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The BBC News item Barometric pressure in London 'highest in 300 years' at least begins with

Overnight, Sunday into Monday, London's Heathrow Airport recorded a barometric pressure of 1,049.6 millibars (mbar).

It's very likely the highest pressure ever recorded in London, with records dating back to 1692.

It links to this Met Office tweet

So far Mumbles Head in Wales appears to have recorded the highest pressure during this current settled spell, with 1050.5 hPa earlier this evening. This remains a little short of the 1050.9 hPa recorded at Benbecula 16.1.1957, and the UK record of 1053.6 hPa, Aberdeen 31.1.1902

and then continues:

Met Office hourly observation reports recorded 1050.3 mbar at Liscombe in Devon, at 2100 GMT on Sunday evening. 1,050.2 mbar was recorded at Dunkeswell in Devon, and 1,050.5 mbar at Mumbles, in South Wales, shortly after.

None of these measurements breach the 1,053.6 mbar recorded at Aberdeen Observatory at 2200 GMT on 31 January 1902, which remains the national record, but the events of the past 24 hours certainly marked the first time for over 60 years that 1,050 mbar has been attained anywhere in the British Isles, said Mr Burt.

To discuss these records it is necessary to compare barometric pressure measurements to 0.1 mbar accuracy out of 1000 mbar, which is at the 100 parts-per-million level.

Question: How are barometric pressure measurements traceable over centuries to 100 parts per million accuracy? What was the procedure used to calibrate barometric pressure centuries ago that could be reproduced and compared to today's standards, or is there a very old standard absolute barometer somewhere that is believed to have remained calibrated?

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    $\begingroup$ Note that the 1692 measurements could have been far less accurate while still being lower than 1,049.6 $\endgroup$ – JollyJoker Jan 23 at 12:50
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Mark's answer about mercury barometers is correct - but it is not the complete story.

Barometers were initially constructed using water, not mercury, on the manometer principle. Water is lighter than mercury, so a water barometer requires a 10.3m tube. 1mbar is therefore roughly 1cm, and 0.1mbar measurement is clearly trivial using a ruler with 1mm gradations. Of course the exact units of measurement would depend on the country in which the measurements were taken, but it should be clear that measurement to this precision was perfectly possible, albeit with an inconveniently-large barometer!

Mercury barometers, with around 75cm height of mercury at 1bar, were significantly more convenient. As Mark says, this does require a much more accurate ruler to measure it though.

But the accuracy of a simple ruler is not the complete story either. The Vernier scale was invented (or at least popularised) around the same time. With a floating Vernier marker, mercury barometers could be read accurately to around 1/10th of the ruler scale. For a ruler marked in 1mm intervals, a 75cm column of mercury could therefore be read to 133ppm accuracy, which is close enough in accuracy to the original water-based barometers.

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  • $\begingroup$ Oh that is fascinating, a floating Vernier mercury barometer! A quick check of the internet for "floating mercury barometer" finds this and this on this page but I still don't understand what it is or how it works. Maybe that can be asked as a separate question... $\endgroup$ – uhoh Jan 21 at 17:43
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    $\begingroup$ @uhoh The basic principle is that humans are really good at spotting when things line up, because we're all basically genius-level pattern-matching machines, and even a slight difference is fairly easy to see. A Vernier scale uses that to let you resolve distances more accurately than it is possible to print or engrave lines. $\endgroup$ – Graham Jan 21 at 18:53
  • $\begingroup$ I've used Vernier scale all the time on mechanical calipers and micrometers, but I don't know what a floating mercury barometer looks like and how it works, so I've just asked What does a floating mercury barometer look like? How does it work and how is buoyancy used? $\endgroup$ – uhoh Jan 21 at 23:22
  • $\begingroup$ To compensate for thermal expansion of water / mercury, the temperature needs to be known to about 0.5 kelvin accuracy. But with some barometer models, that can be directly measured from the total length of the fluid column. $\endgroup$ – jpa Jan 22 at 14:22
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A mercury barometer is a simple, easy-to-build barometer that turns the problem of calibrating a pressure standard into one of calibrating a length standard. Accuracy of length standards has long been a concern of merchants and those regulating them; in 1692, the standard would probably have been a brass prototype yard in the possession of the Royal Society.

Historically, atmospheric pressure in the UK was measured in "inches of mercury"; a pressure of 0.1 millibars is about 0.003 inches of mercury (roughly 1/256th of an inch). I haven't been able to find evidence of rulers of that accuracy in 1692, but ones divided to the 64th of an inch (0.5 millibars) certainly were available.

In short, 100 parts per million? Probably not. 500 parts per million? Certainly.

