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Old 09-16-2000
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Michael Carr is on a distinguished road
Weather Information Sources


Keeping tabs on the weather can help you avoid embarrassing and even dangerous situations at sea.
An embarrassing experience was my wake-up call to pay more attention to the weather and the means to better understand it. I was heading south in the Florida Straits during a winter passage on a large schooner with cold, dry winds blowing against the north-flowing current. The seas built quickly, and one night, we did an uncontrolled jibe (of course I was at the helm) and busted the vessel's boom gallows. Apart from the physical damage, this was a blow to my self-esteem and was all due to my not watching the weather closely enough, especially since we were in the Gulf Stream where the weather is routinely wild and woolly. This was 10 years ago and since then I watch the weather like a hawk!

But how do we obtain the weather charts, text, and voice products needed to make routing decisions and avoid such embarrassments? These items come to us via a variety of electronic methods depending on their format and are intentionally duplicated so we will always have different ways to obtain the necessary data. Here is a general list of weather-information products and their broadcast formats:
  • Voice via VHF and SSB (Single Sideband).
  • Text via Navtex, SSB, and the Internet.
  • Charts via Weatherfax, SSB, e-mail attachment, Internet, and Inmarsat.
  • Satellite images via direct-satellite capture or Weatherfax rebroadcast.
  • Compressed, custom-generated weather files via Inmarsat.


    The author checks his laptop in the nav station for an update on the forecast.

    Weatherfax charts that provide pictures of weather analyses for the present and the near future should be your first choice on this list. A picture is worth at least a thousand words when examining weather. Fortunately, Weatherfax charts are easily available using the Internet and Weatherfax-capture systems.

    All Weatherfax charts covering Atlantic and Pacific waters are produced by the Marine Prediction Center (MPC), which is part of the National Weather Service. These charts are made available on the Internet and via SSB Weatherfax broadcast. Charts covering surface-weather features, upper air, and sea state are produced every six hours and placed on the MPC's Internet site www.mpc.ncep.noaa.gov, and
    are simultaneously broadcast over SSB Weatherfax courtesy of the US Coast Guard. Coast Guard broadcast stations are located in Marshfield, MA, Bell Chase, LA, and Pt. Reyes, CA.

    A subtle difference between the Internet dissemination of weather charts and Weatherfax broadcast is that charts on the Internet remain in place until the next updated version becomes available, while Weatherfax charts are available only at the time of the broadcast. Thus, if you depend on broadcast charts, you must know of the broadcast schedule and frequencies being used for transmission. This information is, appropriately, available on the MPC's Internet site and broadcast each day by the Coast Guard stations themselves.


    "Laptop computers can do the job of capturing weather fax charts."

    Not too long ago Weatherfax charts were captured using a Weatherfax machine, but now laptop computers using demodulators (usually in the form of a PCMCIA card), which outwardly look like the modem card for Internet access, can do the job. By hooking a laptop to a SSB receiver via a Weatherfax modem, software charts can be captured directly into the laptop and stored as time-dated files. There are several reasons why this method of chart capture is superior to dedicated Weatherfax receivers, the most significant being:

  • No need for rolls of Weatherfax paper.
  • Ability to change frequency during chart capture to obtain improved picture.
  • Ability to clean up and manipulate the chart once captured; computers can capture charts at the same time they perform other functions.
  • Ability to reference the charts relative to geography once captured, using GPS input to the computer.
  • Ability to arrange Weatherfax charts alongside navigational and oceanographic charts, such as the Gulf Stream.


    Real-time satellite images are one of your best sources of weather information.


    Additionally, a laptop can be used in port to download charts directly from the Internet, removing the need to monitor broadcast schedules or select an appropriate frequency. Internet access is available at sea through communications systems such as Inmarsat, where the data rate is adequate to download charts in a timely manner. From personal experience using Inmarsat M, it is possible to download approximately three MPC charts in one minute, and with connection time costing approximately $4 a minute, this is a reasonable procedure. Looking into the future, it is low-cost Internet access via wireless communications that will replace Weatherfax, and it will not be long before Internet communications will be the backbone of disseminating marine information.

    Aside from the tremendously data-rich Marine Prediction Center charts, there are voice broadcasts via USCG stations over VHF and SSB. These voice broadcasts originate from coastal broadcast towers as well as the same stations that broadcast charts (Marshfield, Belle Chase, and Pt. Reyes) as well as Honolulu, HI, and Kodiak, AK. Details on broadcast times and frequencies are found on the MPC homepage www.mpc.ncep.noaa.gov). Voice broadcasts
    cover selected coastal, offshore, and high-seas regions, whose limits are shown on regional charts found at the MPC website. Knowing the forecast area in which you are sailing is paramount to using these weather products properly. All too often sailors do not reference weather information adequately to their location and time and subsequently make analysis and forecast errors.

    Textual weather information comes via e-mail, SSB, and Navtex. Navtex is a coastal dissemination system, broadcasting over a single frequency out to between 300 and 400 miles from the coast. Navtex is easy to receive via a dedicated Navtex receiver (the cost is around $500), or via a SSB into a laptop. Navtex use is required on all commercial vessels, and is also appropriate for offshore and coastal sailors since its broadcasts are continuous, there is no required tuning or schedule; just turn it on and out comes text weather for coastal and offshore conditions.

    Satellite images are truly the height of weather information since they provide stunning presentations of your actual weather. The true power of satellite images is this:

  • Images are real-time, i.e. when you capture an image on board it is the weather at that moment since the satellite overhead is sending data to you as it flies over. There is no time delay to account for as there is in Weatherfax charts, voice, or text products.
  • Both infrared and visible images can be received day and night, with infrared images showing heat and visible images showing texture and reflected light. (Visible images are not taken at night for obvious reasons instead two infrared images are provided.)

Images can be geo-referenced using GPS input and then used as a navigational chart. Clouds and ocean currents are revealed, so by using zoom and color enhancement each distinct weather feature can be analyzed and tracked on board.


On-site observations regarding sea state and wind strength serve to confirm the information you get from other sources.
So how do you decide which weather products you should receive? Consider where you are going. If you sail beyond the range of Weatherfax charts, then satellite imagery is a must, because it will be the only way you can obtain "pictures" of your weather. If you sail coastal, you will be fine with Weatherfax and voice.

Any ocean crossing begs for redundancy in all systems, including weather products. You might consider a laptop computer coupled to a SSB receiver for charts and antennae for satellite capture, as well as VHF and a stand-alone Navtex as a backup. And don't forget to take on-scene observations—it's important to
develop situational awareness. Your barometer and senses are really the starting point for all weather analysis.

There is no set procedure for gathering and analyzing weather, you must consider your intended voyage, your vessel's capab
ilities, and your skills in understanding voice, text, and Weatherfax charts. Once you get the handle on this, though, you'll be able to avoid embarrassing moments like mine and keep yourself, your crew, and your vessel a step ahead of the weather.

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