Tools of the Trade: Barometer Basics

Barometers were an important part of keeping track of weather aboard ship in the days before international GPS and other forecasters of potential trouble. When I was young, we had a barometer that as it turns out was more about looks than function. It hung nicely on the off-white walls of what ever house or apartment we happened to be occupying, three clock-like faces announcing simple weather events like “Rain”, “Sun”, “Winds” or “Change”. Little did I know how effectively useless that wood-and-brass showpiece was.


Working barometers tend to look more like thermometers than clocks, with the liquid mercury falling or rising according to the effects of wind or humidity; or both. As Peter H. Spectre wisely notes in A Mariner’s Miscellany, the words and even the number the hand on modern, decorative barometers is pointing to should be ignored all together. “What you are interested in is the rate and direction of change in the barometer readings.”

Here, then, is a by no means all inclusive list of the general changes one might see in a functional barometer and what they indicate:

When the barometer rises, keep these factors in mind: a rise with southerly winds is a sign of fine weather. If the rise comes with winds from the north, note the air and temperature; dry, cold air will portend better weather on the way, particularly in summer, while humid air with cool temperatures is a sign of windy rain. A gradual rise in the barometer usually means the weather will stay steady; a rapid rise means change – and possibly lots of it – on the way.

For a falling barometer, the same types of modifiers apply. Increased humidity and heat is a good indicator of weather coming up from the south. If in the cooler months the air is dry and cold when the barometer falls, snow is on the way. A falling barometer after a lengthy calm spells rain and possible gales, particularly if the air is warm. If the winds are from the north as the barometer falls, plan on rain or hail in the summer, and snow or sleet in the winter. A rapidly falling barometer is a sure sign of a storm.

When the barometer is steady, fair weather can usually be counted on; this in particular if the air temperature is appropriate for the latitude and season.

Keep in mind that all wind directions are for the Northern Hemisphere, and should be reversed when in the Southern.

As noted, these are simply a few points to keep in mind. For more in depth information about seafaring tools and the nautical life in general, Spectre’s book (and his annual Book of Days series) is an invaluable resource.

Header: Weymouth Bay by John Constable c 1817

Tools Of The Trade: The Mercury Is Up

The barometer has become something of an antiquated and almost quaint instrument hung in family foyers and boat rental shops as a reminder of ye olde days of sailing. What with all the computer weather maps and so on, the humble barometer seems out of its league. In the great age of sail, things were very different. The barometer in her hey day was the weather computer.

The invention of the barometer is attributed to Italian scientist and sometime alchemist Evangelisto Torricelli around 1643. In fact a lot of people had opinions about what made the device – originally a 35 foot tube – work and figuring out what held water in the tube steady was what made the barometer possible. Great minds like Rene Descarte and Galileo Galilei got in on the act, finding out that it was not, as originally thought, a vacuum in the end of the tube that held the water up. In fact, it was the then unfathomable pressure of air.

Prior to the 1600’s, science believed that air had no weight. Not clearly understanding how high the sky really was, and the implications of changes in altitude, they simply assumed that because human’s felt nothing heavy bearing down on them from the sky, air must be weightless. In 1646, French scientist Blaise Pascall changed all that by switching the liquid in his tube – now a manageable 32 inches – to mercury and testing it’s steadiness at various heights on a climb up the mountain known as Puy de Dome in Auvergne, France. The mercury level dropped lower in the tube the higher up the mountain Pascal’s brother-in-law went.

By the 1790, Torricelli’s mercury barometer was in use at sea. It consisted of a glass tube 36 inches long. The top of the tube was open and filled with what was then known as quicksilver, refined mercury. On the lower end is a reservoir and the tube is generally mounted in a case of some kind (as with the circa 1790 examples above from the Musee des Art et Metiers in Paris). A scale is attached next to the tube for reading the mercury. Because a vacuum is formed at the top of the tube, the mercury will adjust to the atmospheric force exerted on the reservoir, thus giving a reading of air pressure. High pressure, logically, places more force on the reservoir and low pressure does the opposite.

Barometers have a reputation for being testy and there is good reason for this. Changes in temperature affect the fluidity of the mercury upon which the mechanism depends. Thus a barometer that was made in Ireland and sailed off to the Caribbean will not read the same in Clew Bay as it does in Nassau Port. The scale for reading the height of the mercury must be adjusted by the reader in such cases, so a clear knowledge of the behavior of the barometer is as important as knowing the scale. By the Napoleonic Wars, a thermometer was routinely mounted in the barometer case for ease of reference.

The scale itself is relatively simple once you get the hang of it. 1 atmosphere was equivalent to 29.9 inches and a mercury barometer would generally read between 28 and 31 inches. This gives you an idea of just how sensitive these “primitive” instruments were. For example, the average atmosphere in Seattle, Washington in 29.6, a relatively low number. As we know from watching the local weather, a “low” indicate clouds and precipitation. A high barometer may seem like a good thing but wind often slacks with what a landsman might call fine weather, and no sailor likes a dead calm.

In practice, seamen would take into account all signs that could lead to a good guess at upcoming weather. The barometer would be consulted but then adjusted to temperature and wind direction, which also affected its reading. The sky would be glanced at, the waves noted and yesterday’s weather offerings pondered. In the final analysis, nothing could match experience for accurate prediction of the weather.

Of course, like all technology, barometers improved with expert tinkering, leading to the barographs and aneroid barometers (those round kind that look rather like a clock) that we are familiar with in the modern era. It’s worth remembering, though, that the barometer was once a much coveted instrument, and even a symbol of status. Wealthier naval Captains (and ostentatiously successful pirate commanders) would have finely tuned and beautifully engraved barometers of brass or even silver hanging in their cabins. There they would be checked regularly, admired by guests dining aboard, and polished to gleaming brilliance by stewards or cabin boys.

As Torricelli put it, in poetic language with a hint of the nautical: “We live submerged at the bottom of an ocean of elementary air.” And his barometer proves him right.