High Pressure - Low Water?

[RichardTaylor]

I know the current high pressure will decrease the predicted tide height by around 10cm, that is well documented, but on the other end of the scale if the predicted low water is 1.0m what effect does the high pressure have on it?

 

[VicS]

I know the current high pressure will decrease the predicted tide height by around 10cm, that is well documented, but on the other end of the scale if the predicted low water is 1.0m what effect does the high pressure have on it?

the same

 

[RichardTaylor]

I was wondering if it raised it - making the tidal curve more like neaps, however you are saying it is depressing both high and low waters.

 

[jac]

I was wondering if it raised it - making the tidal curve more like neaps, however you are saying it is depressing both high and low waters.

Yes the higher pressure squeezes the water away. Imagine a bath full of water with equal pressure on the surface. Then put a balloon on the bath and press down a bit. The water will be displaced by the downwards pressure to those areas where pressure is lower.

Basically tidal predictions are based on average pressure. In high pressure they are lower. In low pressure they are higher.

 

[NormanS]

The units have long been superceded, but the principle remains the same. One inch of mercury difference in atmospheric pressure, makes a difference of about one foot of water. It's not just HW. High pressure pushes the water away all the time, and low pressure allows it to rise.

 

[nigelmercier]

The units have long been superceded, but the principle remains the same. One inch of mercury difference in atmospheric pressure, makes a difference of about one foot of water. It's not just HW. High pressure pushes the water away all the time, and low pressure allows it to rise.

In current units, the rule of thumb is: 1 hectoPascal changes the height by 1cm

 

[VicS]

In current units, the rule of thumb is: 1 hectoPascal changes the height by 1cm

Or in units most people are familiar with, 1mb changes the level by 1cm.

 

[William_H]

As said water levels should be lower at both high and low astronomical tide. Just like a low pressure cell will produce high water (storm surge it is often called) at both high and low tide times.
The other factor that cna influence water level is the prevailing winds. ie a wind blowing onto shore will also raise water level while a wind blowing from off the shore will lower water levels.
Around here we have small astronomical tide change but still get water level changes from winds and atmospheric pressure. It seems about similar degrees of influence from each affect.
So in summer we get winds from the shore (inland) in the mornings that along with the high pressure tend to give very low tides in the morning. A sea breeze from the ocean in the afternoon will then raise the water level a lot. Though no where near as much as winter low pressure with gales from the sea. So the end effect is that tide charts for astronomical tide predictions are not so useful. olewill

 

[jac]

As said water levels should be lower at both high and low astronomical tide. Just like a low pressure cell will produce high water (storm surge it is often called) at both high and low tide times.
The other factor that cna influence water level is the prevailing winds. ie a wind blowing onto shore will also raise water level while a wind blowing from off the shore will lower water levels.
Around here we have small astronomical tide change but still get water level changes from winds and atmospheric pressure. It seems about similar degrees of influence from each affect.
So in summer we get winds from the shore (inland) in the mornings that along with the high pressure tend to give very low tides in the morning. A sea breeze from the ocean in the afternoon will then raise the water level a lot. Though no where near as much as winter low pressure with gales from the sea. So the end effect is that tide charts for astronomical tide predictions are not so useful. olewill

Indeed. Always amuses me to think of RYA teaching dazed kippers to calculate so precisely when even in UK the effects of high pressure or storms will mean that the calculated to the minute and cm heights are out s need a big round safety margin added in.

 

[nigelmercier]

Or in units most people are familiar with, 1mb changes the level by 1cm.

You are quite right of course, I'm just trying to wean myself off mb

Always amuses me to think of RYA teaching dazed kippers to calculate so precisely ...

I thought this as I was doing the YM shore based course, they seriously want me to do this to the minute and deduct 0.1m due to the secondary port? Did it for the exams, never since.