Barometric pressure and sea level

[Humblebee]

I should know this but here goes...
Can someone please remind me of the relationship between barometric pressure, as measured in millibars, and sea level? I seem to recall a simple relationship such as 1mb rise equals 1mm drop in sea level but perhaps I am out by a factor of ten?
Thanks!

 

[VicS]

I should know this but here goes...
Can someone please remind me of the relationship between barometric pressure, as measured in millibars, and sea level? I seem to recall a simple relationship such as 1mb rise equals 1mm drop in sea level but perhaps I am out by a factor of ten?
Thanks!

yes out by a factor of 10. Aprox 1mb = 10mm water

 

[caiman]

My understanding is that atmospheric pressure is regarded as 1013 mb as standard @sea level.A mb difference + or - = 1cm difference in water height.ie 1014 means the water level is 1cm lower than the tidal prediction,1012 is 1cm higher.The National Oceanograpy Centre web site publishes a predicted tidal curve and superemposes the actual tidal level due to meteorogical effects at various locations around the UK.The 'real time' graph is usualy about half an hour behind actual time.The site is well worth a visit IMHO.Very educational.
Hope this helps.
Cheers

Vic S is on the ball as usual.

 

[VicS]

My understanding is that atmospheric pressure is regarded as 1013 mb as standard @sea level.A mb difference + or - = 1cm difference in water height.ie 1014 means the water level is 1cm lower than the tidal prediction,1012 is 1cm higher.The National Oceanograpy Centre web site publishes a predicted tidal curve and superemposes the actual tidal level due to meteorogical effects at various locations around the UK.The 'real time' graph is usualy about half an hour behind actual time.The site is well worth a visit IMHO.Very educational.
Hope this helps.
Cheers

National Tidal and Sea Level Facility http://www.ntslf.org/

 

[Humblebee]

Thanks guys, that's what I needed to know!

 

[Uricanejack]

As previously stated 1013 is standard for average sea level.
how much the pressure changes with altitude is also dependant on tempreture.

A rough guide an average on an average day about 33ft or 10m per milli bar.

little airplanes (not knowing much about big ones assume it may be similar) set their altimeters to 1013 until they get the updated pressure from the airport.

 

[GrahamM376]

little airplanes (not knowing much about big ones assume it may be similar) set their altimeters to 1013 until they get the updated pressure from the airport.

Unless things have changed since I gave up flying, 1013.2 is used by everyone when flying in airways, so they maintain separation. On the ground it's UK practice for many to set the altimeter to zero feet (QFE) on the ground, then go to local pressure setting (QNH) after takeoff. Some would set altimeters to QNH which means the altimeter would read the runway height above sea level but I preferred wheels on the ground at zero feet indicated so i didn't have to remember what height the airfield was.

 

[nigelmercier]

A rough guide an average on an average day about 33ft or 10m per milli bar...

Errr ... I think you will find it is one foot per 30mB

 

[BobE]

On a KISS basis I have
1013 mb= 0
1003 =+0.1m
1023 =-0.1m
so one tenth of a meter change for 10 hectopascals
which is 3.937 inches (say four)
and sailing from Christchurch its pretty important !!!!
Cheers Bob E...

 

[VicS]

Errr ... I think you will find it is one foot per 30mB

I think he was suggesting that 10mb per 33ft ( or 10m ) was the variation with altitude....... but that' s not right either its 11.4mb per 100m.

 

[VicS]

On a KISS basis I have
1013 mb= 0
1003 =+0.1m
1023 =-0.1m
so one tenth of a meter change for 10 hectopascals
which is 3.937 inches (say four)
and sailing from Christchurch its pretty important !!!!
Cheers Bob E...

KISS ???
The sweet and simple answer to the OP's question is what I said in #2

 

[nigelmercier]

KISS ???
The sweet and simple answer to the OP's question is what I said in #2

Quite so, but 1cm for 1mB is easier to remember.

 

[VicS]

Quite so, but 1cm for 1mB is easier to remember.

Whichever, but does not switch to a mix of inches and hectopascals

 

[Roberto]

My understanding is that atmospheric pressure is regarded as 1013 mb as standard @sea level.A mb difference + or - = 1cm difference in water height.ie 1014 means the water level is 1cm lower than the tidal prediction,1012 is 1cm higher.

Actually, tide predictions in the UK (and some other countries) are not given against a standard of 1013 but referred to "local average pressure", which means that a couple of additional precious centimetres may be gained or lost in the calculation


http://easytide.ukho.gov.uk/EASYTIDE/EasyTide/Support/faq.aspx
http://noc.ac.uk/f/content/using-science/Info_Tides_and_meteorological_effects.pdf

 

[oldbilbo]

Sheesh! To think I took up weekend sailing as a rest from the complexities of weekday Altimetry calculations using the ICAN Standard Atmosphere and doing Bernouilli Counts with the SenMetO's calibrated can at the end of the duty runway, in the rain....

 

[VicS]

Actually, tide predictions in the UK (and some other countries) are not given against a standard of 1013 but referred to "local average pressure", which means that a couple of additional precious centimetres may be gained or lost in the calculation


http://easytide.ukho.gov.uk/EASYTIDE/EasyTide/Support/faq.aspx
http://noc.ac.uk/f/content/using-science/Info_Tides_and_meteorological_effects.pdf

Thanks for that. Made a note of the links.
I hope Dylan realises that when he eventually gets to the round the north of Scotland section of KTL he will have 2cm less water everywhere than he thinks

 

[Uricanejack]

I think he was suggesting that 10mb per 33ft ( or 10m ) was the variation with altitude....... but that' s not right either its 11.4mb per 100m.

No I was suggesting 1mb per approximately 10m.
Which while vague.

Is close to your 11.4 mb per 100m.

I will take your word as the more accurate number

In reality I completely misread the original question.

 

[GrahamM376]

Have a read - http://weather.mailasail.com/Franks-Weather/Pressure-And-Tides

 

[caiman]

This is the graph of the water levels during the recent 'weather'.It is from the site that VicS provided the link to.The red line is the tidal prediction,the blue line is the actual water level as recorded from the tidal gauge located on Barmouth bridge.Interesting stuff IMHO.
Cheers

 

[Uricanejack]

Interesting graph.

Although the pressure may have been low, on the days with the largest difference perhaps the wind direction may have had a bigger influence on some of those days particularly if it was blowing from offshore up a narrowing channel.

I have heard a wind from the north will increase sea level at south end of north sea considerably and if coinciding with a high tide results in the barrier being closed on the Thames.

I have heard there is a similar effect with a westerly on the Bristol Channel, I wonder if the eastern end of the English Channel is susceptible.