Twister_Ken
Well-Known Member
1017 mb?
Extra centimeter of water for every mb less?
Minus a centimeter for every mb more?
Extra centimeter of water for every mb less?
Minus a centimeter for every mb more?
Standard air pressure for tidal calcs?
- Thread starter Twister_Ken
- Start date
FWB
N/A
Std Press is 1013.2
andyo
Well-Known Member
I get a bit confused about any discussion on how the height of tide is dependant on air pressure - no offence intended to Ken or Dylan.
Water is relatively incompressible. Wikipaedia quotes "The low compressibility of water means that even in the deep oceans at 4 km depth, where pressures are 40 MPa, there is only a 1.8% decrease in volume."
From another source "An increase of pressure by 1 atmosphere (= 1013mbar = 14.7 psi) causes a decrease of the water volume by 5.3*10-5 of the original volume." So, to my mind, double the air pressure and the volume decreases by 5.3*10-3 % (= 0.0053%).
So how can slight changes in atmospheric pressure affect the tide heights?
Water is relatively incompressible. Wikipaedia quotes "The low compressibility of water means that even in the deep oceans at 4 km depth, where pressures are 40 MPa, there is only a 1.8% decrease in volume."
From another source "An increase of pressure by 1 atmosphere (= 1013mbar = 14.7 psi) causes a decrease of the water volume by 5.3*10-5 of the original volume." So, to my mind, double the air pressure and the volume decreases by 5.3*10-3 % (= 0.0053%).
So how can slight changes in atmospheric pressure affect the tide heights?
ancientsailor
Well-Known Member
Admiralty Sailing Directions have the answer(!)
Reeds usually has an indication. The copy I had propping up one corner of the desk says:
UK South Coast
1017 mb July
1014 mb January
Wick
1013 mb July
1007 mb January
Now my desk is wobbly
Reeds usually has an indication. The copy I had propping up one corner of the desk says:
UK South Coast
1017 mb July
1014 mb January
Wick
1013 mb July
1007 mb January
Now my desk is wobbly
colingr
Well-Known Member
It's not the water that is being squashed but the volume of water moving up and down with the tide. Higher pressure counteracts the gravity effect of the sun and moon so reduces the high tide level, and presumably you get a lower low tide as well.
FWB
N/A
To "andyo"
The water isn't compressed it is flattened. A large heap of atmosphere on top of the ocean will flatten out the gravitational effects. Similarly less atmosphere will allow a higher tide.
Another effect on tidal heights is that of the wind.
The water isn't compressed it is flattened. A large heap of atmosphere on top of the ocean will flatten out the gravitational effects. Similarly less atmosphere will allow a higher tide.
Another effect on tidal heights is that of the wind.
ancientsailor
Well-Known Member
You are quite right - compression isn't the issue (trying to compress water leads to all sorts of problems when it gets into engine cylinders!)
A water bed could be used to illustrate(!) Put pressure in one area, the water goes down in that area. Attach a vacuum cleaner and the water comes up in that area.
A picture is worth a thousand words ... (I have to declare an interest - I do sell discs about tides - lots of pictures but none of water beds!)
AntarcticPilot
Well-Known Member
It affects it by affecting the vertical forces acting on the sea surface; it has nothing to do with the compressibility or otherwise of the sea. It isn't really the value of the air pressure that is the factor, it is it's variation in a particular tidal basin. So, what really makes a difference is a strong pressure gradient across a basin such as the North Sea. If you have a large area of fairly constant high pressure, it won't make much difference to the tidal height. As with winds, it is tight isobars that change tidal predictions.
Ricd
Well-Known Member
I take absolutely no account for atmospheric pressure when doing tidal calculations and will continue to do so. I do however take sensible account of wave peak and troughs a dame good look at wave heights.
AntarcticPilot
Well-Known Member
I take absolutely no account for atmospheric pressure when doing tidal calculations and will continue to do so. I do however take sensible account of wave peak and troughs a dame good look at wave heights.
Neither do I - on the West of Scotland, where other factors dominate and tidal basins are relatively small (and in any case, there if it isn't safe at low water, it isn't safe anytime). But it can make a BIG difference on the East Coast, as survivors of the 1953 East Coast floods will attest. Storm surges - which despite their name are primarily driven by differences in air pressure - can affect tidal heights by several metres - up to 3m on the East Coast.
ancientsailor
Well-Known Member
I think very few people do take much account of pressure (except in exams!).
Talking of Scotland ... of course Wick is reported to be subject to seiches.
Here's a very good description from the Enclyopedia of Chicago:
"When a strong and rapid change in atmospheric pressure takes place on one side of a lake, ... it can cause the water level to drop. As a result, the water level rises at the other end of the lake, in a motion that sometimes resembles a tidal wave. This sudden rise in water levels is potentially dangerous ...
Eight people drowned ... on June 26, 1954; a ten-foot wave swept seven people off the rocks at Montrose Harbor and an eighth from North Avenue Bridge. Since then, there have been numerous seiche scares and reports of smaller seiches ... "
Talking of Scotland ... of course Wick is reported to be subject to seiches.
