The trade winds were named for paths, not commerce, and they fertilise the Amazon
Trade once meant a track, and the trade winds were simply the steady routes across tropical oceans. Portuguese sailors learned to loop with them in the 15th century, shaping which coasts empires reached. Today those same easterlies carry millions of tons of Saharan dust across the Atlantic, delivering phosphorus to Amazon soils.
The trades are the prevailing east-to-west winds of the tropics. They start where air sinking around 30 degrees latitude, in belts of high pressure called subtropical ridges, flows back toward the equator along the surface. Earth's rotation deflects that flow westward, so the winds blow from the northeast north of the equator and from the southeast south of it, meeting in the calm, low-pressure band sailors called the doldrums. Every tropical ocean except the northern Indian Ocean has broad trade-wind zones.
The sinking air is dry, having shed its moisture over the equator, and it forms a warm lid over the moist sea air below. This trade wind inversion keeps clouds as shallow cumulus, rarely taller than about 4 kilometres. The winds blow harder in winter, as in the Guianas' windy season from January to April, and strengthen when the Arctic oscillation is in its warm phase. When they slacken, nearby lands such as Central America get more rain. They also steer tropical storms across the Atlantic, Pacific and southern Indian Oceans.
Sailing ships lived by them. Returning from West Africa, the Portuguese swung west and northwest into open ocean, then northeast toward the Azores and home, a manoeuvre they called the volta do mar. To reach South Africa they headed out toward Brazil before turning east around 30 degrees south, since hugging the African coast meant beating into the wind. Europeans did not grasp the full Pacific circuit until Andrés de Urdaneta's voyage of 1565, and Manila galleons could not sail against the wind at all. By the 18th century, English speakers linked the name with commerce, and between 1847 and 1849 Matthew Fontaine Maury charted winds and currents for the world's oceans.
The Saharan dust these winds carry is a mixed gift. Rich in nitrate and phosphate, it feeds depleted Amazon soils, yet over Florida and the Gulf Coast it whitens the sky, suppresses rain, reddens sunsets and worsens air quality. Dust outbreaks have intensified since 1970 with African droughts and have been linked to declining coral reef health across the Caribbean and Florida.
Source: Trade winds
