Definition of Trade Winds
Simply put, trade winds are winds that reliably blow east to west in roughly 30-degree bands north and south of the equator. These winds are usually very predictable and close to the surface.
Why are they called that?
Contrary to what some may think, the name “trade winds” has nothing to do with their use by trading ships (all sailing ships on trade routes use wind for propulsion). According to historian Lincoln Paine, the trade winds get their name from an old use of the word “trade” meaning steadily or regularly.1 The exact origins of the term are unknown, but the use of the term “trade winds” to refer to regular or steady winds probably occurred sometime in the 18th century.2
*Note: Winds are named for the direction from which they blow. Contrast this with water currents which are named for the direction towards which they flow. (e.g. an easterly wind blows from the east, whereas a river or an ocean current would flow towards the east.)
The northeast trade wind in the North Atlantic also goes by the name Passat, which is a mid-16th century German word. This particular trade wind helped Christopher Columbus on his first voyage to the West Indies in 149.3
What Causes Trade Winds?
The trade winds are the result of a cycle of warm, moist air being heated by the sun and rising near the equator. As the air rises, it becomes cooler and denser to the north and south of the equator where it descends back towards the surface. This cool air then rushes in to fill the area vacated by the rising, warm air and completes the cycle. This process creates what’s known as a Hadley cell (as seen in the above diagram). Note that the warm, wet air rises to form clouds and frequent thunderstorms around the equator.4
So based on the Hadley cell, why don’t the trade winds only blow north and south? Well, if the Earth didn’t rotate, then this is what would happen. However, due to the rotation of the Earth (counterclockwise), the resulting Coriolis effect forces the rising warm air on both sides of the equator to curve to the east (forming the Westerlies) where the air cools and descends, as seen in the above diagram.5 The speed of the rotation of the Earth is greater at the equator versus at higher latitudes. The resulting incoming trade winds are therefore diverted back towards the west and hence trade winds blow northeasterly and southeasterly.6
Also, note in the above diagram that there is a band of air around the equator known as the doldrums (officially the Intertropical Convergence Zone). This is an area where there’s very little to no wind and the air is warm and moist from the sun heating and evaporating the water. Clouds and thunderstorms are common in this area, but winds are light to almost non-existent.7 Sailors have long feared their (sailing) vessels becoming becalmed in the doldrums where there’s little (or no wind) to propel their ships. They effectively have become victims of the convection of the warm air in the Hadley cell.
1. Lincoln Paine, The Sea and Civilization: A Maritime History of the World (New York: Alfred A. Knopf, 2013), 18.
2. John Rogers, Origins of Sea Terms (Mystic, CT: Mystic Seaport), 183.
3. Peter Kemp, The Oxford Companion to Ships and the Sea (Oxford Oxfordshire: Oxford University Press, 1994), 635.
4. “What Are Trade Winds?,” NOAA, Accessed August 21, 2022, https://scijinks.gov/trade-winds/. Hereafter referred to as NOAA.
6. Peter Kemp, The Oxford Companion to Ships and the Sea (Oxford Oxfordshire: Oxford University Press, 1994), 882.
Kemp, Peter. The Oxford Companion to Ships and the Sea. Oxford Oxfordshire: Oxford University Press, 1994.
Paine, Lincoln. The Sea and Civilization: A Maritime History of the World. New York: Alfred A. Knopf, 2013.
Rogers, John. Origins of Sea Terms. Mystic, CT: Mystic Seaport, 1985.
“What Are Trade Winds?” NOAA. Accessed August 21, 2022. https://scijinks.gov/trade-winds/.