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A solution to finding the elusive tropical convergence zones

A bit over ten years ago I cruised in the South Pacific among the Solomon Islands and Vanuatu (formerly New Hebrides). One of the characteristics of these places was the lack of navigational aids which placed heavy reliance on the best one you can use … the eyeball. But another characteristic of places such as the Solomon Islands was the poor visibility of the water, making safe passage among the reefs best done in clear, sunny weather.

In those days we placed heavy reliance on the movement of the Tropical Convergence Zone (TCZ). When the TCZ was in our area the weather was characteristically rainy and overcast, making movement among the reefs and islands a bit challenging, and in some cases downright dangerous. Though weather faxes don’t show TCZ information we found it to still be available, albeit in a cumbersome fashion.

The good news was that a daily morse code broadcast from Wellington, New Zealand (ZKLF), on their weather fax frequencies, gave you the ability to draw a weather map using the IAC Fleet Code format. The information originated from the weather office in Nadi, Fiji. The bad news is that ZKLF has discontinued broadcasting the IAC Fleet Code though it is still produced by those dedicated souls in Nadi.

The IAC Fleet Code format is stylized information in five number code groups which, when used with a decoding book, permitted hand drawing of weather maps by ‘connecting the dots’ to show the location of pressure, frontal and tropical systems. It’s the latter which contain the TCZ information we were seeking – the information that was lacking from the available weather faxes. After a few months listening to the broadcasts, writing down the five number code groups, decoding the location and plotting the lines between the dots we in fact became adept enough to plot the dots as we listened to the morse code broadcast.

Here’s a sample from one of the Nadi broadcasts:

10001 33388 01012 99900 85221 63348 63348 00910 99900 81303 62767 62767 01215 99900 85219 73372 73372 00910 99900 81211 73161 73161 01015 ....

Cumbersome as it was, this was an effort well worth doing. We could plan, within a day or so, when it should be possible to shift locations, go snorkeling or dry the laundry

Now enters the age of computers with email and display programs. Together they provide a nearly instant determination of that sought after information - the convergence zones.

Obtaining fleet code bulletins.

Instead of listening to a broadcast and copying, by hand, the broadcast data, today’s cruising boats equipped with an email capability have two choices of getting the information: Sailmail/Winlink 2000 and YOTREPS.

By using either Sailmail or Winlink 2000 you can subscribe to a free service called ‘Saildocs’. Saildocs is an automated server which supplies weather bulletins and other documents via email. The Saildocs service was created by the author of Airmail, Jim Corenman, and is supported and operated by the Sailmail Association (www.sailmail.com) for its membership. In addition to a wide variety of text weather bulletins, Saildocs also makes available grib weather-data files. Saildocs is an email-based system; for more information send a (blank) email to: info@saildocs.com.

A fleet code bulletin is available from Saildocs titled “fleet.nadi”. This is the very same information that we laboriously copied via morse code years ago. The sample shown above is from one of the ‘fleet.nadi’ messages received from Saildocs. The area of coverage of the Nadi information is from the equator to 35 degrees South latitude and from 150 degrees East to 120 degrees West longitude.

This same information is also available from the YOTREPS list server, operated by Mike Harris at PANGOLIN (http://www.pangolin.com.nz). By sending an e-mail to yotreps@pangolin.co.nz with the words "FIJI FLEET" in the text body a message is posted back to you. You can ‘join’ the list and receive regular copies of the postings as they are issued, until you ‘leave’ the list. Help on the use of the YOTREPS list server can be obtained from www.pangolin.co.nz/yotreps/list_manager.php or by sending an e-mail to yotreps@pangolin.co.nz with just the word 'HELP' in the text body.

Displaying the data.

Displaying the information received from either source is effortless, compared with the hand plotting of years ago. A free program, Phys Plot (Version 1.002 BETA release), is available for download from PANGOLIN and does this quite nicely.

The bulletins received from either Saildocs or YOTREPS are contained within a message and must be cut and pasted into a file (I use Notepad) and saved in .iac, .txt, .zip or .exe format (whatever you choose).

When the saved file is opened with the Phys Plot program you get a display such as this one. City locations are displayed by clicking on the yellow dots. I’ve drawn arrows to show the pressure and frontal information



Although this pressure and frontal system activity is normally shown on weather faxes, the thing we’re after is shown by the orange lines … the tropical systems.

If you run the elevator down on the left you will display the information concerning the Tropical Systems. And there we find the lines and zones of convergence!




For this displayed date I would expect sunny weather in the western Solomon Islands but not in the eastern region around Honiara.

In addition to the movement information on each bulletin, the changes in the convergence zone can easily be tracked by keeping daily records. (Taking snapshots of the screen and saving them in a file is a technique described in a related article, ‘Image Logs’)