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Trade Winds, Horse Latitudes, and the Doldrums

Global Atmospheric Circulation and its Related Effects

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A map of the tropics, ITCZ, doldrums, trade winds, and horse latitudes.

A map of the tropics, ITCZ, doldrums, trade winds, and horse latitudes.

Matt Rosenberg
Solar radiation warms the air over the equator, causing it to rise. The rising air then proceeds south and north toward the poles. From approximately 20° to 30° North and South latitude, the air sinks. Then, the air flows along the surface of the earth back toward the equator.

Doldrums

Sailors noticed the stillness of the rising (and not blowing) air near the equator and gave the region the depressing name "doldrums." The doldrums, usually located between 5° north and 5° south of the equator, are also known as the Intertropical Convergence Zone or ITCZ for short. The trade winds converge in the region of the ITCZ, producing convectional storms that produce some of the world's heaviest precipitation regions.

The ITCZ moves north and south of the equator depending on the season and solar energy received. The location of the ITCZ can vary as much as 40° to 45° of latitude north or south of the equator based on the pattern of land and ocean. The Intertropical Convergence Zone is also known as the Equatorial Convergence Zone or Intertropical Front.

Horse Latitudes

Between about 30° to 35° north and 30° to 35° south of the equator lies the region known as the horse latitudes or the subtropical high. This region of subsiding dry air and high pressure results in weak winds. Tradition states that sailors gave the region of the subtropical high the name "horse latitudes" because ships relying on wind power stalled; fearful of running out of food and water, sailors threw their horses and cattle overboard to save on provisions. (It's a puzzle why sailors would not have eaten the animals instead of throwing them overboard.) The Oxford English Dictionary claims the origin of the term "uncertain."

Major deserts of the world, such as the Sahara and the Great Australian Desert, lie under the high pressure of the horse latitudes.

The region is also known as the Calms of Cancer in the northern hemisphere and the Calms of Capricorn in the southern hemisphere.

Trade Winds

Blowing from the subtropical highs or horse latitudes toward the low pressure of the ITCZ are the trade winds. Named from their ability to quickly propel trading ships across the ocean, the trade winds between about 30° latitude and the equator are steady and blow about 11 to 13 miles per hour. In the Northern Hemisphere, the trade winds blow from the northeast and are known as the Northeast Trade Winds; in the Southern Hemisphere, the winds blow from the southeast and are called the Southeast Trade Winds.

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