From the following what would be the maximum effect of barometric pressure on tidal heights

How to read tide tables

Glossary of terms:

Where to find tidal information?

Using EasyTide Website Data:

Date: 14th April 2020:

Tidal Range = 5m

Tidal Range = 4.4m

Using Almanac Data:

16th September:

Tidal Range = 1.9m

Tidal Range = 2.2m

+ 1 hour has been added to adjust for ‘Summer Time’ as 16th September is in a ‘non-shaded’ area.

To calculate heights of tide between LW and HW see the tidal curve on EasyTide website or complete a Tidal Curve (see our Tidal Curves Tutorial)

Calculating ‘actual’ depth:

Chart Datum (L.A.T.) is found on a chart of the area you’re planning on going afloat. See Charts Tutorial for more info.

Chart Datum + Tide Height = Depth of Water

7.2m @ 10:14

0.5m @ 10:14

Spring & Neap Tides:

Occur approx every 14 days near a full or new moon.

Spring tides are the highest high tides and lowest low tides, where the tidal range is at its largest which also means the tidal flow will be at its strongest..

Occur approx 7 days after springs and around the first and last quarter moon phases.

Neap tides are the lowest high tides and highest low tides, where the tidal range is at it’s smallest.

How to find out if it’s a Spring or Neap Tide from a Tide Table?

Many (though not all) tide tables will have small symbols showing the moon phases (see adjacent image).

If your tide table does not show the Moon Phases you can still figure out whether it is Spring or Neap tides by looking at and comparing the tidal ranges across a 4 week period on a tide table.

Example:

Tidal Range on 16th May = 1.3m (Neap Tide)

Tidal Range on 6th June = 3.7m (Spring Tide)

Time Zones:

UT = Universal Time

GMT = Greenwich Mean Time

BST = British Summer Time (i.e. + 1 hour)

Effect of Barometric Pressure on Tidal Heights:

Extremely high pressure will make the tide heights less

Extreme low pressure will make the high tides more

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.

I look forward to your reply.

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.

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.

I look forward to your reply.

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.

Meteorological effects on tides

Unusually high or low barometric pressure, or prolonged periods of strong winds can result in variations between actual sea level and the predicted heights.

Differences between predicted and actual times of high and low water are caused mainly by the wind.

Barometric pressure: Tide predictions are computed for a standard barometric pressure of 1013 hectopascals (hPa) or millibars. A difference from the average of 1 hPa can cause a difference in height of 1 centimetre. A low barometer will allow the sea level to rise and a high barometer will tend to depress it. This phenomenon is often described as the inverted barometer effect. The water level does not, however, adjust itself immediately to a change of pressure; it responds to the average change over a considerable area. Changes in sea level due to barometric pressure alone seldom exceed 30 centimetres but, as such circumstances are usually associated with adverse weather conditions, the actual change in sea level is often much greater.

Wind: The effect of the wind on sea level, and therefore on tidal heights and times, is variable and depends largely on the topography of the area. In general it can be said that the wind will raise the level of the sea in the direction towards which it is blowing. This effect is often called wind setup. A strong wind blowing onshore will pile up the water and cause the sea level to be higher than predicted, while winds blowing off the land will have the reverse effect.

Storm surges: The combination of wind setup and the inverted barometer effect associated with storms can create a pronounced increase in sea level. This is often called a storm surge. A long surface wave travelling with the storm depression can further exaggerate this sea level increase. A negative storm surge is the opposite effect, generally associated with high pressure systems and offshore winds, and can create unusually shallow water. This effect is of great importance to very large vessels which may be navigating with small under-keel clearances.

Active Angling New Zealand

For Anglers Who Want to Fish More Actively

THE BAROMETRIC PRESSURE MYTH

Lead image: Paul Smith

For some time now I’ve been fascinated by the claim that fish feeding behaviour is largely influenced by changes in barometric pressure. The fascination started when I read Ronald Reinhold’s book “Predicting the Bite” (ISBN 978-0-578-04734-8) which discusses how to predict when fish will be feeding based largely on changes in barometric pressure. Reinhold is convinced that barometric pressure is responsible for fish feeding activity and has developed a comprehensive set of rules linking barometric pressure to weather patterns and other factors which identifies the “state of ableness” of fish to feed.

While Reinhold obviously put an immense amount of work into developing his rules, they seem to be very specific to his location in the USA and of limited value anywhere else on the planet. I also got the unerring feeling that Reinhold’s theory is predominately based on anecdotal data and not scientific experimentation. He seems to have developed his model based on his diary records and observations and he uses science selectively to reinforce his propositions. It is a powerful dissertation that is unfortunately scant on experimental data analysis.

The problem with Reinhold’s approach is that it seems he was convinced that barometric pressure was central to feeding behaviour from the outset and did not design scientific experiments to prove or disprove his hypothesis. Because of this, his findings are probably flawed due to confirmation bias. Confirmation bias is the tendency to search for, interpret, favor, and recall information in a way that affirms one’s prior beliefs or hypotheses. People display this bias when they gather or remember information selectively, or when they interpret it in a biased way. The effect is stronger for desired outcomes, for emotionally charged issues, and for deeply-entrenched beliefs. People also tend to interpret ambiguous evidence as supporting their existing position.

