Friday, April 19, 2013

Modern Barometry and its Important Role in Marine Navigation

Talk presented at the commissioning of the World's Tallest Barometer at Portland State University on Apr 18, 2013.  (Videos of the instrumentMore news.)

I am grateful to Tom Bennett, designer and builder of this fine instrument, and to Portland State University for the opportunity to talk to you about barometers. It has been a favorite subject for many years, which started with my participation in ocean yacht racing in the early 80s. Barometric pressure is the key factor in the selection on an optimum ocean sailing route.

This facility promises to be a wonderful teaching tool. When you see the atmosphere push this column of liguid up three flights of stairs, there is no better way to appreciate Torricelli's insight that "We live on bottom of an ocean of air."  In just an hour of discussions with Tom as he demonstrated the instrument, we discovered a half dozen new informative study projects that could be added to the long list he had started.


When I mentioned I was giving a talk in the shadow of the World's Tallest Barometer, one of my associates said that he and a couple navy buddies learned about tall barometers on the deck of a ship, 35 feet above the water line. They had lowered a hose over the side to pump water onto the deck, and then could not figure out why the pump would not work; they had just used in on the dock that morning. Then his basic physics came back to him, and he realized they can’t pump water 35 ft, no matter how good the vacuum is at the top of the hose. Water will only rise 31 ft. This same observation is what ultimately led to the discovery of a working barometer in the early 1600s.

Now if the ocean had been made of this special vacuum oil used here, then their pump would have worked just fine, and no one would have learned anythingso we see the science sneaking into even the first thoughts of such an instrument.

And indeed, one of the things I would like to show is that the public understanding of barometers and barometric pressure is lagging way behind its prominence in our lives... and especially in our language.

The first thing Sarah Palin said to 70 million people in a vice presidential debate was “The barometer of the economy is a soccer mom.” Jerry Seinfeld said “The barometer of a relationship is the answering machine.” I am not sure what either of those statements mean, but the term is used as the holy grail of measurements, without anyone knowing what it means.

The public comments to the online article in The Oregonian about this project are another example. One stated: “If they wanted a barometer, they should buy one at Home Depot”which is cute enough, but misses the point more than they realize.

Barometers are not like thermometers. If you buy a thermometer at Home Depot it will probably workat least on some useable level. But if you bought a barometer at Home Depot it almost certainly would not work. It might go up and down to some extent, but it probably will not go up and down the right amount, and indeed it might only go so far, and then just stop. In fact, where the pressure matters most, at the two ends of the dial, the common consumer grade barometers are most likely not to work properly. In short, most such units are decorative, not functional.

But most barometer owners don't know they are not functional, because we do not know what function they are suppose to provide.

There is a certain awareness that pressure is related to weather: low pressure usually means bad weather and high pressure means good weather, and therefore a changing barometer means changing weather, but that knowledge is not useful if your barometer does not work right.

Ask most folks with that knowledge how high it has to be for good weather or how low for bad weather, or what is a fast rate of change compared to a slow rate of change, and there won't be many valuable answers... and it is not right to say that the right answer is there is no right answer, because there has to be an answer. You could say there is no short answer.

The problem is if your barometer does not work, you do not get any useful data to compile this answer from. Compare that with temperature. We know 90º is hot and 30º is cold. Going from 50º in the morning to 70º in the late afternoon is a big change, but not unreasonable. But 30º in the morning and 70º at noon is not possible, etc.

On top of that, there are perceived functions of barometric pressure that are demonstrably wrong, such as the correlation between atmospheric pressure and good fishing. There are tons of Fishing Barometers on the market. Yet if a fish changes depth an inch or two the pressure change it experiences is far more than would ever change with the weather. There could well be some weather conditions that do affect fishing that correlate with the pressure, but it is certainly not the pressure itself.... not to mention that fishing barometers are typically in the category of not working in the first place.

There are also Harvard Medical School scientists who have correlated large pressure changes with headaches, but it takes a black belt in statistics to appreciate the results. On the other hand there are tons of anecdotal evidence of that correlation.

There is also much anecdotal evidence that low pressure can cause the water to break earlier than expected in near full term pregnant women. That, it would seem has a reasonable model behind it, and one that should be easy to study from existing medical and pressure records, but I do not know of any real data.

There are of course many bonafide applications of accurate barometric pressure beyond its fundamental role in meteorology and weather. I will review a couple we have learned about over the past few years.


(1) Weather is of course No. 1. Pressure is a key input to atmospheric computer models used to predict the weather. I will come back to this one.

