2020 6:00 pm

Amphidromic Points, as i am sure we all know well, are points on the surface of the earth palpated by the moon (that's close enough I hope).
At an Amphidromic Point there will be zero up and down tidal range (caused by the moon), but at which the time at which the side to side tidal movement changes rapidly.
Away from Amphidromic Points he time at which the side to side tidal movement changes is more gradual, but the up and down tidal range is larger and larger.
I am grateful to John Dempsey of FECKers for the attached chart.
The more you know the more your wonder grows ............
DWalsh
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Amphidromic_Points_Chart.png
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johnd
Posts: 97
Joined: Sat Oct 26, 2013 9:37 am

Re: Amphidromic Points

Post by johnd » Tue Dec 15, 2020 6:18 pm

Following in the steps of the master, I came across this in Oileain:
The amphidromic point near Cahore [Point, Wexford] reduces the tidal range and ensures the times change rapidly over a relatively short distance.

I got curious. According to Wikipedia:
An amphidromic point [...] is a geographical location which has zero tidal amplitude for one harmonic constituent of the tide. The tidal range [...] for that harmonic constituent increases with distance from this point

The tide is caused by a number of factors the most important of which is the moon; this factor is referred to as 'M2' by those in the know.

The image below shows the variation in the M2 component of tide heights in the Irish Sea:
m2-tidal-elevation-amplitude.png
m2-tidal-elevation-amplitude.png (86.43 KiB) Viewed 848 times

The phase of the tide changes very rapidly moving from one side of an amphidromic point to the other:
m2-elevation-phase-in-degrees.png
m2-elevation-phase-in-degrees.png (80.42 KiB) Viewed 848 times

The tidal flows into and out of the Irish Sea are very different to elsewhere on the coast of Ireland.
ireland-tidal-flow.png
ireland-tidal-flow.png (247.63 KiB) Viewed 847 times

Click here for an animation of tidal flows around Ireland.

;)

knotman
Posts: 134
Joined: Sat Oct 26, 2013 9:37 am

Re: Amphidromic Points

Post by knotman » Wed Dec 16, 2020 9:29 pm

Most interesting Dave and John.

To visualise an amphidromic [node] point, just rock your Christmas GLASS OF WINE gently over and back.
I suggest the first one :lol: .

The amphidromic point is where there is no up/down movement at the glass EDGE,
even though the sides are rocking up and down. Think see-saw.
If you don't drink wine, then slosh some water over and back in a bath, if anyone still has one.

This phenomenon accounts for the very low range of tidal height in Arklow,
10 CENTIMETERS the last time I checked.
Arklow sits close to this Cahore nodal point.
.
Sennen

DaveWalsh
Posts: 138
Joined: Sat Oct 26, 2013 9:37 am

Re: Amphidromic Points

Post by DaveWalsh » Thu Dec 17, 2020 10:23 am

Tidal range today in Galway varies between 4.1m and 4.5m.
Tidal range today in Ballycastle county Antrim is less between 0.5m and 1.0m
DW
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