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The Weather Triangle

Analyzing and predicting the weather at sea is an ongoing process of gathering, analyzing, and displaying information. To ensure this process is fruitful and productive we use a concept called the "weather triangle." The weather triangle happens when you use three independent sources for weather information, and use them to verify each other. These three sources are:

  1. On-Scene Observations
  2. Official Analyses and Forecasts
  3. Satellite Imagery

Satellite imagery, like this high-resolution image from, makes up the third side of the weather triangle.

In addition to gathering and using weather information, we need to verify that information is complete as well as accurate. The weather triangle assists with this need as well. Former President Ronald Reagan is supposed to have once said: "Trust but verify," and regardless of your party affiliation, this is a useful slogan in both politics and weather!

What are we trusting and yet verifying? We trust official analyses and forecasts, as well as our onboard instruments such as the barometer and the anemometer, but we are verifying information from these sources with actual observations and human senses. Our verification is assisted by satellite imagery, which can be captured on board directly by using systems such as SeaStation 2000 (, Weatherfax, or via Internet connection.

Here is an example of how the weather triangle works: say we have departed Annapolis, MD, in October and are heading to Bermuda. On our second day out we observe a swell building from the south. The swell period (the time in seconds between successive crests) is increasing as the day progresses; it starts with a six to eight-second period and by evening is 12 to14 seconds and the seas are building in height. What is happening? Well, we have one piece of the weather triangle established—the on-scene observation—as we have detected a swell and noted its changing period.

The second side of the weather triangle comes from official forecasts.
Now the second side of the triangle, the official analysis and forecasts, needs to be established. We obtain information on marine weather from the National Weather Services Marine Prediction Center (MPC), which produces all the coastal and offshore weather charts and text information (called products) for Atlantic and Pacific waters. MPC charts cover three necessary data fields: (1) upper-air, jet-stream flow; (2) surface-weather conditions; and (3) wind and wave conditions. We obtain MPC products via Weatherfax broadcasts over the single side band (SSB), by Internet (using satellite-communication systems such as Inmarsat), by high-frequency (HF) voice, Navtex, and Inmarsat Safety Net broadcasts. And as communications systems continue to blossom, there will be additional communication methods available in the near future.

By examining surface and wind wave charts, we will be able to determine what type of weather system is producing this large swell, most likely a developing tropical system since the swell is coming from the south and increasing in strength, as noted by increasing swell height, which brings an increase in period.

However, before we can be certain of this tropical system's nature, we need to fulfill the third side of the weather triangle, which is satellite imagery. Why do we need this piece? Because black and white, two-dimensional weather charts do not show us the extent of the clouds and structure of weather systems. MPC forecasters know the power of satellite images and use them as the background for constructing and drawing weather charts. Look at the weather channel and what do you see almost constantly? Satellite images. So how do we obtain these data-rich, data-intensive sources of weather information? We can capture satellite images on board by using a laptop computer hooked to a satellite receiver (these are the size of a cell phone) that is connected by coaxial cable to a satellite antenna. Antennae weight less than three pounds and are so compact they fit on a stern rail.

These systems pick up the VHF-FM signal from low-orbiting polar satellites (the same satellites that carry sensors for our 406 EPIRBs) and transform that signal into stunning, visible, and infrared imagery on the laptop's screen. I can see you doubting the ease and use of these images, and as a Whitbread sailor said years ago, "Satellite imagery is just amorphous cloud shapes of little use." But this is not true! The winning boats in this year's Newport to Bermuda Race, as well as the most recent Whitbread and Around Alone contest all relied on real-time satellite imagery for weather analysis and verification.

With a GPS unit hooked into your onboard laptop, your boat's position can be displayed on the image and a precise lat-long grid, including landmasses, can be overlaid on each captured image as well. This geo-referenced imagery is analogous to receiving your own custom Weather Channel as sea.

Too good to be true? No, it is true! The price for a satellite system is around $2,000 and such systems also interface with your SSB to capture Weatherfax with an Internet download capability. (To see samples of satellite images visit the SeaStation 2000 website at

If you're not able to accomplish a direct capture, then use re-broadcast satellite images that are sent via Weatherfax. These images are static black and white, and not interactive in the way of actual captured images, but they will enable you to visualize the weather and compare the clouds and texture of weather features with the weather charts and text weather that you receive.

So, ideally you scan the weather charts, compare them with your observed on-scene conditions (which should include wind speed and direction, barometric pressure, swell and sea height and direction, clouds, and water temperature) and compare those with the analysis and forecast charts, and your recently received satellite image. You should note the valid time for each piece of information—when its features are accurate—and note your location on each product. Then answer these questions:

  1. Do your actual conditions match those shown on the analysis products and do the trends match with the forecast conditions. Please note that MPC forecasts go out 120 hours, five days, so there really is no excuse for being surprised by a weather system!
  2. Does the satellite image correlate with the surface and upper air charts? Do the clouds or lack of clouds match with the conditions your are seeing? Yes, or no, and if no then why? The conditions might be changing rapidly and you can see yourself being overrun by a rapidly advancing front or low-pressure system.
  3. What adjustments need to be made to your intended course based upon this analysis and verification process?

Does this process really work? You bet, and here is an example. I assisted in the delivery of a Whitbread 60 from Ft Lauderdale, FL, to Southampton, UK, prior to the start of the last Whitbread. On board, we had a Weatherfax, laptop computer, and direct-capture satellite imagery. As the navigator, my daily routine was to examine surface-analysis charts, compare those with the satellite imagery and our on-scene conditions, and then adjust or modify our course to keep the winds abaft the beam. I was always looking over my shoulder, back to the west so I could see the low-pressure systems coming and overtaking us. We would adjust our course so that as the lows came over us we would always be to their south and thus always in southerly, westerly, and then northerly winds. We flew along, covering over 1,000 miles in the final three days of the crossing. Our elapsed crossing time was just under 13 days, with an average daily mileage of around 280 miles. Of curious note is that our arrival in Southampton was on the evening of the final day that all yachts participating in the Whitbread had to be in the UK. We might call this technique just-in-time arrival. And the weather triangle put us there on time!

Michael Carr is offline  
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