To confirm my doubts that barometric pressure was central to feeding behaviour I decided to run a 12 month experiment to see if there was any difference in catch rates between the periods Reinhold identified as optimum and the periods which were supposed to be sub-optimal. To cut a long story short, there was no statistically significant difference between catch rates in the optimal and sub-optimal periods. I could find no pattern. For the record, I used a single lure throughout the 12 month period to avoid bias and collect meaningful data. For more details click on:- The single lure experiment

Surprised at the results I then decided to see if I could find an explanation why. During my investigation I found an excellent article in MidCurrent written by Dr. David Ross entitled “The Pressure myth” which provided some insights. Ross writes:-

“FISHERMEN sometimes have ideas or opinions about the marine environment that do not stand up to scientific scrutiny. For example, many anglers believe that changes in barometric pressure strongly influence fish behavior—most notably their willingness to cooperate with anglers. Some have even written that fish can detect a change in barometric pressure before it occurs. An interesting notion, perhaps, though in almost all instances it is incorrect.

A rise or fall in barometric pressure, such as with an approaching cold front, usually means a shift in the weather pattern. And it is the change in the weather, not any fluctuation in barometric pressure, that affects both the fish and the fishing. In fact, most saltwater species probably aren’t even aware of barometric variations.

Pressure, whether in the air or in the ocean, is expressed by scientists as units of “atmosphere.” One atmosphere is defined as the pressure caused by the weight of all the overlying air at sea level—or 14.7 pounds per square inch (psi). Atmospheric pressure is often called barometric pressure because it can be measured by the height of the mercury column in a barometer. Changes in barometric pressure, therefore, indicate capricious weather. In general, low-pressure systems bring unstable conditions, often with precipitation and clouds. A rising barometer means high-pressure is approaching, the harbinger of stable and clear skies.

Pressure in the ocean, called hydrostatic pressure, increases with depth due to the weight of the overlying water. Water is almost 800 times denser than air; thus, hydrostatic pressure increases much more rapidly than atmospheric pressure. If you swim or dive just a few feet below the water’s surface, you feel this rapid increase in pressure.

At a depth of just 32.8 feet in the ocean, the hydrostatic pressure is equal to the pressure from the entire weight of the earth’s atmosphere as measured in pounds per square inch. In other words, at 32.8 feet, the total pressure, due to the weight of both the atmosphere and the water, is two atmospheres. At 65.6 feet it’s 3 atmospheres, and so forth. Fish can tolerate hydrostatic pressure because they have a swim bladder containing a volume of gas, which they adjust to equal their environment. This enables most fish to comfortably make small and quick up or down movements in the water column.

In the ocean, four main factors can change the hydrostatic pressure in the fish’s world.

It’s a happy notion that one could simply consult the mercury column each morning to know whether it’s a better day for work or fishing, but it’s unlikely that barometric pressure alone can trigger the sudden bite that angling’s common wisdom often asserts”.

I also discovered an excellent article by Fisheries Scientist, Ralph Manns, at In-Fisherman.com entitled Barometric pressure and bass which also challenges “one of the most persistent myths in fishing is that barometric pressure controls the activity of bass and other gamefish”.

Manns states that “Although many researchers have tried, scientific studies have been unable to demonstrate that such a relationship exists. Every scientific report we’ve seen, in which barometric pressure was studied, reached a similar conclusion: no direct relationship is evident. This consistency results mainly because no way has been found to isolate barometric pressure influences from simultaneous weather phenomena. We need observations of fish behavior when air pressure changes are the only variable. But significant barometric changes are rare without accompanying changes in wind, temperature, and sky conditions.

The typical weather front is preceded by dropping barometric pressure and increasing cloudiness, while postfrontal conditions usually are clear skies, bright sunlight, and higher air pressure. Although barometric pressure might directly trigger gamefish responses, no mechanism for detecting these changes has been seriously postulated by scientists”.

Manns goes on to describe his field studies in detail and provide some excellent interesting insights into the possible relationship of barometric pressure, weather, and bass behavior namely:-

The last point above is probably the most important of all. If a fish is full then it does not matter what is happening to the barometric pressure it will not be inclined to feed actively. Fish digestion takes much longer than fishermen realise and is heavily influenced by temperature. If the temperature is sub-optimal then it can take days, sometimes weeks, to digest prey which explains why fish in winter are so torpid. I recently wrote an article for AANZ on how fish digestion influences feeding behaviour and this quantifies the effect of temperature on digestion. Click on the following link to read it:- Fish digestion and how it drives feeding behaviour

In summary, there are many factors which govern when fish feed and barometric pressure alone is not an accurate predictor. The science seems to suggest that fish digestion rates, water temperature, weather patterns, water clarity and solunar phase all contribute to when and how aggressively fish feed so it is a wise strategy not to rely on any single indicator. The best plan is to fish as often as possible, even when the conditions and indicators may not appear conducive to fishing. Confining your outings to days when the conditions are “barometrically ideal” means you are missing out on many opportunities.