Trade winds

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Trade winds, also called trades are the constant and regular winds that blow in belts on either side of the equator to about 30 degrees north and south over the three oceans that exist at those latitudes. They are caused by air moving in to replace the hot, rising air in the permanent low pressure band at the equator—frequently called the doldrums. They move in south-easterly (northern hemisphere) and north-easterly (southern hemisphere) directions instead of due south or north because of the Earth’s rotation.

Above and below them are belts called the “variables” before bands of prevailing westerlies (called the “Roaring Forties” in the southern hemisphere) occur between about 40 and 60 degrees.

Their existence of all these “belts” has had a profound effect on the history of trade, and European exploration through history:

  • The Spanish and other Europeans used the trades to reach the Americas then returned with the help of the Variables and the north-west setting Gulf Stream.
  • The Portuguese, and later the Dutch and British used the trades in the Indian Ocean in their return from India and the East Indies and much later with the tea clippers from China.
  • Pedro Cabral came across Brazil whilst reaching across the Atlantic South-east trades whilst on his way to India.
  • The Triangular trade used the trades for the “middle passage” then sailed north and out of them for the return to Europe.
  • All seafaring nations used the trades from the Americas west across the Pacific, then used the westerlies to return.
  • The Dutch used the westerly “Roaring Forties” on their voyages from the Cape of Good Hope. Their occasional miscalculation about when to turn north for Batavia (modern Djakarta) was instrumental in the discovery of Western Australia, then known as New Holland.
  • The British and French used the African route sailing to Australasia, then returned via Cape Horn, making use of the westerlies on both trips.

There have been exceptions. Trade across the north Indian Ocean between India and north-east Africa and the Red Sea made use of the African and Indian monsoons which occurred at different times of the year. Also, currents have been used: as well as the Gulf Stream, the currents running up the west coasts of Africa and South America were a boon to mariners in the age of sail.

The trade winds have always been a moderator of the tropical heat; a blessing to the islands and coasts in their path. John Masefield’s poem "Trade Winds”, later put to music by Frederick Keel, evokes this beautifully:

"In the harbour, in the island, in the Spanish seas,
Are the tiny white houses and the orange trees,
And day-long, night-long, the cool and pleasant breeze
Of the steady Trade Winds blowing.

There is the red wine, the nutty Spanish ale,
The shuffle of the dancers, and the old salt's tale,
The squeaking fiddle, and the soughing in the sail
Of the steady Trade Winds blowing.

And o'nights there's the fire-flies and the yellow moon,
And in the ghostly palm trees the sleepy tune
Of the quiet voice calling me, the long low croon
Of the steady Trade Winds blowing."


References

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