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A powerful weather resource

Jun 8, 2012
Loaded with information via words, numbers, symbols and colors, pilot charts are compilations of long-term weather data rather than present or forecast conditions.

Loaded with information via words, numbers, symbols and colors, pilot charts are compilations of long-term weather data rather than present or forecast conditions.

History is littered with stories of ships lost at sea, frequently due to encountering extreme weather. Luckily for mariners, though, significant advances in meteorology have made long ocean passages significantly safer. In the effort to use the latest weather techniques, however, voyagers shouldn’t ignore a still useful tool for voyage planning: the pilot chart. These charts contain vast amounts of historical weather data that voyagers can put to good use.

Today, of course, ongoing advancements in meteorology, along with more powerful computers, have delivered better tools for more accurate, longer-range forecasting into the hands of meteorologists. The advent of better communication methods, particularly through the Internet, has made the forecasts these meteorologists produce much more available to mariners on the high seas.

There was a similar leap in the availability of information in the middle of the 19th century. Prior to that time, knowledge of the weather patterns over the world’s oceans was transmitted mostly by word of mouth from generation to generation among sea captains, augmented by some written records for some portions of the oceans over some of the months of the year.

The presence of the trade winds in lower latitudes was well known, as was the trend for generally westerly winds in higher latitudes, and the monsoon circulations of the Indian Ocean. It was known that stronger storms occurred in the North Atlantic during the winter season, and hurricanes were known to occur in the tropical latitudes during the late summer and fall with occasional forays into more northern latitudes at times. However, none of this information was well organized or quantified into a form where it could be of maximum benefit to the mariner.

Enter Matthew Fontaine Maury, dubbed the “Pathfinder of the Seas.” Maury joined the U.S. Navy before the age of 20 and sailed extensively around the world for many years. After suffering an injury which ended his seagoing career, he transitioned to shoreside work, eventually becoming the first superintendent of the U.S. Naval Observatory in 1842. He quickly set out to put information into the hands of ship captains which would allow them to make better choices for ocean passages, both in terms of routing and in terms of the times of year to be favored (or avoided).

Extracting data from logbooks
Maury accomplished this by studying logbooks of prior voyages, compiling the wind and current information they contained, and presenting it on a chart. He first published his Wind and Current Chart of the North Atlantic in 1847. He then solicited ship captains to provide him with their current wind and current information in return for providing them with his chart. He received a great deal of additional information from the captains in the ensuing few years, and expanded his charts to cover most of the world’s oceans. His career with the Naval Observatory ended in 1861 when the Civil War began. Maury was a Virginian and aligned himself with the Confederacy, but by then his contribution was made and the oceanic pilot charts were established.

Modern day oceangoing mariners, both professional and recreational, have access to long-range (one to two weeks) weather forecasts that are reasonably accurate. These forecasts can be accessed in many different ways, including e-mail, the Internet, VHF and HF radio, NAVTEX, and more. Forecasts from government meteorologists and from private meteorological consultants can be obtained. In fact, via the Internet, many of the tools used by meteorologists (i.e., computer models) are now directly available to mariners, although these need to be used with caution.

About a generation ago, however, the longest-range operational forecasts for the ocean looked only three or four days ahead, and the reliability of those forecasts was not what mariners have come to expect today. Since ocean crossings take longer than four days, the pilot charts were the only resource that mariners could turn to in order to gather some sense of what could be expected for these crossings.

With the advent of longer-range forecasts, there is a temptation to discard the pilot charts since they depict long-term averages of data, and not the actual expected weather on a passage. That would be a mistake, though, because these charts still contain a wealth of information.

Bound by ocean
Pilot charts are sold in a bound, chart-sized (roughly 20 by 30 inches) volume for each major ocean basin with one chart for each month of the year. The North Atlantic volume has three charts per month with additional charts for the Caribbean Sea/Gulf of Mexico and the far north Atlantic. The charts can also be downloaded as pdf files at no charge. However, the large size of the charts makes them difficult to view on most standard computer monitors, requiring significant zooming-in to be able to see details, at which point it is not possible to view the entire chart. To get the most value out of the charts, obtaining the printed volume is a good investment. (See sidebar How to obtain pilot charts).

