Windswept_ The Story of Wind and Weather - Marq de Villiers [37]
Starting from the equator and going north, these belts are the doldrums, the trade winds, the horse latitudes, the prevailing south-westerlies of midlatitudes, and the northeasterlies of high polar latitudes. In the southern hemisphere, the same belts exist but the wind directions differ.
The doldrums straddle the equator and girdle the earth, a belt of low pressure, static and ever-present, a windless region known to all sailors. The doldrums are more properly known as the intertropical convergence zone (ICZ), or thermal equator, which generally occupies about 5 degrees each side of the equator, though it migrates somewhat north or south with the seasonal position of the sun. (It also shifts farther to the south over land masses such as South America, Africa, and Australia, and farther to the north over open water, such as the Pacific or Atlantic.9 It can occasionally reach beyond the 30th parallel.) Confusingly, it is sometimes called the equatorial convergence zone or the intertropical front.
The trade winds are next, bounded on the doldrums side by a zone of sharply rising winds, creating towering cumulonimbus thunderclouds and torrential rains. The trade winds flow from the next band out, the subtropical high-pressure zone called the horse latitudes, toward the low-pressure zone of the doldrums, and are "turned" westward by the Coriolis force. They were named, obviously enough, for their useful ability to push sailing vessels quickly and economically across oceans; they blow steadily at about 12 miles an hour between the ICZ and the 30th degree of latitude. In the northern hemisphere, the trade winds are northeasterlies; in the southern hemisphere, they are southeasterlies.
The horse latitudes are bands of calmer air, zones of stillness that have caused many a mariner to rue his profession. Maritime lore has several explanations for the name, all of them more or less implausible; the most popular is that ships became becalmed for so long in this zone that sailors and colonists were forced to toss their horses overboard, no longer having enough water to keep them alive. The horse latitudes are sometimes called the calms of Cancer in the northern hemisphere and the calms of Capricorn in the southern hemisphere. On land, most of the world's great deserts lie in this region.
Outside the horse latitudes are the prevailing westerlies (southwesterly in the northern hemisphere, northwesterly in the southern hemisphere) that cover most of Europe, North America, China, and similar latitudes south of the equator. They're as consistent—and as useful—as the trade winds. In the southern hemisphere they're closer to the equator and are more vigorous than their counterparts in the north, and are known as the roaring forties. In the northern hemisphere, this is where most of the North American and European weather is generated.
In high latitudes the flow reverses, and easterlies are the dominant wind patterns of both Arctic and Antarctic regions.
The most turbulent, changeable and perversely complicated weather on the planet occurs in the midlatitudes, where the warm equatorial air and the cooler polar air intersect in an apparently patternless turbulence. This is the downside of living in temperate regions, otherwise the most benign climate on Earth. The intersections of these winds—frontal zones—cause violent storms, tornadoes, thunder cells, and gales, as well as mild breezes and balmy temperatures. Mark Twain was moved to comment on the weather in northeastern America: "The people of New England are by nature patient and forbearing," he said in a speech to the New England Society in 1876, "but there are some things which they will not stand. Every year they kill a lot of poets for