0.02953 inches to the hectopascal

When scientists talk about pressure, they measure it in Pascals (Pa: the SI unit for pressure). For atmospheric pressure, 1Pa is an inconveniently small number, so we lump them together in groups of 100 and talk about hectopascals (hPa: 1hPa=100Pa). The atmospheric pressure at sea level is usually given as 101325 Pa, which is approximately 1000 hPa; so 1 hectopascal is also referred to as 1 millibar – when you hear your weather forecaster talking about millibars, hectopascals are what he’s really using. The ships, however, don’t measure pressure in hectopascals or even millibars; they measure it in inches. This is an artefact of the way they measure the pressure – with a mercury barometer.

Back in the early 17th century there was much discussion among the scientists of the day about why it was impossible to pump water more than about 10m upwards. It was Evangelista Torricelli, in 1643, who realised not only that the height to which the water rose was determined by the weight of the surrounding air, but also that you could use this effect to measure changes in the air pressure. A 10m column of water is a nuisance to work with, so he switched to the much heavier mercury as his working medium, and made the first ever barometer measurement.

We’ve been measuring air pressure in the same way ever since – balance the weight of a column of mercury against the weight of the surrounding atmosphere, and the taller the column the higher the atmospheric pressure. At sea-level, the column will be about 76cm (29 inches) high, and the changes in atmospheric pressure as the weather changes cause fluctuations of up to a few inches. The pressure is proportional to the height, so we can get the pressure in hPa by multiplying the height in inches by 33.86389.

Of course, making precise measurements requires great care (very pure mercury, no air in the tube, careful calibration, …) but by our period (1914) barometer manufacturers were making very good instruments. There are, unfortunately, still a few complicating factors which we need to be aware of:

  1. The weight of a column of mercury changes with temperature – the weight of 760mm of mercury is less when it’s hot than when it’s cold, so we need to adjust for this when calculating pressure from height. A further complication is that the column height is usually measured using brass measuring rods, and the length of brass rods also changes with temperature. So we apply a correction from a table or an empirical formula – these tables vary slightly depending on the barometer design, but in OldWeather we don’t usually know the make of barometer in use so we use a generic table. To make this temperature correction we need, of course, to know the temperature of the barometer: Almost all mercury barometers have a thermometer attached and it is usual to record the barometer height and attached thermometer temperature together – as is done in many of our logs. Moving from 0C to 35C (Arctic to the tropics or February to July in the UK) would introduce a change of about 0.5% (2 tenths of an inch).
  2. The weight of a column of mercury changes with latitude. We launch satellites from French Guiana, rather than Europe, because satellites weigh less in French Guiana than they do in Europe. Moving from Plymouth to Singapore would reduce your weight by about 0.2% (about 8 hundredths of an inch)
  3. We generally want the pressure at sea-level. We usually keep the barometer above sea-level, so we need to add a little to the pressure to adjust for this. Every 80 or 90 feet above sea-level reduces the pressure by 1 tenth of an inch.
  4. It’s usual to measure the position of the top of the mercury column. As the mercury rises in the tube, the level of the mercury in the cistern at the bottom of the tube will fall. Because the mercury column balancing the atmosphere runs from the top of the level in the tube to the level in the cistern, we need to add a little to measured height changes to allow for this.
  5. If the glass tube containing the mercury column is narrow (to reduce weight and to damp oscillations) the height of the mercury will be reduced by capillary action. We need to add a little to the measured height to allow for this.

We call these, respectively, the temperature correction, the gravity correction, the height correction, the capacity correction and the capillary correction. By 1914, with a good barometer, the last two should have been allowed for in the instrument’s calibration and operation, and the third is small for ships, but we still need to make the first two corrections. The changes involved are small compared with the changes associated with short term weather, but they are important for correctly representing the more subtle, longer-term changes.

Mercury barometers are great for fixed, stable, weather stations. They are however expensive, difficult to read accurately in a ship in motion, a terrible nuisance to carry around, and really too fragile for service in a warship. So much ingenuity has been spent on devising cheap, portable, alternatives. The aneroid barometer is essentially a sealed metal bellows that grows and shrinks as the air pressure rises and falls, coupled to machinery to amplify its movements and display them on a scale. These first appeared in 1843, but it took a long time to make them accurate and reliable enough for serious use. By 1914, however, they were coming into use, and it’s clear from the logs that our ships used both mercury and aneroid barometers. Aneroids don’t require gravity, capacity, or capillary correction – and are mostly deliberately designed to be insensitive to temperature changes, so they don’t need an attached thermometer measurement. Nowadays aneroid barometers report pressure in hPa, but back in 1914 most gave readings in inches of mercury. So far I’ve only seen one ship reporting pressures in hPa – HMS Glowworm.

Were the aneroids on our ships less accurate than mercury barometers? more accurate? different in some subtle way? I don’t know – but I look forward to finding out. So if you see any reference in the log to the type or make of barometer in use, please transcribe it. We don’t need to know what they were using, as we can guess with good accuracy, but it does help. A few ships record both mercury and aneroid barometer readings – if you see this, please transcribe both of them; the comparison between them helps us estimate the accuracy of the measurements.

6 responses to “0.02953 inches to the hectopascal”

  1. John Dulak says :


    I must take exception to part of your blog entry on barometers.


    “We launch satellites from French Guiana, rather than Europe, because satellites weigh less in French Guiana than they do in Europe.”

    That is true to a small extent but the REAL reason you try to launch satellites from as close to the equator as possible is to take advantage of the liner rotational speed of the earth. At the poles the liner rotational speed of the earth is almost nill. However, near the equator the liner speed of rotation is substantial.

    1 revolution 25,000 Miles
    ———— X ———— = ~1000 miles/ hour
    24 hours Revolution

    This represents energy that you do NOT have to provide via the launch vehicle so most satellites are launched from as close to the equator as possible. The kenitic energy of a moving body scales with the velocity SQUARED so it pays to take advantage of this.

    John Dulak

  2. Bunts says :

    Just when I think I’ve got an improved handle on the nature of your activity, thanks to your detailed history of the science, along come A, B, C, D and E.
    I never cease to marvel at the layers of complication and refinement that are built on previous discovery and invention.
    Disregarding any unpleasant connotation, “Standing on the shoulders of giants” is not inappropriate. I wonder what the the thoughts of your predecessors would be if they knew of the development of their work.
    Remarkable stuff.

  3. studentforever says :

    I have usually recorded the make and model no of the barometer when this has appeared. I have sometimes recorded the accuracy but desisted when the figures were always the same and how did they assess it anyway.
    I have posted to the forum two occasions when inaccuracies in the readings were noted in the logs and recorded it in the transcription under EVENT-OTHER. How you deal with these ships I don’t know, one of the advantages of not being involved in the actual research is that I only have to report it! Interesting and useful explanation of the workings of the barometers.

  4. Yvan Dutil says :

    I was wondering. Is there any effort to homogenize the data? In theory, ship moving from place to place could be used to carried the reference trough the world. A similar process was used in the past to synchronize clocks around the world. Hence, Greenwich and Harvard exchanged hundred chronometer the establish the longitude of Harvard to Greenwich. The same thing should be doable with pressure measurement.

  5. Philip says :

    We are not doing homogenization as part of OldWeather, it’s desirable to keep the raw records as well as any processed versions of them, and our primary aim is to recover the actual measurements made.

    But other people will subsequently work to homogenise the data – ship systematic errors and point measurement uncertainties are estimated as part of the data assimilation process when running reanalyses. And there is a project being planned to produce a homogenised version of the entire ICOADS (http://icoads.noaa.gov) – our observations are going into ICOADS.

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