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Plenty of weather data – How do you use it?

Oct 1, 2009

The amount of weather information available to mariners today is nothing less than staggering. Detailed text forecasts as well as weather charts depicting actual and forecast data are provided regularly by government weather services of many countries around the world. These products are available for high seas and coastal areas, bays and inlets, and also for inland waters. Some of the government data is repackaged by private vendors and offered in slick displays, providing the mariner yet another manner in which to observe the meteorological information. The availability of forecast services from private consulting meteorologists adds yet another possible level of information for boaters.

While the amount and quality of meteorological information has increased dramatically over the last decade or so, of equal or perhaps greater impact has been the communications revolution that has swept through the marine industry. The wide availability of affordable e-mail communication for vessels at sea as well as full Internet access has made it possible for mariners to receive much the same comprehensive weather information at sea as they can while tethered to the dock. This communications revolution has also made it more feasible to communicate directly with a consulting meteorologist on a regular basis throughout a voyage, and also allows frequent updates of repackaged government data in proprietary software displays.

The wide availability of more comprehensive weather information begs the question: “How is the mariner to make the best use of all this data?” At this point, many articles would now head in the direction of giving pointers about how to interpret the various available chart products, the best ways of finding the data that are most useful, and other tidbits to improve the mariner’s ability to figure out what to do with the mountain of available information.

The focus of this article will be somewhat different. While knowledge of the products that are available and how to interpret them is, of course, very important, it is equally important for the mariner to know how to appropriately apply the data to his/her situation, and as a result, make the proper decisions.



First step in decision making

The first step in applying the available weather data is to have a thorough knowledge of your vessel, yourself, and your situation. Notice that this stage has nothing to do with looking at weather data. This is deliberate. It is essential for the mariner to have a firm understanding of the type of conditions that are appropriate for each situation.

First, consider the vessel. It is necessary to know how the vessel will perform in certain conditions, and the upper limits of a vessel to handle certain conditions. For most vessels, this will depend largely on sea state, but wind speed and direction along with other factors will also come into play. If the vessel is one with which you are quite familiar, then you likely have a good feeling for the upper limit of conditions in which the vessel remains safe. If it is a new vessel, you may need to rely on other owners of similar vessels, or on data provided by the builder. In any case, it is essential that there is a complete understanding of the vessel’s limits in place prior to evaluating weather conditions for a voyage.

Next, consider your personal limits. A knowledge of your ability to handle (or to be comfortable with) certain conditions is necessary. Conditions to be considered are wind velocity (speed and direction), sea state, and other weather conditions such as precipitation or limited visibility. Upper limits for these conditions need to be determined. Perhaps you are willing to push yourself all the way to the limits of the vessel, but, more likely, your personal limits will be somewhat below the limits of the vessel. Again, a complete understanding of these limits is essential prior to any voyage planning.

Finally, the situation must be considered. This has to do with the type of voyage being contemplated, the ability and experience of any crew, and the goals of the voyage. If you are in a situation where you need to deliver a boat from one port to another, have ample, well experienced crew, and the goal of the passage is simply to get the boat from one place to another without damaging it, then you may be willing to push yourself to your personal limits as determined above. If, however, you are in a situation where you are taking a cruise for pleasure, your crew is not as experienced, and includes family members, and the goal of the passage is enjoyment, then the limits of the conditions will likely be lower. In fact, for this type of passage, even if winds and seas are forecast to remain well within acceptable limits, if rain and fog are forecast for most of the passage, you may decide to change your plans on that basis.

I have defined six categories of conditions that are shown in Table 1. The conditions at any given time during a voyage will fall into one of these categories. The definition of these categories will be different for each voyage, taking into account the vessel, your personal abilities, and the situation as just described. Therefore it is useful to use a form like Table 2 to define these categories as carefully as possible prior to each voyage. Ideally, this procedure will be done well in advance of each voyage, and prior to evaluating any weather data for the voyage.

The reason for going through this procedure well in advance of the voyage is that it prevents what I call “limit creep.” This is a situation where, when looking at weather data prior to a trip, and finding that conditions might feature stronger winds or higher seas than are desired, that, due to the desire “to get the delivery done” or “not to ruin the vacation” a determination is made to go ahead with the passage even though forecast conditions might indicate that a different decision would be more prudent.

If the upper limits of conditions have been carefully defined ahead of time, the decisions can be much more objective, and the safest options will be chosen. For this reason, it is important to physically put the pen to the paper and write the limits down, filling out the chart as shown in Table 2. Once this is done, then you have at your disposal a set of parameters for the trip, and this will make the decision making process much more effective and safer.



Examining the data

The examination of actual weather data should begin several days in advance of the scheduled departure. Whether you are using data directly from government sources, repackaged data, or information from a private consultant, you should see how the forecast conditions fall into the categories which you have previously defined, and use this information to make determinations concerning the passage. Of course, the easiest situations are those when the forecast conditions all fall within the Ideal or Favorable categories, or within the Unacceptable or Perilous categories. In these cases, the passage will either proceed as planned, or will obviously need to be delayed, proceed with an altered route or schedule, or cancelled altogether. The more difficult situations will be passages where conditions are forecast to fall into the Acceptable or Marginal categories, either for short periods of time, or for a good portion of the passage. These situations will require more deliberation and thought, and decisions will, to a great degree, depend on the particular situation.