The information on the charts is presented for five-degree latitude/longitude sections (roughly squares), and just as it was in the mid-19th century, the information is based on actual observations from ships. As long as there exists a large number of observations over a long period of time, meaningful averages can be calculated. On the main part of the chart, information about wind speed and direction is given along with information about predominant currents and the frequency of significant wave heights exceeding 12 feet.

In Figure 1, a portion of the North Atlantic pilot chart for the month of January is shown. The blue symbols provide wind information; the green arrows show predominant currents; and the red lines indicate the frequency of higher seas. In particular, for the square between 35° N and 40° N, and 65° W and 70° W, the most frequent wind direction during this month is from the northwest (shown by the longest of the wind barbs) with an average speed of force 6 on the Beaufort scale (shown by the number of feathers at the end of the staff). The other wind directions are less frequent, as indicated by the lengths of their shafts (there is a scale on the chart for measuring these), and the average wind speeds from those directions are indicated by their feathers. The number in the middle of the circle indicates the frequency of calm conditions (in this case 1 percent). The green arrows show the most likely direction of the surface current, and the number with the arrows indicates the average speed of the current in knots. The red lines indicate the percentage of time that significant wave heights can be expected to exceed 12 feet. The lines are drawn in 10 percent increments. Note the label on the line just south of Long Island; when looking at the entire chart, all of the labels can be seen. For the square where we evaluated the wind, this percentage would range from about 15 percent in the northwest to more than 25 percent in the east.

Figure 2 shows the same area, but for the month of July. One can clearly see that the average wind speeds are lighter, the distribution of the wind directions is different with southwest being the most likely direction, and the percentage of seas greater than 12 feet is less than 10 percent with no red lines showing up.

By spending some time looking at each monthly chart through the entire ocean basin, a mariner can learn a great deal about the progression of weather patterns through the year, and can make some determinations about which months of the year favor certain passages, and which routes would be more favorable. Indeed, this was Maury’s goal for commercial ship captains when he first compiled the charts in the 1840s.

Additional information available
There is even more information available on these charts today. Magnetic variation is shown on the main chart, and can be seen in Figures 1 and 2 by the gray lines running generally from northwest to southeast. These lines are labeled on the full chart, but the labels do not happen to fall on the small sections shown in these figures. Information about ice conditions in higher latitudes is also shown.

There are also smaller inset charts for each month. These charts show average surface barometric pressure (blue lines), average tracks of low pressure systems (red lines) and tropical cyclones (green lines) and the percentage of time that gale force winds have been observed in each five-degree section (red numbers). Magnetic variation is also shown on this chart inset. Other inset charts present information for average temperatures, both of the air and the sea surface, percentage of time when visibility is less than two miles, and the probability of having at least one tropical cyclone at some point during the month in each five-degree section.

There are extensive explanations on each chart about how the chart should be interpreted and used, and also some general summaries of the parameters and comments about the average weather patterns for the month of the chart. One important caveat is given on every chart: “It should be kept in mind that most ships tend to avoid areas of inclement weather. The frequency of gales and high waves is generally greater than that which is actually reported.”

A set of pilot charts for an ocean basin allows the user to see a great deal of information about the conditions that can be expected through that ocean basin through the year. While the advent of more accurate longer-range, real-time forecasts for the oceans means that these charts are not as heavily relied upon as they once were, they remain a very valuable resource. There is so much data presented on them that any mariner who is contemplating an ocean passage would be well served by obtaining these charts and spending some time studying them well in advance of the passage.

Ken McKinley earned a bachelor’s degree in atmospheric science from Cornell University in 1980, and attended graduate school in meteorology at MIT. After working as a meteorologist for nearly 10 years he founded his own meteorological consulting firm, Locus Weather, in Camden, Maine.

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Jul 12, 2012 11:28 am
 Posted by  BajaDan

Fascinating. As a boater, instead of a mariner or yachtsman, I really enjoyed this article and learning about Pilot Charts, which I had never heard of. Perhaps one day I will be a blue water sailor and can really put the information that I learn from your excellent magazine to good use. Until then, I thoroughly enjoy the learning process.

Thanks,

Dan

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