Time of year when trade winds blow

The Charles W. Morgan built in 1841 is the last of an American whaling fleet that once numbered more than 2,700 vessels. The ship currently restored and anchored at the Mystic Seaport Museum Mystic CT.

Ships like the Morgan often used routes defined by the trade winds to navigate the oceans.

Elladikdik iuṃwinTūṃur ekūtañtañin eṃṃaan. “Under the windstorms of Tūṃur, a man is reduced to inchworm at sea.” Thus, goes the proverbial Marshall Islanders saying that describes that time of the year when the sea begins to churn from the east and diminish a man, his craft and his course to that of the tiny inch worm tossed about by its mountainous swells. The initial rising of the star Tūṃur (Antares) just before dawn right around Christmas marks the beginning of the windy season across the Pacific and thus it has always been. 

Charles W. Morgan at Mystic Seaport

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 drove 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? They form between about 30 degrees north and 30 degrees south of the equator, in a region called the horse latitudes. They call these the horse latitudes because they are subject to periodic doldrums. Doldrums occur during the summers of the northern hemisphere and during the winters in the southern hemisphere. They called them the horse latitudes because the crew would release the horses to the sea as they suffered from stillness and drought. This is when the trade winds stop blowing and the sea turns into a sheet of glass. The doldrums were the subject of the Ryme of the Ancient Mariner by Samuel Taylor Coleridge:

Water, water, everywhere

And all the boards did shrink;

Water, water, everywhere,

Nor any drop to drink.

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. In the summer this band drifts north deep into the Marshall Islands bringing rain and variable winds. It is an appropriate time to sail between the islands.  During the winter the band drifts back south to the equator once Tūṃur makes his appearance just  before dawn and the cycle repeats.

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