Standard air pressure for tidal calcs?

[Danny Jo]

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

Oh shucks! Wrong again. I owe an apology to Phil, for suggesting that he had been misled by Reeds, and to Reeds for even contemplating that it might be wrong. [Edit - more fool me for not spotting the URL in ancientsailor's signature; I would have thought twice if I had known I was taking on a pro!]

In mitigation, the UK hydrographic office practice (of basing tidal height predictions on the mean barometric pressure for the locality and the season) is not universal - in New Zealand, at least, it is based on a standard pressure of 1013 kPa. (Clicky for LINZ page "Meteorological effects on tides"). And the references to the 1013 kPa standard on web pages apparently written by UK-based individuals (this one, for example), make me wonder whether the UKHO might have introduced this refinement relatively recently.

 

[ancientsailor]

Oh shucks! Wrong again. I owe an apology to Phil, for suggesting that he had been misled by Reeds, and to Reeds for even contemplating that it might be wrong.

That's very handsome of you! The forum was becoming increasingly complex and I have learnt a lot.

Next person to come along may well prove me wrong!

Whether it's recent Admiralty practice, I don't know. I still have my 1980s Admiralty Manuals. When I am a loose end I might have a look if no-one else has the definitive answer first

 

[timbartlett]

Next person to come along may well prove me wrong!

Whether it's recent Admiralty practice, I don't know. I still have my 1980s Admiralty Manuals.

Admiralty Manual of Navigation 1987 Vol 1 refers to "average" barometric pressure
Admiralty Manual of Navigation 1964 Vol 1 mentions that barometric pressure has an effect, but doesn't specify "average" or "standard".