500 MB Chart for Mariners

The 500mb chart can be a useful weather tool for the weather savvy mariner along with the more familiar surface pressure charts. The 500mb chart is a constant pressure chart which means that everywhere on the chart the air pressure is the same (500mb). This occurs in our atmosphere, on average, at a height of about 5600 meters or about 18,000ft above sea level but varies from place to place due to the density of the air column.

The heights depicted on this chart represent the level at which the air pressure reaches 500mb or about one half the normal surface pressure of about 1013mb). The lines depicted on the chart are lines of equal height and are given in “tens of meters” above sea level so that the “540 line” on the chart means that the 500mb level is located at a height of 5,400 meters above sea level. Heights increase when the air is warmer and less dense and fall when the air is colder and denser so that the distance between these height lines indicates the slope of the 500mb surface.

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This is the key element of the “Advanced Meteoroology” course used to qualify for STCW certification as Chief Mate and Master. This is the same course that representatives of a large segment of the industry mocked and ridiculed as being irrelevant and unnecessary to a working mariner.

Having been a weather routing professional, I know better.

One of my old Captains was explaining the 500mb chart and asked me if I ever made a fort out of sheets when I was a kid. He said that I should imagine the 500mb like that - peaked in some areas, dropping in others - not a static flat layer. What a great visual!

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Good introduction or review of the use of upper air charts in Dec 2008 Mainer’s Weather Log

Mariner’s Guide to the 500 – Millibar Chart

The 500 mb charts were available by weather fax but it’s easier to get these charts now if the ship has internet access.

Marine Radiofax Charts

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This is the same course that representatives of a large segment of the industry mocked and ridiculed as being irrelevant and unnecessary to a working mariner.

I don’t know if it was the towing sector of the industry that you’re referring to, but unfortunately that would be a typical response by most of our mariners, too. If ever there were a pair of courses that ought to be mandatory for all deck officers, regardless of tonnage or trade-restriction, basic & advanced meteorology would be it, excepting those with Rivers endorsements to basic met only. I know that’s a blasphemous statement, but it’s true.

Too many among us just listen to or look at the basic NOAA marine forecasts, taking them at face value, and then go semi-blindly forth. Especially on the longer coastwise trips. It really is shameful when people who ought to know better do this as a matter of course and complain about after-the-fact “over-regulation” as a result of accidents & incidents, if not outright disasters. It’s even worse when their mates learn from the example set for them by masters who don’t need no steen-king training.

This is one of the few subjects that generally needs to be initially taught from the classroom. Trying to teach/learn it on-the-job is very, very difficult and unnecessarily slow. Been there, done that. Yeah, I know, “school sucks, waste of time, blah, blah, blah.” Bullshit.


I agree, best to be introduced in the classroom.

What’s changed now is weather routing software which displays the upper air charts updated twice daily is more widely used… The upper air charts can also be downloaded by email if there’s no internet access.

If there is internet access the upper air charts can be downloaded plus self-teaching is a bit easier with maternal from the internet such as this article from Starpath.

I carry Heavy Weather Avoidance and Route Design: Concepts and Applications of 500 Mb Charts on my kindle.

I keep a hard copy of that book on the tug for permanent reference. We don’t enjoy weather routing services, but we can and do download the 500mb charts right off NOAA’s website. The Mariners Guide to 500mb Charts I have printed and in plastic pages in a binder.

I’ll have to check out the Starpath article. People really need to learn and relearn this stuff. And the longer I’ve been doing this job the more I’ve seen where, overall, weather knowledge is middling-to-weak or worse. The weather in Alaska & PNW tends to root out those with weak wx knowledge and ability. The East and Gulf Coasts are generally much more forgiving of it, which is great until it isn’t. Every once in a while an example gets set: Scandia, Valour, Sentry & La Princesa, Bounty, etc. But people forget soon enough and get sloppy, or sloppier.

In this day and age, we need a new regulation that requires tugboat owners to provide good onboard internet access, and weather routing services. Too many owners are too ignorant and too cheap to provide this without being forced to by a regulation. How many times have you heard “its not USCG required, we don’t have to provide that” ?

The situation we have today, with little or no internet access and no weather routing, is similar to what tugboatmen had to endure back in the 1920’s when tugboat owners were too cheap to install radio receivers so that their tugs could listen to radio weather reports.

The very best sat internet system costs what $70,000 ? ---- less than filling the fuel tanks a quarter full one time. A $1000 a month internet bill is not much, when your burning $1000 worth of fuel every few hours. Good weather information only has to save a few hours a month to more than pay for itself.

Good internet access and weather information actually save savvy tugboat owners real money. Its too bad that the USCG does not require tugboat owners to take STCW Basic and Advanced Meteorology and Leadership and Management. Now that would be a very beneficial requirement.



Yes. The T. J. Hooper, is exactly the seminal Learnard Hand case I was thinking of, although I could not remember the name of the case, when I wrote the above. The tug was held liable for failure to have a radio receiver to get weather reports, although no regulation required a radio. I have yet to meet a DPA, port captain, or tugboat owner who has heard of that case, or the legal principle that it it established.

In simplified form, the T. J. Hooper held: Tugboat owners are legally required to provide the best readily available, and cost effective equipment for safe navigation, even if the USCG regulations have so far neglected to require it, and the use of such equipment has not yet become customary in the trade.

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Here is the corrected link for the original post http://oceanweatherservices.com/featured_blog_posts/the_use_of_the_500_mb_chart_at_sea

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