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The doldrums is a colloquial expression derived from historical maritime usage, which refers to those parts of the Atlantic Ocean and the Pacific Ocean affected by the Intertropical Convergence Zone, a low-pressure area around the equator where the prevailing winds are calm. The doldrums are also noted for calm periods when the winds disappear altogether, trapping sailing ships for periods of days or weeks. The term appears to have arisen in the 18th century, when trans-equator sailing voyages became more common. Since this zone is where two trade winds meet, it is also called the Intertropical Convergence Zone. They roughly lie between latitudes 5° north and south.
In maritime usage, the low pressure characteristics of the doldrums are caused by the expanding atmosphere due to heating at the equator, which makes the air rise and travel north and south high in the atmosphere, until it subsides again in the horse latitudes. Some of that air returns to the doldrums through the trade winds. This process can lead to light or variable winds and more severe weather, in the form of squalls, thunderstorms, and hurricanes. The doldrums are also noted for calm periods when the winds disappear altogether, trapping sail-powered boats for periods of days or weeks.
Colloquially, the "doldrums" are a state of inactivity, mild depression, listlessness, or stagnation. The word is derived from dold, an archaic term meaning "stupid", and -rum(s), a noun suffix found in such words as "tantrum."
The doldrums are notably described in Samuel Taylor Coleridge's poem The Rime of the Ancient Mariner (1798), in Patrick O'Brian's novel Desolation Island (1978), and in Laura Hillenbrand's non-fiction book Unbroken: A World War II Story of Survival, Resilience, and Redemption (2010). Additionally, the Doldrums are a fictional place in Norton Juster's novel The Phantom Tollbooth (1961).