Monday, March 18, 2024

A New Revolution in Barometers

We have worked for many years promoting the use of accurate pressure in marine navigation, which had literally fallen out of all standard texts on marine weather twenty years ago. The word "barometer" was barely mentioned. We would see occasionally that a falling barometer means bad weather, but nothing more, and certainly nothing about how fast it must fall for bad weather. And all of these books state—they are all still in print—that the value of the pressure does not matter; it is just a question of rising or falling, fast or slow, but never with any numerical values.

Accurate pressure was crucial in the late1700s and early 1800s when much of global marine weather was first learned and understood with the aid of accurate mercury barometers used at sea. But they were unwieldy and difficult to use and happily set aside with the development of aneroid barometers in the mid 1800s. That revolution took place without the full recognition that with the great convenience of the aneroids came a notable loss of accuracy over the higher and lower ends of the dial, which typically matter the most in routing decisions—a fact that has followed aneroid use into modern times. Thus began the doctrine that only the change in the pressure matters, not its actual value.

Now it remains as it was then: only the high-end, expensive aneroid units can be counted on for accurate pressures over the full range we care about in marine navigation. I would venture to guess that most barometers on vessels today are there primarily for traditional reasons, and not referred to for routing decisions.

We began our goal to change that with the first edition of Modern Marine Weather and had gone into the interesting history of how this came about in The Barometer Handbook. Both books show how important it is to know accurate pressure to evaluate numerical weather predictions that we ultimately rely on for routing. 

Accurate pressure is also often the fastest way to detect a change in the weather or the movement of a High pressure system we are carefully navigating around. Responding to the motion of a High is often a key decision for sailors in an ocean crossing.

In the tropics, where the standard deviation of the seasonal pressure is just a couple millibars (mb), we can know from accurate pressure alone whether or not a tropical storm is approaching—and we can know this before we see notable changes in the clouds or wind. Needless to say, we navigate in such waters primarily based on official forecasts and tropical cyclone advisories, but an accurate barometer gives us early notification that forecasted storm motions are on time, early, or late. On the other hand, any loss of wireless communications makes the barometer even more important.

In the hurricane zone between Panama and Hawaii, we would expect a July pressure of about 1012 mb, with a standard deviation of 2 to 2.8 mb.  A measured pressure of 1007 mb (2.5 standard deviations below normal) has only a 0.6% chance of being a statistical variation and a 99.4% chance of being an early tropical storm warning.  This type of analysis does not work at higher latitudes because the standard deviations are much larger.

Pressure statistics needed for this type of analysis are included in our Mariners Pressure Atlas.

We developed a sophisticated electronic barograph that was quickly adopted by the NWS for use on the voluntary observing ships  (VOS). We later sold that product to another company.

To further support the use of accurate pressure, we became the US distributor for the state of the art Fischer Precision Aneroid Barometer, used by those who want the best of the best in a mechanical unit, including the Navies, Coast Guards, and Weather Service vessels around the world, including the US. Fischer is one of the last sources for accurate, hand-made aneroid barometers.

To follow up on that, we developed both a free Marine Barometer app and low-cost Marine Barograph app for iOS and Android mobile devices. 

In short, we have worked on barometers for over 20 years now, but I felt we still did not have the unit that could have the biggest impact on marine navigation, which is what lead to the development of the Starpath USB Baro.

Not all vessels can invest in the high-end units. The mobile apps, while providing a convenient backup that can indeed broadcast pressure data to a navigation program, still rely on a device that must be charged and protected. Also running it full time does put a strain on the phone's battery life.

The New Revolution

Our goal was to develop a barometer that was first and foremost highly accurate and dependable, plus we wanted it to be easily portable. Finally, we wanted to produce it at a low enough cost to be attractive to all mariners, even those using it as a backup. For mariners we also need the output signals to be in the NMEA standard to match navigation electronics and software.

The result is the Starpath USB Baro for $49, which includes a metal transport case. It can be read in any Navigation program, or use our free USB Baro app for Mac or PC.

In stock and ready to ship from the link above.

Below shows how the pressure appears in three popular navigation programs. Video setup procedures for each are shown in the link above.

We can compare this with official pressure data from the West Point Lighthouse (NDBC WPOW1), which is 1.6 nmi from where the USB Baro data were accumulated.

The red square marks the data corresponding to our measurements with the USB Baro. We can now overlay that data with what we measured, as shown below.

So, we see that with this simple device we have access to the same pressure data that NOAA relies on to make their official forecasts and numerical weather predictions.  

The difference between1023.0 mb indicated in the Lighthouse value and the 1017.2 mb observed in our office can be accounted for to the tenth of a mb, because of the elevation of the USB Baros compared to the sea level data from NOAA.  All of the Nav apps used offer the option to incorporate this offset so the instrument reads sea level pressure directly. Our free Marine Barograph apps made for the USB Baro also have that option.

Our Guarantee

If you have now a common aneroid barometer and then compare what it reads with the known accuracy of the USB Baro over a pressure variation of 30 mb or so, you will be very pleased to own the USB Baro. 

You will either show that your aneroid is accurate, effectively calibrating it, which otherwise costs $195, or you will learn that you did indeed need a more accurate source of pressure for your boat or home.


Below we see the same comparison between the official NOAA data from West Point Lighthouse and the USB Baro measurements using the free computer app, rather than a nav app.

Again we see the finest detail in the atmospheric pressure variation measured 1.7 miles apart being captured in the USB Baro and its free computer app for Mac or PC.


Mike said...

Hello David,
There is a question about the pressure trend arrow conventions you recommend to all barometer manufacturers.
Can these symbols be freely implemented to support the standard, or some copyright/proprietary limitations do apply?
I have emailed this question to you few days ago, perhaps it ended up in the spam folder.

David Burch said...

I apologize for the delay. Your email was passed on to me then I misplaced it.

The trend arrows we use are ones we developed ourselves over many years of marine weather study. They are presented in our online marine weather course and we have used them in all the barometers and barometer apps we have produced. They can be seen with full instructions in the online help files at

You or anyone is welcome to use them. The best citation would be simply “Compliments of Starpath School of Navigation.”

The definition of the symbols are unique and tied to our standard pressure alert of “4-5-6,” explained in that help file and in our textbook Modern Marine Weather.

Note the delay in your original email came about be because the actual symbol included in the email was not one of our trend arrows, but a sample of the pressure tendency characteristic code symbols, and these are indeed public domain. They are a WMO standard, described in the NWS Observes Handbook, available at

It was going to take awhile to write that up (now done!) and that is how I got distracted. Again, I apologize.

I hope that helps