Let’s look at an example. Refer to Table 3, which has been completed with the limits for a trip from New York to Annapolis, Md., on a sailboat (let’s assume about 45 feet or so). This particular trip is for a family vacation with the family members serving as skipper and crew. This trip would have two possible routing options, one where the vessel would sail south to the Hampton Roads, then back north through the Chesapeake Bay to Annapolis, and the other where the vessel would sail to Annapolis via Delaware Bay and the Chesapeake and Delaware Canal.

Taking a closer look at how each category of conditions has been defined, notice that in the Winds column, ranges of wind speeds have been noted, and also acceptable periods of no wind have been indicated. Since this is a sailboat, and depending on fuel capacity and range under power, longer periods with no wind become less favorable. In the Seas column, notice that both wind waves and swells have been noted since the longer period swells will affect the vessel differently. In the Weather column, both precipitation and visibility have been noted, and if the passage were occurring during the cold season, temperature information might also come into play.

Using Table 3 and available weather forecast information, the skipper can evaluate each route option and determine whether one or the other is more appropriate, or, whether neither route is right for the scheduled departure date.

For example, if conditions on the Atlantic were forecast to deteriorate into the Acceptable or Marginal categories 24 hours after departure and to remain in these categories for a few days, then taking the route through Delaware Bay and the Chesapeake and Delaware Canal might be the best decision. In particular, if the reason for the deterioration of conditions was primarily due to sea state issues, sailing in the more protected waters later in the passage might allow conditions to remain in the Favorable category. In any event, having a chart like Table 3 completed well in advance of the trip will make the decisions more clear cut, and in this case, will likely lead to a pleasurable trip, or will likely avoid a case where departure occurs as scheduled, but the family has a miserable time.

Even with well-defined limits, some subjective analysis will still be required during the evaluation of weather forecast information. For example, it will be necessary to make a determination of how long a period of time conditions in the Acceptable and/or Marginal categories can be tolerated. If it appears that conditions in the Marginal category will prevail for an hour or two on a three-day passage, then a decision may be made to go ahead with the passage, but on the other hand, if it appears that these type of conditions will last for 12 hours, that may lead to a different decision. But again, even with these more subjective evaluations, having the well defined limits in place will lead to an easier decision making process.

Let’s look at how Table 3 might be constructed differently in different situations. First, consider the same vessel, but instead of a family vacation, think about a delivery passage from Annapolis back to New York with a hired crew with lots of experience. What will change? The Ideal category will likely change very little, perhaps slightly higher wind speeds to make the boat go faster. Somewhat stronger winds will likely show up in the Favorable category as well, and perhaps slightly higher seas. Conditions for the Acceptable category will likely feature stronger winds and higher seas along with a greater tolerance for inclement weather. This is mainly due to the fact that having a good time has been removed as a goal for this voyage, in fact the goal is simply to get the boat back to New York without any damage or injuries to the crew.

These adjustments will push the Acceptable category closer to the Marginal category, which itself may be nudged a bit higher for winds and seas. With a professional crew, the Unacceptable category will likely be pushed higher toward the Perilous category. The Perilous category may be increase a bit as well, due again to the more experienced crew, but will still need to take account of the limits of the vessel.

If the vessel involved was a motor yacht, the categories would also be defined differently. In this situation, periods of time with no wind change from being liabilities to being strong assets. Sea state, particularly wind waves, may become more of a liability when heading into the wind due to the higher speed of the vessel and the resulting pounding that can occur in these situations. Thus the limits for sea state as one goes up the category list may be lower than for our sailboat trip. The Ideal category would likely show wind direction nearly due astern rather than abeam.



Power voyagers

In most cases, for larger vessels, the definition of each category will tend to have higher winds and seas, with smaller vessels the opposite case will prevail. In fact, this procedure can be used very effectively for making decisions for trans-oceanic passages for large commercial vessels like tankers as well. Obviously, in these situations, the definitions of each category will be significantly different than for our sailboat trip, but the procedure to construct the table, and then to apply it using available weather forecast information will be much the same.

To summarize, well ahead of any passage, make a determination about how weather information will be obtained. As noted at the outset, there are many possibilities for this. Also, well ahead of any passage, define the limits of conditions for the passage using a table like Table 2. Then, as the passage approaches, use the weather forecast information from your chosen source and apply it to the defined limits of conditions, and this will allow for a strongly objective decision making process which will assist in meeting the goals of a passage, or in avoiding an unsatisfactory or dangerous passage.



Ken McKinley earned a bachelor’s degree in atmospheric science from Cornell University in 1980, and attended graduate school in meteorology at MIT. He founded his own meteorological consulting firm, Locus Weather, in Camden, Maine, in 1991.

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