Sea heights

[whiteoaks7]

I keep on hearing how changes in atmospheric pressure affect the height of the sea surface. I also hear of yachtmaster practical candidates being asked to include this factor in their calculation of tidal heights (not something I did in my theory exam)!! My own calculation, based on the bulk modulus of seawater, indicates that for 100m depth each 100 mB change in pressure give less than 0.5 mm (yes millimeter!) change in sea surface height. Does anyone have a better handle on this? It seems I should be wrong somewhere.

David

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[alex_rogers]

The high pressure doesn't compress the water - it just pushes it somewhere else. Hence your calculation doesn't really apply. I've heard it suggested that very high pressure might make a difference of a 10-30 cm but even this doesn't seem too much to worry about.

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[snowleopard]

think you need to re-visit the principles of your calculation. 1000mb (1Atm) approx equivalent to 10m head of water so 100mb equivalent to 1m water.

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[extravert]

I don't understand your objection. I did the same calculation on a similar post this week, and my figures agree with whiteoaks7's. The volumetric compressibility of water is approx 50 parts per million per atmosphere of pressure change.

Therefore 100mb (0.1 of an atmosphere approx) would be 5 parts per million change in height, or for 100m depth the change due to compression would be 5 * 100 * 1000 / 1000000, which is 0.5mm.

For the rest of the pressure induced height difference we see I also agree with alex_rogers' post.

<hr width=100% size=1>One day, I want to be a real sailor. In the mean time I'll just keep tri-ing.

 

[snowleopard]

imagine a u-tube filled with water. if the air pressure on one side is 1000mb and on the other side it is 970mb, the difference between the water levels will be 300mm.

the earth's surface is just a big version of that. comressability is insignificant by comarison.

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[Fill]

11mb = 10cms change. (ish)

SO a good low of around 975 to a real high of around 1030 could be 0.5m change in depth. Unfortunately I can't remember what the standard pressure is meant to be but am sure it was in a thread which gave a reference to it.

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[MainlySteam]

I agree with snowleopard for stationary lows where the rule of thumb (which matches snowleopard's calc) is that 1 millibar depression results in 1 cm increase in tide and vice versa. But for a moving depression the effect is much less and is significantly reduced by the speed of the depression and the shallowness of the water.

John

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[whiteoaks7]

Thanks, seems the most sensible answer. I guess some guys need to refresh their physics though !! :-0

DB

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[hamish]

Hi

the change in height of tide is 1 cms to 1mm, which we seem to agree with.

However what is not stated is that depth of water on charts which we all know as LAT is measured at 1013mb.

Hamish

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[charles_reed]

Baffling theory

In tidal waters such as Atlantic coasts the effect of barometric pressure is negligible, about 100mm for 30Mb change.
However in large virtually enclosed areas (such as the Morbihan) the effect of changed barometric pressure is exaggerated and you can double this.

In the Med, with about 300mm between HW/LW Springs in the centre, barometric pressure has far more effect than tides on water levels. In Porto Vecchio, Corsica they've recorded 600mm water-level change for a 30Mb change in pressure. allied to storm-surge this can make a painful difference to levels.

If your theory exam is going into such detail I would expect the variation for the latter, and for rivers in spate, to be included in the calculation.

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[AndrewB]

Re: Baffling theory

As I posted earlier, something of this magitude also occurs at Venice. There is a diagram at the tidal station in St Mark's Square that shows the expected effect of low pressure on the height of high water.

It is believed that even the strength of the solar wind can affect the tide height at Venice - now that's something they don't mention in yachtmaster courses!

 

[Guest]

To berth or not to berth ....

I used to have aberth at Thornham Marina many moons ago and I could berth on all high tides EXCEPT when high pressure went through .....

This happened a number of times when with high pressure I would ground approaching the pontoons. I got used to checking the depth just of the bank that was before the marina .... if it was xx depth - I would anchor and not waste time trying to dredge my way to berth !!

In the end I cancelled my berthing contract with them and left.

The difference experienced due to barometric pressure was significant and certainly greater than the few mm's mentioned in this thread ....


<hr width=100% size=1>Nigel ... and of course Yahoo groups :
http://groups.yahoo.com/group/gps-navigator/

 

[extravert]

Yes, thank you for that. I do understand why sea level changes when air pressure changes. The large part of the changes is as you say. However, there is a small compressibility part, insignificant as it may be, but it is still there, and I still agree with whiteoak7's original calculation.


<hr width=100% size=1>One day, I want to be a real sailor. In the mean time I'll just keep tri-ing.

 

[snowleopard]

when considering other small corrections, high pressure generally results in higher temperatures and thus reduced density of surface water, wind-induced surface movements will be subject to corioli's force. and artificial satellites have a finite gravitational effect.

then again, it reminds me of the mouse and the elephant crossing the bridge "we sure shook that didn't we" said the mouse.

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