Tides & Baro Pressure

BobE

BobE

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Just wonder if you "Gurus" can sort this out for me..

Dear old Christchurch S C, of which I'm a member, say in their "Navigational notes..."
QUOTE
During periods of "high pressure" the depth of water could be reduced by a foot or more. With a normal range of only about 3 to 5 feet, this can dramatically reduce the available depth!
UNQUOTE

Now I know from many experiences that this is true but.....
Where does the missing water go? If it's lower here it must be higher somewhere else?
I assume the tides are lower at low water as well as high water?
So the "range" is presumably unchanged?
I think the high pressure makes the HW time late... But I'm not sure about the LW time?
Does the fact that the water is being "pushed down" by the high pressure mean that the tide ebbs quicker? (If HW is late it rises slower unless LW is also late)

And despite all this we're expected to be within 0.1 of a metre in our secondary port calcs for the RYA YM exams!!

Lots of ?s above.
How many can you answer??
Comments ( polite & constructive ;-) ;-) ) welcomed.
Funny old world
Cheers Bob E..

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The water obviously "goes" somewhere, but as the average depth of the seas is about 3800m then a foot or so of water in some coastal areas can easily be accomodated elsewhere without a huge amount of difference. Also remember the opposite happens in areas of low pressure, hence the flooding which occurs during depressions and tropical storms.
One could also have the same discussion about the atmosphere, i.e where has all the air gone if the pressure is low? Obviously in the atmosphere the pressure imbalance causes the presure gradient wind, so maybe you could assume similar fo the water, i.e if the tide is lower or higher than usual there is some 'gradient' current as well as the normal tidal current?
I'm not sure if you can assume the range is unchanged, especially in somewhere as complex as the thames estuary or the solent, where there is alsorts of 'stuff' in the way of the tides.


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What about the water getting squashed by the extra pressure? I know that water is virtually incompressible, but it will compress a little bit. 0.3m in 3800m is only 1 part in 13000. Anyone know the compressibility of water?

<hr width=100% size=1>One day, I want to be a real sailor. In the mean time I'll just keep tri-ing.
 
Good question. It seems to me that when the high tide is depressed by the weather, the low tide is also LOWER than predicted, and vice versa*. I'm sure that this is true of wind driven variations to the tide around the UK (e.g. "storm surges"), but not certain with regard to pressure variations.

To be honest, despite living right by the sea for many years around Kent, I've not noticed that pressure alone really makes much difference to tide height here. It might be different for Christchurch which is a rather exceptional area with regard to tides. It is certainly known to make a difference in Venice on the Adriatic where I was a few weeks back, but even slight variations are critical there.

*i.e. the opposite of springs and neaps, where a high, high tide is accompanied by a low, low tide and vice versa.
 
You have to compress water by a lot more than a few millibars of pressure compress it to any measurable degree. Pressure varies all over the world, and so does mean sea level. It is not so much a question of pushing water from here to there but of restricting or increasing its ability to rise and fall under tidal influences. High pressure makes low and high tides lower; extends the period of low water; makes the flood more sluggish and the ebb sharper.

Within the normal range of pressure in this country the differences are very small but a sharp winter storm or a prolonged period of high pressure in summer will need to be taken into account.

In the Med, where, by and large, there are no tides, the effect of pressure changes can be seen clearly and, particularly in the Greek islands, can lead to some strong and unpredictable (to me) currents.

<hr width=100% size=1>JJ
 
The compressibility calculations...

A quick Google finds the compressibility of water as 46 parts per million volume change per atmosphere.

Really low pressure to really high pressure is approximately 100mb, or 0.1 atmosphere.

Therefore volume change is 4.6 parts per million

Average ocean depth is 3800m, which is 3.8 * 10^6 mm

Multiply this by 4.6, gives 17mm depth change for a 100mb atmospheric pressure change.

In other words, a pretty small sea height change for the absolute extremes of amospheric pressure.

>Within the normal range of pressure in this country the differences are very
>small but a sharp winter storm...

or a typical Wimbledon fortnight storm, like at the moment.



<hr width=100% size=1>One day, I want to be a real sailor. In the mean time I'll just keep tri-ing.
 
So, in an enclosed area like the Mediterranean, if pressure falls near, say, Turkey, but the average pressure falls more over the rest of the sea, does the tidal height go up off Turkey?

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Just so there is no confusion here (ha), the effect of barometric pressure is approx 1cm per millibar with a standard 'normal' pressure of 1013 if I recall correctly. As has been elegantly demonstrated above, that's not a result of water compression; the water is in fact going somewhere else, possibly even France.



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