(2) Altimetry is a close second. As you rise above the surface or climb a hill, the pressure drops at a rate of about 0.44 mb per 12 ft (chosen so we can say “point four four per floor”). Altimeters used in aviation are barometers. They usually measure elevation more accurate than a GPS can do, except possibly in special cases using the WAAS satellites. Thus aviators, hikers, surveyors, and sky divers often rely on accurate barometric pressure measurements.

And now it is difficult to rank significance, so this is just a list, with no meaning to the order.

(3) The inverse barometer effect. It was known to mariners in the 1800s that “the fog nips the tide,” meaning that in foggy conditions the tide is not as high as normal. We know now this is not the fog at work, but the pressure. Fog is often associated with High pressure. When the tide rises, it has to lift the atmosphere and in High pressure the air is heavier so the tide does not go up as much. This is a small effect (1 mb of extra pressure inhibits the tide by 1 cm), but still a crucial one to many applications and again to our understanding of the role of pressure in our lives.

It is, for example, crucial to understanding the rise in global sea level, since this factor is much larger than sea level changes over the years. It has to be carefully factored out of all measurements so we learn what the sea level really is.

The roll of this factor is also strangely mis-represented on occasion by experts who know better. The head of NOAA stated after the record storm surges following Storm Sandy that this would not have been so bad had it not been for the global warning induced rise of the sea level. This is frankly not the case. Storm surge is water coming ashore that is much higher than predicted by the tides. The vast majority of the surge they saw was due to very Low pressure and strong onshore wind. In fact, tide surge by definition cannot be related to sea level height because the predicted tide heights are referenced to the known sea levels. From our context here, this is example of not giving atmospheric pressure the attention it deserves.

(4) Mining safety. The safe ventilation of mines relies on measurements of relative pressure throughout the mines, which are ultimately referenced to the station pressure at the surface entrance. This can be a tricker measurement than many interested parties are fully aware of. The issue is that at high-elevation mines (some are well above 7,000 ft) even some quality barometers might not work properly, because an are calibrated to work at sea level. This application requires careful consultation between mining engineers and the sellers of the barometers they use.

(5) Calibration and adjustment of various instruments.

Oncology labs that provide radiation therapy use dosimeters to calibrate the radiation intensity. There are several ways to calibrate these, but one popular method relies on knowing an accurate value of the station pressure at the time of calibration.

Crime labs must periodically calibrate the breath analyzers used throughout their jurisdictions, and as of last year, the method of choice now requires an accurate station pressure.

High precision Pick and Place assembly line robots use a vacuum to do the lifting, and the setting of these pumps require an accurate station pressure during calibration.

Inflatable life rafts must be certified for leakage under strictly controlled conditions, which include by law an accurate value of the station pressure at the time.

High performance racing car engines are tuned to meet atmospheric conditions before each race. We learned from one organization that they had a definite advantage over their main competitor because he knew the competitor was using his barometer incorrectly. There is often a misunderstanding about the relative role of station pressure and sea level pressure. All of these applications mentioned need station pressure at the time and location of the event. They do not care at all about the (sea level) pressure the local radio station is reporting, which could be dramatically different.

In analogy with the race car engines are the very much bigger engines used to generate power in electric power stations. The output from these engines is remarkably sensitive to pressure. A 0.3% error in the pressure causes a 0.5% error in the output. Considering the stations cost $100M, this is a real number.

(6) And then we come to a favorite of mine, world records in long track speed skating. For years it was thought that high elevation skating venues held all of the world records and athlete’s best times because of the “fast ice” at high elevation. There were even multiple theories published about why the ice is so much faster at high elevation. But it is not fast ice; it is “fast air.” This is such a fine tuned sport with records differing by tenths or hundredths of a second, that it is easy to show that the lower atmospheric pressure at high elevation reduces the drag on the skaters by more than enough to account for these observations. It is for the same reason that more home runs are hit in Boulder, CO than in Oakland, CA. The same hit goes about 20 ft father on average in Bolder.

During our research of this topic we found a former olympic skater who had been preaching this for years, but no one listened to him. He was right all along. To his credit he was telling the sport years ago that they should be recording the atmospheric pressure with every record set.

Marine Weather

The history of meteorology as a science owes much to the work of early mariners. Their logbook records of wind and pressure during the age of discovery were crucial to the development of modern meteorology. Their records led to early theories of global weather and to an understanding of storm formation and behavior.

The very concept of a weather forecast came from mariners, specifically Robert FitzRoy, the captain of Beagle who took Darwin around the world. (Darwin was originally there to provide the captain with intelligent company at dinner, but it obviously evolved into rather more.) It was FitzRoy's concept to have observations on the west coast of England telegraphed to the East coast so fishermen could be prepared. It was known by then that midlatitude weather moved toward the east.

