Jimmy Cornell Talks About His Ocean Atlas
Jimmy Cornell answers our questions about his latest publication--Cornell's Ocean Atlas: Pilot Charts for All Oceans of the World.
Jimmy Cornell, the dean of bluewater voyaging and planning and a CW contributing editor, has, along with his son, Ivan, just released Cornell’s Ocean Atlas: Pilot Charts for All Oceans of the World (2011; Cornell Sailing Publications, $100). CW editor Mark Pillsbury had the following question-and-answer exchange with Jimmy, who is soon to embark on a tour of U.S. boat shows that includes appearances at Strictly Sail events in Chicago, Miami, Oakland, and at next fall’s U.S. Sailboat Show in Annapolis.
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CW: Jimmy, for sailors who are perhaps headed offshore for the first time, can you explain what the Atlas is and how you might use it for voyage planning?
J.C.: All sailors, even experienced ones, planning to set off on a shorter or longer offshore passage are concerned about what kind of weather they might encounter. The wise navigator will prepare for the worst eventuality but do everything possible to avoid it. The main role of the Atlas is to allow you to check out wind conditions for every month of the year in any ocean of the world and thus gain an immediate overall knowledge of what you can expect at any given time of year.
The Atlas contains 60 monthly up-to-date pilot charts that show wind speed and direction, current rates and direction, the most common tracks of tropical storms, the mean location of high-pressure cells for each hemisphere, and the approximate extent of the Intertropical Convergence Zone, commonly known as the doldrums. This is the kind of information you need even before you start actually planning a specific voyage, as it will allow you to decide straight away whether your plan is feasible. Once you’ve decided that it is, you can proceed to the next stage, and that’s to plan a voyage along a route that has the best chance of favourable sailing conditions. And this is where the rest of the Atlas comes to your help. Side by side with the monthly charts are 69 detailed charts of the most commonly sailed transoceanic routes. Even sailors with limited offshore experience will find it easy to plan a route that takes best advantage of the prevailing weather conditions at the chosen time of year. Sidebars with tactical suggestions have been added to the months when most passages are undertaken. The comments and tips on tactics, as well as weather overviews for each ocean, were contributed by meteorologists and routers specializing in those oceanic areas. Furthermore, the Atlas also contains a comprehensive description of weather conditions in every ocean.
CW: So by looking at a body of water month by month, sailors can determine either the best time to make a crossing or the likelihood of the wind and weather they might expect?
CW: Can you explain how you arrive at the wind averages for a particular month and how to read the arrows, or wind roses?
J.C.: The wind and current data was obtained from observations made by a network of meteorological satellites since 1987. Daily samples of the average conditions measured across the globe using various remote sensing techniques were averaged and collated by a computer program. The resulting data is displayed onto the underlying maps in wind-rose form.
The ocean charts in the Atlas show the mean wind speed and direction for every month of the year in each of the world’s oceans. On the monthly ocean charts, each wind rose is located in the center of a square covering 5 or 10 degrees square of longitude and latitude and shows the distribution of the winds that prevail in that area from eight cardinal points, both their direction and speed.
The arrows fly with the wind, and their lengths show the percentage of the total number of observations in which the wind has blown from that cardinal point. The number of feathers shows the force of the wind, which has been recorded most frequently from that sector. The wind force is measured on the Beaufort Scale, with each feather being equivalent to one unit of wind force, so that four feathers represent an average Force 4 wind from that direction for that particular month. In areas with prevailing winds, the resulting arrow would be too long to be shown in its entirety, in which case for percentages higher than 30, the percentage is shown numerically on the shaft. The figure in the center of each circle gives either the percentage of calms (less than Force 2) in blue or the percentage of storms (more than Force 7) in red, whichever is greater.
CW: Based on your experience, how closely can you rely on monthly wind and weather predictions that can be drawn from the Atlas?
J.C.: I need to stress that the pilot charts in the Atlas, and pilot charts generally, only show statistical data based on observations made over a long period of time. Such observations are averaged out for a given month, and as in the case with any statistical data, their relative accuracy depends on the number of observations, the reliability of those observations, and the method of calculation that was used. This is, in fact, the main difference between the old pilot charts and those used in this Atlas, as the data shown in some of the old pilot charts may have been derived from a relatively small number of observations, and their reliability may not have been too high. One important point in case is the direction of the wind, as the navigators who made some of those earlier observations may not have paid much attention to the difference between true- and apparent-wind speeds and direction, so it’s quite likely that a navigator on a fast clipper, or a fast container ship, may have erred on both counts. Compared to this, the data obtained from the satellites used in this program was based on millions of observations.
CW: In your experience, how accurate will the Atlas be in a given geographical area? In other words, if a sailor wanted to avoid gales in, say, an area of the North Atlantic, by how much of a distance would that area have to be skirted?
J.C.: The frequency of gale-force winds is indicated by a red figure in the center of each wind rose. Not surprisingly, the figures in high latitudes, especially in winter, tend to be high in each ocean. The frequency of gales in mid-latitudes in summer is far lower, and therefore they may not show up among the mean data calculated over an entire month, even if there may have been some spells of stronger winds. So the information contained in this Atlas may help you avoid the worst areas by choosing a route that avoids areas with a high percentage of winds stronger than Force 6. But the only way to be sure of avoiding gales in the North Atlantic is to sail a route that passes well to the south of known stormy areas.
But I must point out that anyone planning a passage through an area where there’s a probability of gale-force winds needs to be prepared, and able, to deal with such winds should they occur. A passage from North America to Europe across the North Atlantic is a good example, as even at the height of summer, it’s very unlikely that one will be able to sail some 3,000 miles without at least once having to put up with winds over 30 or even 35 knots. But at least they’ll be from behind and, as the song goes, storms never last. They certainly don’t!