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:
- 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).
- 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)
- 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.
- 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.
- 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.