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To insure safety at sea, the best that science can devise and that naval organization can provide must be regarded only as an aid, and never as a substitute for good seamanship, self-reliance, and sense of ultimate responsibility which are the first requisites in a seaman and naval officer.

~Admiral Chester W. Nimitz




 

What are the Trade Winds?

Early commerce to the Americas relied on the trade winds—the prevailing easterly winds that circle the Earth near the equator.

Known to sailors around the world, the trade winds and associated ocean currents helped early sailing ships from European and African ports make their journeys to the Americas. Likewise, the trade winds also drive sailing vessels from the Americas toward Asia. Even now, commercial ships use "the trades" and the currents the winds produce to hasten their oceanic voyages.

How do these commerce-friendly winds form? Between about 30 degrees north and 30 degrees south of the equator, in a region called the horse latitudes, the Earth's rotation causes air to slant toward the equator in a southwesterly direction in the northern hemisphere and in a northwesterly direction in the southern hemisphere. This is called the Coriolis Effect.

The Coriolis Effect, in combination with an area of high pressure, causes the prevailing winds—the trade winds—to move from east to west on both sides of the equator across this 60-degree "belt."

As the wind blows to about five degrees north and south of the equator, both air and ocean currents come to a halt in a band of hot, dry air. This 10-degree belt around Earth's midsection is called the Inter-Tropical Convergence Zone, more commonly known as the doldrums.

Intense solar heat in the doldrums warms and moistens the trade winds, thrusting air upwards into the atmosphere like a hot air balloon. As the air rises, it cools, causing persistent bands of showers and storms in the tropics and rainforests. The rising air masses move toward the poles, then sink back toward Earth's surface near the horse latitudes. The sinking air triggers the calm trade winds and little precipitation, completing the cycle.

 

What are the Doldrums?

The "doldrums" is a popular nautical term that refers to the belt around the Earth near the equator where sailing ships sometimes get stuck on windless waters.

Known to sailors around the world as the doldrums, the Inter-Tropical Convergence Zone, (ITCZ, pronounced and sometimes referred to as the “itch”), is a belt around the Earth extending approximately five degrees north and south of the equator. Here, the prevailing trade winds of the northern hemisphere blow to the southwest and collide with the southern hemisphere’s driving northeast trade winds.

Due to intense solar heating near the equator, the warm, moist air is forced up into the atmosphere like a hot air balloon. As the air rises, it cools, causing persistent bands of showers and storms around the Earth’s midsection. The rising air mass finally subsides in what is known as the horse latitudes, where the air moves downward toward Earth’s surface.

Because the air circulates in an upward direction, there is often little surface wind in the ITCZ. That is why sailors well know that the area can becalm sailing ships for weeks. And that’s why they call it the doldrums.

 

What are the Roaring Forties?


A NOAA research vessel braving the winds of the Roaring Forties.

Sailors call the latitudes between 40 and 50 degrees south of the equator the Roaring Forties.

During the Age of Sail (circa 15th to 19th centuries), these strong prevailing winds propelled ships across the Pacific, often at breakneck speed. Nevertheless, sailing west into heavy seas and strong headwinds could take weeks, especially around Cape Horn at the southern tip of South America, making it one of the most treacherous sailing passages in the world.

The Roaring Forties take shape as warm air near the equator rises and moves toward the poles. Warm air moving poleward (on both sides of the equator) is the result of nature trying to reduce the temperature difference between the equator and at the poles created by uneven heating from the sun.

This process sets up global circulation cells, which are mainly responsible for global-scale wind patterns. The air descends back to Earth’s surface at about 30 degrees’ latitude north and south of the equator. This is known as the high-pressure subtropical ridge, also known as the horse latitudes. Here, as the temperature gradient decreases, air is deflected toward the poles by the Earth’s rotation, causing strong westerly and prevailing winds at approximately 40 degrees. These winds are the Roaring Forties.

The Roaring Forties in the Northern Hemisphere don’t pack the same punch that they do in the Southern Hemisphere. This is because the large land masses of North America, Europe, and Asia obstruct the airstream, whereas, in the southern hemisphere, there is less land to break the wind in South America, Australia, and New Zealand.

While the Roaring Forties may be fierce, 10 degrees south are even stronger gale-force winds called the Furious Fifties. And 10 degrees south of the Furious Fifties lie the Screaming Sixties! We can thank the intrepid sailors of yore for these wildly descriptive terms.

 

What are the Horse Latitudes?

The horse latitudes are subtropical regions known for calm winds and little precipitation.

The horse latitudes are located at about 30 degrees north and south of the equator. It is common in this region of the subtropics for winds to diverge and either flow toward the poles (known as the prevailing westerlies) or toward the equator (known as the trade winds). These diverging winds are the result of an area of high pressure, which is characterized by calm winds, sunny skies and little or no precipitation.

According to legend, the term comes from ships sailing to the New World that would often become stalled for days or even weeks when they encountered areas of high pressure and calm winds. Many of these ships carried horses to the Americas as part of their cargo. Unable to sail and resupply due to lack of wind, crews often ran out of drinking water. To conserve scarce water, sailors on these ships would sometimes throw the horses they were transporting overboard. Thus, the phrase 'horse latitudes' was born.

David L. Heiserman, Editor

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Revised: June 06, 2015