Here's a very good description from the Enclyopedia of Chicago:
"When a strong and rapid change in atmospheric pressure takes place on one side of a lake, ... it can cause the water level to drop. As a result, the water level rises at the other end of the lake, in a motion that sometimes resembles a tidal wave. This sudden rise in water levels is potentially dangerous ...
Eight people drowned ... on June 26, 1954; a ten-foot wave swept seven people off the rocks at Montrose Harbor and an eighth from North Avenue Bridge. Since then, there have been numerous seiche scares and reports of smaller seiches ... "
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ditchcrawler
Well-Known Member
Here on the East Anglian Coast as Antpilot says pressure does seem to make a difference and we do seem to have less depth when the pressure is high.Mind you the direction of strong winds seems to make a bigger difference and anything with a decent east to s/e in it holds up the ebb in the rivers and estuaries
30boat
N/A
A good example of the dramatic effect pressure has on tides is when there's a Typhoon and there is a surge in the water level especially when it coincides with high water.I remember an occasion in Macau when some streets flooded and the car shops were busy for months afterwards rebuilding waterlogged engines.
Skysail
Well-Known Member
The tripper boats that go up the Dart to Totnes certainly take account of atmos pressure when choosing their route across the shallows, and when we launch from Galmpton it makes an appreciable difference to the 0.6 metre clearance at HWS; I would like low pressure on Friday, please.
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dt4134
Well-Known Member
It delayed the opening of the flap gate by about 35 mins at Peel the weekend before last.
I wouldn't ignore it. Having said that I tend not to attempt to calculate the effect, just allow a bigger margin if it's likely the high pressure will lower the tidal height.
I wouldn't ignore it. Having said that I tend not to attempt to calculate the effect, just allow a bigger margin if it's likely the high pressure will lower the tidal height.
capt_courageous
Well-Known Member
Ken
This gives a table showing the effect. It is towards the end of the article
www.sailtrain.co.uk/navigation/theightanchoring.htm
This gives a table showing the effect. It is towards the end of the article
www.sailtrain.co.uk/navigation/theightanchoring.htm
Danny Jo
Well-Known Member
Reeds misleads on this question?
Rather uncharacteristically, the coverage of this topic in Reeds is sketchy, if not actually misleading [Edit - I have since learned that it is not misleading - for an explanation of my recantation see subsequent posts]: "Over a period of time and across a wide area sea levels are lowered by high pressure and raised by low. The former is of more practical concern to mariners. Mean sea level pressures are quoted in Admiralty Pilots: 1017mb, for example, along the UK south coast in July; 1014mb in January. 1013mb at Wick in July; 1007mb in January." There follows a paragraph on seiches, waves with periods between a few minutes to an hour or so, caused by intense local variations in pressure. I take issue with this statement not on grounds of accuracy - Scuttlebutt rightly sneers at pedantry - but because it does not help one either understand the phenomenon or estimate its magnitude.
The original post's question assumes, like I do, that tables of tidal height predictions are calculated on the basis of a single standard atmospheric pressure. To base them on local mean atmospheric pressures would add unnecessary complexity to critical tidal height calculations. (Sod's law dictates that the more critical the calculation, the more likely I am to get it wrong.)
Frank's Weather Window provides a brief and informative answer. As he points out, the lowest pressure recorded in the British Isles, 925mb, would give tidal heights nearly a metre above the tabled predictions and the highest, around 1050mb, about 40 centimetres lower. He quotes the standard as 1013 and the rule of thumb 1 centimetre for each millibar.
Assuming that this statement reflected a confusion between the standard atmospheric pressure used in drawing up the tidal height tables and mean atmospheric pressure, and being a great admirer of Reeds Nautical Almanac, I reached for my own copy (2009).
Rather uncharacteristically, the coverage of this topic in Reeds is sketchy, if not actually misleading [Edit - I have since learned that it is not misleading - for an explanation of my recantation see subsequent posts]: "Over a period of time and across a wide area sea levels are lowered by high pressure and raised by low. The former is of more practical concern to mariners. Mean sea level pressures are quoted in Admiralty Pilots: 1017mb, for example, along the UK south coast in July; 1014mb in January. 1013mb at Wick in July; 1007mb in January." There follows a paragraph on seiches, waves with periods between a few minutes to an hour or so, caused by intense local variations in pressure. I take issue with this statement not on grounds of accuracy - Scuttlebutt rightly sneers at pedantry - but because it does not help one either understand the phenomenon or estimate its magnitude.
The original post's question assumes, like I do, that tables of tidal height predictions are calculated on the basis of a single standard atmospheric pressure. To base them on local mean atmospheric pressures would add unnecessary complexity to critical tidal height calculations. (Sod's law dictates that the more critical the calculation, the more likely I am to get it wrong.)