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  • $\begingroup$ Ah! So 1.0000 mbar is 0.75006 mm of mercury. Okay, with a standard tube inner diameter and protocol for interpreting the shape of the inevitable meniscus, a simple barometer built today will perform the same way as one built centuries ago. I don't know why I didn't think of that. I guess because mercury is dangerous and long thin glass tubes break easily, we don't see too many mercury barometers around. Thanks! $\endgroup$ – uhoh Jan 21 at 3:44
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    $\begingroup$ Note that the millibar is also a deprecated unit. The modern unit of pressure is the pascal (1 hPa = 1 mbar). The Met Office uses hPa, I don't know why BBC News chooses to use mbar. $\endgroup$ – gerrit Jan 21 at 9:01
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    $\begingroup$ @gerrit because it's approximately 1000 times more convenient. $\endgroup$ – hobbs Jan 21 at 16:28
  • $\begingroup$ @hobbs Hardly, it is just a name, the general public will hear a number and som gibberish. They just need to remebr the actual name of the thing. He tye news just uses hPa and you won't meet mbar almost anywhere (you will meet bars for tyre pressures though). It is just a convention for a name. When you choose to use other thing like inchHg or torr (still common for blood) you just need to have an idea which nember id high and which low. I am using PSI for tyres and I do not care how many SIs are there in a metre. I just know which number I need. $\endgroup$ – Vladimir F Jan 23 at 15:56
  • $\begingroup$ @hobbs The reason why PSI is good for tyre pressures is the same hundreds of feet are good for aviation. One is a nice number from 0 to 100, the other nice numbers from 1 to 660, not awkward fractional numbers like kilometers or hundreds of meters. The name of the unit is almost irrelevant, one needs to know how much is small how much is big and how to acheve it. The name is just a convention. Here hPas are just fine in the weather news. It is just a different name. $\endgroup$ – Vladimir F Jan 23 at 16:00
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I don't think they are using that level of accuracy for the old readings.

Reporting pressure, in hPa, to one decimal place is being done for recent measurements. There are no values given for very old readings.

As for the record reading of 1053.6 hPa for 1902, I suspect that is a mathematical conversion of a reading that was most likely recorded in inches of mercury - the UK began converting to metric units in the 1970s.

As to the reporting of atmospheric pressure to one decimal place, 0.1 hPa is 10 Pa, which is 1.02 kg/m2, that seems a reasonable level of accuracy for modern barometers.

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    $\begingroup$ There's no loss in precision when converting between inches of mercury, milibars and Pascals, though. $\endgroup$ – Luaan Jan 23 at 9:28
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How are barometric pressure measurements traceable over centuries to 100 parts per million accuracy?

To compare records of barometric temperature, this standard doesn't necessarily have to be met.

Recording barometric pressure to 100 parts per million precision has been possible for centuries because it's reasonably trivial to observe movement of the level in a hydrostatic barometer by small fractions of an inch with the help of simple instrumentation. Over short timescales, the small differences between different readings are important meteorologically and for a single, stable barometer these changes are likely to remain accurate too.

Guaranteeing that the barometric pressure is accurate in absolute terms (and therefore that the various recording instruments that have doubtlessly been used over the centuries can be treated interchangeably) is harder, although the physics, as Mark has detailed, is simple enough to give meteorologists a good shot at it. It's plausible enough that it would be foolish to reduce the accuracy by adding the rounding error of recording the pressure at a lower precision. Yet when we talk about a record like this being surpassed, the premium on the difference between these records taken centuries apart being accurate is not that high.

The fact that one record surpassed another serves primarily a symbolic milestone that helps laypeople grasp roughly what kind of exceptional conditions are being described. Reports of the record don't have to make assertions about accurate comparisons between atmospheric conditions centuries apart at all (and as far as I can see they haven't). They have to choose a record that serves as a representative comparison, but so long as they limit their remarks to the difference between the records they are completely correct. Officials did record values to at least a 0.1 mbar of precision today, and some of values were higher than any that had been recorded by officials before.

These are, after all, merely records of barometric pressures at select points over the area of a whole country. They serve as an indicator of the barometric pressures governing the weather systems governing the country's weather as a whole at the time. There have doubtless been other very high pressure systems in the intervening years, and equally it's very likely that this has been one of the highest (on account of it breaking records where pressure has been recorded) but we don't really have to be absolutely certain that it has been the highest ever by any particular measure.

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The maximum reading was taken at Aberdeen Observatory and they still have a barometer which was in use during the period the reading was taken (the inscriptions dates it to 1888 and after 1922 it was still in use).

It's not a mercury one - mercury is relevant as the article states records go back to 1692, before aneroid barometers were invented, though does not state that records before 1902 were that accurate.

The dial appears to be marked with a resolution of 0.01 inches Hg, or ~0.34 mbar, though it's a little hard to say whether it's that or half that from the photo. It also has more than one scale and a digit window so the range is more than one revolution of the dial, a bit like modern dial indicators which measure down to one micron.

I don't think this will be the instrument that was used - this looks like a portable one but I'd imagine they had a charting ones for the observatory, and it was twenty years out of date by then - but it does set a lower bound on the accuracy of instruments known to be available to the observatory where the reading was made in 1902. The description for the charting barometer in their historic collection doesn't specify dates, and I couldn't read its plaque from the photo.

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