# Tidal Heights/Barometric Pressure

What barometric pressure are tidal height predictions based on?

Tidal height has nothing to do with barometric pressure. Barometric pressure is a reading that is measured. Now if you are wondering what the "standard" pressure is, it is the average barometric pressure at average sea level (average between high tide and low tide), and the value of standard barometic pressure is 760 mmHg, or 101.3 kilopascals.

I agree tidal height has nothing to do with barometric pressure but barometric pressure does affect sea level or put another way the predicted height of the tide.

OK. It takes 33 feet of water to make a average barometric pressue of 760 mmHg. So, if one had a pressure of say 730mmHg, then water could be lifted up to an inceased height of... (30/760 * 33 ft)=1.3 feet in order to equalize the hydrostatic pressure on the water. You are correct in that a large body of water (Atlantic Ocean, for instance) can experience some lateral flow due to differences in atmospheric pressure. What holds water to the Earth is Gravity, and it is mostly equal over the surface. Small differences in barometic pressure along the surface can move water from high pressure areas to low pressure areas. Low pressure increases tide heights, as in the example above.
Now to the physics. Barometric pressure is not a simple thing: The pressure created by gravity on the air mass is the main cause of barometric pressure. There are other things also, such as the partical pressue of the water vapor in the air, and over large water bodies this is significant. Water vapor pressure is caused mainly by water evaporation, which in turn is caused by sea surface temperature. Hotter water, more evaporation, more partial pressure of water, higher barometric pressure, air flows away and water level goes down.
I hope this helps.

There is a small effect also on atmospheric pressure caused by the tidal effect: The force which causes tides, gravity from the Sun or moon, not only pulls on the seas, but it also pulls on the air. This pull can vary air pressure by up to 5mmHg during the day, twice a day. Of course, measuring this effect is difficult, as many other things affect the air pressure during the day (namely the change in water vapor content).

Thank you Bob, all additional information is very welcome. Can you help with my understanding of "Average Barometric Pressure". My understanding is that it is an internationally agreed standard in order to be able to make sense of say the boiling point of water.

In fact there will be a few places on earth where the actual mean barometric pressure will conform to the internationally agreed figure but there will be many more that dont.

I do have some harmonised barometric pressure curves for the UK and it is clear that there will be areas of the UK that do conform. The mean pressure for the north of Scotland being about 1009 hPa/mb and for the south coast around 1015 hPa/mb.

Moving on to my original question "what barometric pressure are tidal predictions based on".

It is generally acknowledged that a variation of pressure from the average of 34 hPa will alter the level of the sea by 300mm.

Additionally we know tidal heights are based on average barometric pressure. The thing is they are not based on the international average we have talked about previously. They are based on studies of average barometric pressure made close to the location the tide tables are to be produced for.

Of course before we can make a calculation of how much the height of tide might differ we need to know 2 things. The barometric pressure at the time we are doing the calculation and the barometric pressure used to make the predictions in the first place.

Problem is there are too many options for the latter.

For the UK I have the mean pressure for whole years some say 1012 some 1013. I also have mean pressures for all the different ports throughout the UK. I also have all this information available on a month by month basis. So if I was calculation for Wick in January would I use 1. The UK annual average of 1013. 2. The annual average for Wick of 1009. 3. The January average for Wick of 1007.

Despite extensive research I have been unable to get o the bottom of this question. I do appreciate the real difference to the answer in this case will only be a few mm but I am trying to establish correct procedures.

Thank you,

Mike.

Can you help with my understanding of "Average Barometric Pressure". My understanding is that it is an internationally agreed standard in order to be able to make sense of say the boiling point of water.Yes, IUPAC some time ago agreed on the vaule of "average sea level barometric pressure.

In fact there will be a few places on earth where the actual mean barometric pressure will conform to the internationally agreed figure but there will be many more that dont.Probably

I do have some harmonised barometric pressure curves for the UK and it is clear that there will be areas of the UK that do conform. The mean pressure for the north of Scotland being about 1009 hPa/mb and for the south coast around 1015 hPa/mb.

Moving on to my original question "what barometric pressure are tidal predictions based on".

It is generally acknowledged that a variation of pressure from the average of 34 hPa will alter the level of the sea by 300mm.

Additionally we know tidal heights are based on average barometric pressure. The thing is they are not based on the international average we have talked about previously. They are based on studies of average barometric pressure made close to the location the tide tables are to be produced for.The last time I did any study on this, the US Navy Tidal charts did take "local conditions" and historical measures into consideration when formulating tidal predictions. This could include varying average barometric conditions, and a number of other things. I do not recall other than this that barometric pressure would be used

Of course before we can make a calculation of how much the height of tide might differ we need to know 2 things. The barometric pressure at the time we are doing the calculation and the barometric pressure used to make the predictions in the first place.

Problem is there are too many options for the latter.

For the UK I have the mean pressure for whole years some say 1012 some 1013. I also have mean pressures for all the different ports throughout the UK. I also have all this information available on a month by month basis. So if I was calculation for Wick in January would I use 1. The UK annual average of 1013. 2. The annual average for Wick of 1009. 3. The January average for Wick of 1007.

Despite extensive research I have been unable to get o the bottom of this question. I do appreciate the real difference to the answer in this case will only be a few mm but I am trying to establish correct procedures.
THe only source of predictions that I have used are from the US Navy. The source for this work is the US Naval Observatory, which works very closely in conjunction with HM Nautical Almanac Office http://www.nao.rl.ac.uk/ in England. Frankly, your detailed questions might be answered best there, but I suspect the real answer lies similar to the above remark, and that average baraometric variances by location are lumped into a category called Local effects when predicting tides. Good luck, this sounds most interesting.

make it short !@#\$%^&

make it short !@#\$%^&

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1. 1013millibars

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posted by john

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