Jimmy Cornell Talks About His Ocean Atlas

CW: How does your Atlas differ from others that are currently in print?

J.C.: The data is far more accurate, as it’s based on the latest weather information gathered by a network of meteorological satellites during the last 20 years. What I need to stress here isn’t so much the importance of the data in the Atlas being as accurate as possible, but how the information shown in the old pilot charts can be misleading or possibly inaccurate. There are many examples to bear this out, but I’ll use only two, as they both refer to some frequently sailed routes and show just how much the information displayed on the new charts differs from the old pilot charts.

To give an idea of how much sailing conditions have changed over the years, I’ll compare the old pilot chart for the area west from Panama for the month of March with the latest data for that same area as depicted on the chart for the same month reproduced in this Atlas. The changes are quite significant, both north of the Galápagos, where northeast winds predominate north of the equator, and south and west of Galápagos, where the proportion of east and southeast winds is now shown as markedly higher than in the past. What the current pilot chart shows is that those sailors bound from Panama for the Marquesas who aren’t interested in stopping in the Galápagos may fare better by sailing southwest out of the Bay of Panama to about 4 degrees north, then continue west from there until the point is reached where the equator is crossed and the Marquesas are approached from an east-northeast direction.

North Pacific Ocean March—old pilot chart:

North Pacific Ocean March—new Atlas pilot chart:

Just as significant are the changes that have occurred along the frequently sailed route from Bermuda to the Azores. In the past, the recommended tactic on leaving Bermuda was to sail northeast or even north-northeast into an area of prevailing westerly winds, make the required easting in that latitude, and only alter course for the Azores when northwest of the islands. This advice accords with the information shown on the old pilot chart for June. But the latest pilot charts from my new Atlas shows that in fact, that advice is no longer valid and that the probability of favourable winds is just as high on the more direct and shorter route. In fact, the proportion of favourable southwest and south winds isn’t much affected even if you happen to stray into the upper limb of the Azores High.
               
North Atlantic Ocean June - old pilot chart:


North Atlantic Ocean June—new pilot chart:

Below, the upper red line shows how the traditional route would’ve been sailed according to the information shown on the old pilot chart, while the lower red line traces a route that takes the best advantage of the information shown in the new Atlas.

I hope that these two examples show why the current pilot charts are of such relevance to sailors planning an offshore passage.

I’d like to add here that those considerable discrepancies can be due to two very different factors. They can be due to climate change but could also be explained by the fact that some of those earlier observations weren’t so accurate or were based on limited sources. This is particularly the case in less frequented areas of the oceans, both in the tropics and high latitudes, where there have been fewer observations over the years than in areas regularly crossed by commercial ships.

Another important point that I’d like to make about the difference between the old and new pilot charts in the Atlas is their clarity. The old pilot charts were conceived for the use of commercial ships, and much of the information they contain isn’t relevant to cruising boats. What’s worse, some of those charts are cluttered with too much detail, whereas the new charts only show information that is of direct relevance to cruising boats. There’s also considerably more detail on the charts depicting specific ocean areas. Whereas on the old pilot charts wind roses were usually displayed at the center of each 5 degrees square, on some of the new pilot charts, their frequency increases as much as sixteenfold.