Tidal height and barometric pressure.

[AIDY]

Is there a scientific way of working tidal height out relative to barometric pressure.

Or is it a bit of a guess knowing that a high pressure system will give a decrease in tidal hights.

TIA

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[woody001]

I dont think so, but if there was it would be a bit pointless going to that accurate detail as you would then have to include a massive amount of factors such as rainfall etc

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[Roberto]

it is said that every 10mB there is a difference of 10cm



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[Landale]

I'd agree in a static system, however that doesn't take account of a surge that travels down with a low. The surge can almost be how long is a piece of string depending on the depth and diameter of the depression and the distance it's travelled etc.

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[oldharry]

Its a bit of a guess; there are so many other variables at work apart from barometric pressure: wind speed and direction being a major one, surface water drain off (or lack of it), local topography and tidal flows, the predicted height of the tide, and the tidal flows associated with the differing predictive heights, temperature, salinity etc, all can make major differences.

Tidal surges are equally difficult to predict with any accuracy, other than knowing that there is likely to be a surge given certain prevailing conditions. Attempting to predict the surge height even from statistics of previous surges is equally fraught. about the best one can say is that in a given set of conditions, it is likely a surge will occur.

However i have seen quite major surges occuring: + 10 or 15% above prediction on a fine calm day following a stormy spell, and with no apparent meteorological cause.

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[geoffatstanpit]

That would tie in with the fact that 1 atmosphere (Bar) is equivalent to 10m of water so 10mB would be 100th of 10m = 10cm

Geoff

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[AIDY]

Thank you all..

<hr width=100% size=1>/forums/images/icons/wink.gif <font color=blue> Regards Andy </font color=blue>

 

[jrt]

The generally accepted figures are that "normal" pressure is 1013 to 1015 mbar (hPa - hecto Pascals). Then for every 34 mbar change in pressure the sea level changes by 300 mm. Pressure up, level down. This is not in the Yachtmaster sylabus, but candidates have been asked questions on it in the practical exam.

Then comes the tricky bit, prevailing winds. These affect times as well as heights. Sea levels tend to rise in the direction towards which the wind is blowing. There is no need to bother with winds less than force 5. Strong winds may alter the times by up to one hour. So much for the RYA teaching of secondary port calculations.

Read more about it in Macmillan Reeds Nautical Almanac. Section 7.8 in the 2002 edition.


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[WayneS]

I have heard that 1" of mercury = 1' of water.

Could be wrong but it makes no diff to me as I am of metric upbringing.


Cheers

Wayne

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[Twister_Ken]

Almost exactly 1 cm per mb

Standard tides are calcualted at atmos pressure of 1013 mb.

Every mb of pressure higher than that will create a tide 1 cm lower than predicted and vice versa.

That may not seem terribly significant but imagine a situation where a weather system passes over during the course of a day. At the first HW a low at 970 mb is overhead, the following day a high with a pressure of 1030 mb has arrived. Day 2 will have 60 cm less water at the top of the tide than day 1.

Which in some manky little port might be enough to stop you getting out.


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[Cornishman]

Re: Almost exactly 1 cm per mb

"Almost exactly" - is this an oxymoron?

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[TheBoatman]

How dare you!

O'i Mike
Are you daring to call my mate Ken a moron?

Shame on you, go stand in the corner.

This is a case to be heard before the "Forum Members Court".

We now need to gather evidence before the trial, so all those members wishing to represent Twister Ken (plaintif) please post your evidence on this site, all those wishing to post evidence for the accused may also post here.

As the presiding judge in this case it doesn't look good for the accused and I'm seriously considering fining the accused 1 Fray Bentos Steak & Ale pie, 1 tin of Jersey Royal pots and 1 tin of mixed carrots/peas.

I am open to persuasion, but not bribery.

Your thoughts please, should we condem this forum member to penal servitude or just let him off with a warning?

Judge
Peter the Boatman.

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[Twister_Ken]

Almost exactly right

Isn't an oxo moron someone who has gravy with their pastie?

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