Using 500mb Charts on Forecasting

Here’s a forecasting trick I found useful when we were planning and looking for a weather window for the passage from Fiji to New Zealand last year:

Usually for our passage-planning I use a combination of raw forecasting models data and man-made weather assessments. More details can be found here. Up to Fiji, I have found this combo to be satisfactory.

However, for the FJ-NZ crossing, I felt the need to be more informed for the decision-making. In particular I wanted something that would:

  • minimize our risk of being caught in truly bad weather – something very possible on those waters
  • maximize the lead time for an upcoming weather window (that is, be able to “see farther into the future”)
  • enable me to have a more critical view of whatever information I gathered – be it from computerized models or man made assessments (in other words, I wanted to be able to make my own decision for the passage, rather than having to believe in someone else’s blindly)

Just when I was getting into this, I came across an article on the Attainable Adventure Cruising website (which is an excellent source of practical offshore cruising information, by the way) which encouraged me to revisit the “500Mb Charts” for our passage planning.

Many relevant weather events that happen on or near the surface are in large part caused or steered by patterns in the higher atmosphere. Naturally, however, looking at the atmosphere from a holistic, 3D point of view immediately gets overwhelming for us and can only be dealt by complex computer models. But it has been observed that one parameter in particular represents a very good surrogate of higher atmospheric conditions which, as a whole, influence surface weather – and this is what the “500mb charts” (also known as “Geopotential Height”) do.

It takes a while to understand What they are, at least to me it did: for any given geographic point, these charts will tell you the height above sea level – in meters – where the barometric pressure is exactly at 500 millibars. In one place it can be, say, 5900 meters high, in others 5200 and so it goes.

I know, it is a mouthful. Articles are written about it and even books. And it was to the latter that the article mentioned previously led me to.

A few years back I had acquired this book, knowing it is considered reference reading about the use of 500Mb charts. It is indeed a very robust book, which I read through at least twice that I remember of, but always getting overwhelmed by the concepts and even falling asleep on some occasions. In the end, the book stayed at the shelf onboard Pesto, and I ended up not using the 500mb charts for our forecasting. But the AAC article mentioned above has some very practical tips on How to read the book and what to take out of it, which I followed.

And, for the purpose of our Fiji-New Zealand passage, here’s what I took away from it:

– the lines on the 500mb charts represent Contours. It’s identical to a geographic map, where Hills and Valleys are also represented by contours.

– where the 500mb lines show a “Hump”, the equivalent of a Hill on a terrain map, that’s known as an “Upper Level Ridge”. Benign weather at the surface tends to form under – or just ahead of – these Ridges.

– where the 500mb lines show a “Trench”, or the equivalent of a deep cut Valley in a terrain, that’s an “Upper Level Trough”. Dangerous weather can form under and just ahead of these Troughs at the surface.

The examples above – taken from actual charts I was using at the time we were planning for the passage – show Surface Highs (conducive of benign conditions) ahead of and under an Upper Level Ridge, and Surface Lows (causing bad weather) also ahead of and under an Upper Level Trough.

The following animation, made with same-day forecasts (aka, nearly Real conditions) show the movement of 500mb isolines and the corresponding High and Low systems down at the surface for just over a month. To me at least it does show that Surface Lows precede Upper Level Troughs, and Surface Highs come ahead and under Upper Level Ridges:

But one question remains: if the forecasting models provide BOTH 500mb AND surface charts, why look at the former and not just stick to the surface forecast? To me, it was a matter of confidence in an area where bad weather can form and get nasty quickly. So, if an Upper Level Trough was forecast to linger over the area, I knew that any surface Highs that might be forecast would be short-lived. And that dangerous Lows could develop quickly. Conversely, the presence of an Upper Level Ridge gave me confidence that surface Highs would tend to be consistent and long-lived, and surface Lows (thus, Bad Weather) far less likely.


And there is one more aspect – a quite important one: the 500mb layer more or less follows a pattern, flowing in planetary-scale waves around the world near both poles, from west to east. So, by looking at the motion of these waves it is possible to make judgements, with a fair bit of anticipation, on what shall be the 500mb conditions over a certain area, quite a few days before the models start predicting it.

Accordingly, in the beginning of November I pulled a long term 500mb forecast covering a broad area extending from the mid Atlantic ocean all the way to New Zealand. This forecast showed an Upper Lever Ridge traveling Eastward. By judging it’s motion, I reckoned that it might reach New Zealand by or around the 3rd week of November.

On November 7th, about a week later, I wrote about it for the first time on this post, which covers our planning as it progressed.

As days went on, the forecast kept validating that the Upper Level Ridge – and the corresponding Surface High, and the favorable passage conditions – would be present over the area around the 20th. And they did.

So, using the 500mb charts, I had at least a hint that underlying climatic conditions could be conducive to favorable weather for a passage towards the end of the month. That was important information, which allowed us to forego sub-optimal interim weather windows, and encouraged us to wait for a proper one. And that was over 3 weeks in advance !

2 Replies to “Using 500mb Charts on Forecasting”

  1. I haven’t looked at Chesnau’s book for a few years. I didn’t find it helpful in Mexico. But I will definitely re-read it before heading south again. Thanks for finding the essence of the 500mb chart!

  2. Hi Phil – same experience as I had.
    With that being said, for the transit in between the South Pacific Isles the 500mb won’t say much – first because that area is basically a Trade-Wind Conveyor Belt, second because the 500mb patterns become more predictable and relevant at latitudes higher than 20 degrees (also a learning from Ma Li’s Book).
    This year, as you sail West, what you WILL want to pay a lot of attention to are:
    – the ITCZ: normally affecting passages between Tahiti and Cooks
    – the SPCZ: hovers over Vanuatu, Fiji, Tonga and Cooks and is to be avoided when too intense
    – large High Pressure systems parading South of 25-30 degrees, creating “squash Zones” of reinforced trade winds (we sailed in these and it is nor pleasant)
    – any off-season Tropical Depressions
    Enjoy Ottawa !!!

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