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The trade winds are a consistent weather pattern of easterly winds which blow near the Earth's equator. The term “trade” in this case refers to a track or path, rather than commerce, with historical mariners being fans of these winds because they could rapidly blow ships to the west. Over time, people started associating the trades, as these winds are sometimes called, with commerce, because the seasonal ebbing and flowing of the trades played an important role in global commerce.
In order to understand how the trade winds form, it is necessary to briefly discuss the general patterns of wind on Earth. When air hits the equator, it heats up and rises, eventually being pulled down to the poles, where it becomes chilled, sinking close to the surface of the ocean and then being pushed back toward the equator by the pressure of cooling air from above. This creates a continuous cycle of air which moves slowly towards the equator near the surface of the Earth, with the air above moving toward the poles.
When air meets at the equator, it is deflected due to a phenomenon called the Coriolis effect, which is caused by the rotation of the Earth. The deflection causes the winds to divert to the West, creating a steady flow of wind which is termed “easterly” because winds are named for the direction from which they originated, rather than the direction in which they are headed.
Mariners found the trade winds of interest both because they could be used to speed sailing to the west, and because they were surrounded by two other interesting wind patterns: the doldrums and the horse latitudes. The doldrums, known to meteorologists as the Intertropical Convergence Zone, are located at the equator, where there are almost no winds at all. In the horse latitudes above and below the equator, there are also periods of minimal wind which can cause ships to become stranded.
Historically, finding the trade winds and sticking with them was extremely important, because ships could be stuck for extended periods of time in the doldrums or the horse latitudes, eventually running out of supplies. Although the trades are no longer of critical importance to merchant ships, since they do not rely on wind power, sailors continue to utilize the trade winds as a sort of oceanic fast lane to cut down on travel times across the Pacific and Atlantic. The trade winds also play an important role in global weather, bringing storms to the western coasts of Asia, Africa, and the Americas.