Solent double tides...does anybody have a straightforward explanation?

[Angele]

That's very interesting - thanks.

I understand the blue tidal-range contours, but what are the red time ones saying?

Pete

Time of high water at springs (I think).

 

[DJE]

The red lines join points where high water occurs at the same time. It came from the French Bloc Marine Almanac.

 

[prv]

The red lines join points where high water occurs at the same time. It came from the French Bloc Marine Almanac.

I realise that, but I couldn't work out what the numbers on them meant.

Since it's French, perhaps it's hours after high water La Rochelle or somewhere similar? Is that the French equivalent to Dover as a reference?

Pete

 

[Angele]

I realise that, but I couldn't work out what the numbers on them meant.

Since it's French, perhaps it's hours after high water La Rochelle or somewhere similar? Is that the French equivalent to Dover as a reference?

Pete

Post #21.

I have always worked on the principle that HW springs is midday in the Solent, and early morning (i.e. before I get out of bed) and early evening in the Channel Islands.

I think I might be correct for once....

 

[prv]

Post #21.

Oh - as in 11h means 1100, 11am? I didn't think of that, I guess it's a more French way of writing it. I took it to mean "11 hours", and wondered "11 hours from when?"

Thanks.

Pete

 

[l'escargot]

Oh - as in 11h means 1100, 11am? I didn't think of that, I guess it's a more French way of writing it. I took it to mean "11 hours", and wondered "11 hours from when?"

Thanks.

Pete

I found an explanation here: http://www.incois.gov.in/Tutor/IntroOc/notes/figures/fig11a5.html but it doesn't say what M2 is

"...Red lines are co-phase lines of the M2 tide, labelled in hours after the moon's transit through the meridian of Greenwich. (There are thus only 25 minutes between the co-phase lines labelled 12 and 0.)..."

 

[DJE]

I'll try to remember to see what it says in Bloc Marine this weekend.

 

[prv]

I found an explanation here: http://www.incois.gov.in/Tutor/IntroOc/notes/figures/fig11a5.html but it doesn't say what M2 is

"...Red lines are co-phase lines of the M2 tide, labelled in hours after the moon's transit through the meridian of Greenwich. (There are thus only 25 minutes between the co-phase lines labelled 12 and 0.)..."

Ah hah, so it wasn't a silly question after all

Pete

 

[Angele]

.... hours after the moon's transit through the meridian of Greenwich.

"Hours after ..... meridian of Greenwich". Just another way of saying "time".

 

[prv]

"Hours after ..... meridian of Greenwich". Just another way of saying "time".

Yes, but only useful if you know when the moon passes over Greenwich, which I don't.

It's obvious from the beginning that the red lines are something to do with time; we're trying to establish exactly what they mean.

Pete

 

[Angele]

Yes, but only useful if you know when the moon passes over Greenwich, which I don't.

It's obvious from the beginning that the red lines are something to do with time; we're trying to establish exactly what they mean.

Pete

Ah but.....

At springs the moon, the Earth and the Sun are in line. So at springs the moon passes over the Greenwich Meridian at midday (or midnight) GMT. So, counting the hours after this happens is simply telling you what time high water springs is.

.... Admittedly, now that we know the complete explanation, the red lines also have a usefulness on the other 13 days in every fortnight that my earlier answer did not identify.

So I was right....

... Partially at least. :encouragement:


p.s. Reeds gives the moonrise and moonset times, so not difficult to work out what time it is overhead if you have a copy. Or else google it.

 

[alan_d]

At springs the moon, the Earth and the Sun are in line.

Not true, there is a lag. Spring tides around the British Isles occur a day or two after the full or new moon.