The Race is On

 

“The general route for these round-the-world races is basically the same for everyone,” explains Cahalan, “because it’s based on the persistent wind belts and calms that are part of the general circulation of the atmosphere.” The general circulation is simply the movement of the atmosphere averaged over a span of time, such as a month, a season, or a year. The patterns repeat so predictably because the underlying factors that drive them either don’t change—for example, the fact that the Earth is spinning or that the equator gets more direct sunlight than the poles—or else change so slowly that they appear unchanging—like the location of the continents.

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Map of Cheyenne's route around the world
 

Cahalan explains it is the major wind belts and calms that sailors are concerned with. “Between the poles and the equator, each hemisphere has three major surface wind belts: the polar easterlies, which extend from the poles to about 60 degrees latitude; the prevailing westerlies, which stretch from about 60 degrees to 35 degrees; and the trade winds, which pick up at about 30 degrees, and blow towards the equator.”

The opposite of these typically windy areas are latitudes where the surface winds—when they exist—are light and unpredictable. One of these calm zones circles the globe between five degrees north and south of the equator, where the trade winds converge. Thunderstorms occur daily, but surface winds are so scarce in the region that the European sailors nicknamed it “the doldrums.” Meteorologists call it the Inter-Tropical Convergence Zone, or ITCZ, for short.

 

The route covered by round-the-world sailing expeditions starts off the coast of France, heads south through the Atlantic Ocean, circles west to east in the Southern Ocean, and returns northward to Europe through the Atlantic. This map shows Cheyenne’s approximate route and highlights milestones along the way. (Map adapted from Steve Fossett Challenges)

  Satellite map of winds in th Atlantic Ocean
 

The other areas of predictable calm occur between about 35 and 30 degrees latitude in both hemispheres. These clear-sky, dry areas are called “the horse latitudes,” which may be a grim reference to the days when the crews of ships stranded without wind at those latitudes were forced to either eat their horses or throw them overboard when food and water supplies dwindled.

In the doldrums, there is little surface wind because the trade winds collide and drive air upward. In the horse latitudes, the opposite occurs: air is sinking back down to the surface from high in the atmosphere. Rising air creates zones of low pressure at the surface; sinking air creates zones of high pressure.

In the Atlantic in the northern hemisphere, this region of high pressure at the horse latitudes is called the Bermuda High; in the Atlantic in the southern hemisphere horse latitudes, the region is called the South Atlantic High. Sailors must avoid these matching areas of high pressure and little wind that lurk in the middle of the Atlantic in both Hemispheres.

“Leaving from the English Channel, the route is south along the Bay of Biscay, staying about 80 miles west of the Canary Islands. Next, you aim to cross the Equator somewhere between 25 and 30 degrees West, a place where the ITCZ [the doldrums] is rather thin. Then you go around the South Atlantic High and head toward the Southern Ocean,” explains Cahalan.

 

Global wind patterns dictate the round-the-world sailing route. Winds blowing from west to east dominate the middle to high latitudes, separated from the east-to-west trade winds by zones of high pressure and low winds. The northern and southern trade winds are separated along the equator by the Inter-Tropical Convergence Zone, or “doldrums.” This image shows ocean winds on February 9, 2004, while Cheyenne was sailing towards the equator. (Image courtesy NASA/JPL Seaflux)

 

When the Cheyenne reached the Antarctic Convergence Zone—where cold Antarctic waters meet the waters of warmer mid-latitude oceans—Cahalan recalls how quickly conditions changed. “It was amazing how it was like a line in the ocean as far as how strong the differences were in air and water temperature,” she says. “We saw icebergs as far north as 48 degrees South when we were near 60 degrees East. We sailed the rest of the way eastward around the Southern Ocean at about 50 degrees South, dipping a little lower to maybe 53-54 south in the South Pacific.” After crossing the empty Pacific, the Cheyenne sailed around the southern tip of South America and in mid-March, they found themselves once again in the Atlantic.

  Photograph of cloud edge

Changes in ocean conditions can lead to abrupt changes in weather. When the Cheyenne crossed into the Southern Ocean, temperature dropped so suddenly that it seemed to Cahalan that they had crossed some invisible boundary line. (Photograph copyright Nick Leggatt)

  Satellite map of Indian Ocean winds on February 29, 2004  

Sustained winds along 50° South latitude propelled Cheyenne across the Indian Ocean—a distance of over 7,500 kilometers (4,700 miles)— in under 10 days. In this QuikSCAT satellite image, high winds (yellow areas) dominate the southern Indian Ocean between Africa (left) and Australia (right). (Image courtesy NASA/JPL Seaflux)

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