Frank's Weather Window provides a brief and informative answer. As he points out, the lowest pressure recorded in the British Isles, 925mb, would give tidal heights nearly a metre above the tabled predictions and the highest, around 1050mb, about 40 centimetres lower. He quotes the standard as 1013 and the rule of thumb 1 centimetre for each millibar.
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Lady Campanula
N/A
Oh, goody! A good 'quibble'....
As usual, some are quite justified in choosing not to use this 'tweak' to Tidal Heights. The point is that one should know of the phenomenon, so as be able to use the calculation/allowance should circumstances and intentions require.
....As in seeking to leave Scilly St Mary's, northbound across the Tresco Flats on a flood tide, bound for Padstow before the ebb prevents entry to the Inner Harbour - or, indeed, over Doom Bar.
....As in seeking to get a boat all the way up the Exe to The Retreat Boatyard, past Topsham, on a neap tide, then go home with the wifelet who's driven down 200 miles to collect you.
And, for the sake of the outrage to be heaped on me....
[pedant] ....I am advised by my one remaining friend in meteorological circles that the world and his uncle gave up on Sir William Napier Shaw's millibars around the time of Muhammad Ali's defeat by Joe Frazier, and the start of the Vietnam War. My little baro-thingy is calibrated in Hectors' Pastilles anyway, and there's quite a lot of them around in a good summer. [/pedant]
As usual, some are quite justified in choosing not to use this 'tweak' to Tidal Heights. The point is that one should know of the phenomenon, so as be able to use the calculation/allowance should circumstances and intentions require.
....As in seeking to leave Scilly St Mary's, northbound across the Tresco Flats on a flood tide, bound for Padstow before the ebb prevents entry to the Inner Harbour - or, indeed, over Doom Bar.
....As in seeking to get a boat all the way up the Exe to The Retreat Boatyard, past Topsham, on a neap tide, then go home with the wifelet who's driven down 200 miles to collect you.
And, for the sake of the outrage to be heaped on me....
[pedant] ....I am advised by my one remaining friend in meteorological circles that the world and his uncle gave up on Sir William Napier Shaw's millibars around the time of Muhammad Ali's defeat by Joe Frazier, and the start of the Vietnam War. My little baro-thingy is calibrated in Hectors' Pastilles anyway, and there's quite a lot of them around in a good summer. [/pedant]
nimbusgb
Well-Known Member
'It has nothing to do with compressibility of water'
Err well it has something to do with it. If you press down here it goes up 'over there' or if you suck it up here it comes from 'over there'. If water was drastically more compressible then air pressure would still affect the height of a tide locally, it just wouldn't affect the height elsewhere quite so much.
Think balloon. If you squeeze it you can affect its shape locally but unless you squeeze the entire balloon at the same time, it tends to squelch and bulge out elsewhere.
Err well it has something to do with it. If you press down here it goes up 'over there' or if you suck it up here it comes from 'over there'. If water was drastically more compressible then air pressure would still affect the height of a tide locally, it just wouldn't affect the height elsewhere quite so much.
Think balloon. If you squeeze it you can affect its shape locally but unless you squeeze the entire balloon at the same time, it tends to squelch and bulge out elsewhere.
ancientsailor
Well-Known Member
Admiralty Easytide FAQs may or may not convince anyone! ...
"Q: Predictions are referred to being computed for average barometric pressure. What is average, and how do tides react to differences from this average?
A: Tidal predictions are computed for average barometric pressure at the particular place concerned. The average barometric pressure for certain places is given in Admiralty Sailing Directions and information is also given in some instances concerning the changes in level which can be expected under different conditions.
A difference from the average of 34 millibars can cause a difference in height of about 0.3m. A low barometer will tend to raise sea level and a high barometer will tend to depress it. The water level does not, however, adjust itself immediately to a change of pressure and it responds, moreover, to the average change in pressure over a considerable area. Changes in level due to barometric pressure seldom exceed 0.3m but, when mean sea level is raised or lowered by strong winds or by Storm Surges (wind-induced long period waves causing higher and lower-than-predicted levels to occur), this effect can be important." http://easytide.ukho.gov.uk/EASYTIDE/EasyTide/Support/faq.aspx
"Q: Predictions are referred to being computed for average barometric pressure. What is average, and how do tides react to differences from this average?
A: Tidal predictions are computed for average barometric pressure at the particular place concerned. The average barometric pressure for certain places is given in Admiralty Sailing Directions and information is also given in some instances concerning the changes in level which can be expected under different conditions.
A difference from the average of 34 millibars can cause a difference in height of about 0.3m. A low barometer will tend to raise sea level and a high barometer will tend to depress it. The water level does not, however, adjust itself immediately to a change of pressure and it responds, moreover, to the average change in pressure over a considerable area. Changes in level due to barometric pressure seldom exceed 0.3m but, when mean sea level is raised or lowered by strong winds or by Storm Surges (wind-induced long period waves causing higher and lower-than-predicted levels to occur), this effect can be important." http://easytide.ukho.gov.uk/EASYTIDE/EasyTide/Support/faq.aspx