FitzRoy was also the pioneer of the use of barometer for weather forecasting and analysis.

Barometer measurements are important to weather in all walks of life, but they have a special significance to mariners at sea, which has persisted for 300 years. Even today, the barometer means more to sea captains than to any other group of users—except maybe managers of power stations who are accountable to their stock holders!

When you are at sea, it is not a matter of just going inside if the weather gets bad. Any tool that can warn of bad weather is crucial so the vessel can be prepared and navigated accordingly. They are not going to pull off the road and wait this one out. The pressure makes the wind, and the wind makes the waves, and the waves can be a threat to any vessel.

Sailors use the barometer for weather warnings, but they also use their barometer to find more wind or more favorable wind more often than they use it to avoid too much wind.

The center of the Atlantic or Pacific ocean in the summertime is usually dominated by a broad mountain of high pressure, which creates a windless desert on the surface. To sail across an ocean, you must sail around this High or get stuck like the Ancient Mariner with no wind for many days. World sailing routes are determined by the intensity, location, and motion of these mid-ocean Highs. With study and experience you learn how how close you can get to the High and still have wind, but you need an accurate barometer to map out the mountain as you go around.

But you can rightfully ask: Don't all mariners now have satellite phones and modern weather maps giving them the best possible forecasts? Why is the barometer still so important?

The key to this answer are the words “best possible.” There will always be a forecast, that is certain. But marine forecasts are not marked good or bad. On land we get 40% chance of rain; but at sea you do not get 40% chance of gail. They either say gail or they do not. (Mountain weather and fire weather have probabilistic forecasts, but marine weather so far does not.)

And in fact, some configurations of the atmosphere lead to much more dependable forecasts than others. Thus it is the job of the mariner to gather all possible data to help them evaluate and correlate the official forecasts with what they observe.

Ironically, the more sophisticated the forecasting has become, the more valuable the barometer on each vessel becomes. A common way to get weather data at sea these days is via digital weather maps sent by sat phone and viewed in a computer program. These are beautiful products with very detailed data, which is easily accessed. But this super-convenient source of data is a direct output from numerical models that is delivered unchecked by human meteorologists. Most of the weather maps you see on TV etc are this same type of datain the US, usually from the Global Forecasting System (GFS) model. This model does a very good job in some cases, but not all.

Thus the mariner needs to check the model data, by comparing what the model predicts for the pressure compared to what they actually observe. If they agree, they can be more confident that the forecasts are good, but if they do not agree they know to be more cautious in their decisions.

Even when the forecasts, no matter what their source, are even right, it is not uncommon that the timing is off somewhat. The storm could have started to move faster than predicted, or slower. By watching the barometer they know how things are actually changing.

The barometer work is especially important for sailors looking for more wind in light air. Regions of light air are not predicted well by the forecasts, and the difference between 4 kts of wind and 8 kts of wind is huge factor under sail.

The other factor important to small craft mariners at sea is independence. They know that all systems, including their electronic connections to civilization, are vulnerable at sea, and if they lose this connection, the barometer becomes paramount to their shipboard forecasting. In the tropics for example, the pressure is very stable and with a small standard deviation of change. A detected change from the normal average daily pressure being observed of just 3 or 4 mb can be a strong warning of a tropical storm approaching, even if no other signs are apparent, especially if accompanied by the onset of a new long, low swell.

I commend Portland State University for building this beautiful instrument.

It is not only a unique teaching tool, it is a monument to the history of science. After all, if you look into who all was involved on some level (Aristotle, Galileo, Descartes, Pascal, Boyle, Hooke, Halley, Locke, Leibniz, Bernoulli) and look at the ramifications it had on the evolution of philosophy, science, and maritime achievements, it is fair to say that the “Barometer of the history of science” is “the history of the barometer itself.

Reference links:


Location of instrument if you get a chance to visit: 45º 30.560' N, 122º 40.880'W 
Go in the door and turn right.

To check for an accurate reference pressure, see
To see the historical accuracy of the various pressure stations nearby see:


Tom said...

David, thanks for posting this - just saw it. Will send you a photo of the top of the barometer - uv light has turned the clear barometer oil a pale ale color, making the column much more visible.

Tom Bennett, Portland State University: Civil and Environmental Engineering Dept.

Ludwig said...

I just bought a barometer for my boat. It has not even arrived yet. I am glad I found this article because I thought a compensated barometer needed to be calibrated once and that was it. Now I have another instument to tweak and tinker like the rest of my collection of navigation instruments. Thank you for the interesting article