Nautical Terminator – Tradewinds (Part 2)

By Edmond Halley –, Public Domain,

By the early seventeenth century, mariners were calling the steady easterly winds near the equator the “trade” winds. But it was not understood why these winds blew with such constancy, and a full explanation had to wait until the early twentieth century.

The earliest hypothesis in the early 1700s, by the astronomer Edmund Halley and the maritime lexicographer and poet William Falconer, was that the air directly under the sun was heated and therefore rose, causing surface air to flow in to fill the vacuum. Halley and Falconer weren’t right, as their theory would predict that the wind would not blow at night, and that in the morning, the wind would blow east, towards the sun. But the trades don’t change direction.

A better hypothesis was developed around the same time by George Hadley, whose brother invented the octant, a predecessor of the sextant. Hadley’s theory languished in obscurity until the late 19th century, but now he is remembered in meteorology by the “Hadley Cell.” Hadley theorized that the linear momentum of the air played a role as the earth rotated beneath it. The wind blew toward the equator, and the earth’s rotation made it also appear to travel from east to west. But the final piece of the puzzle required the addition of an effect described by Gaspard-Gustave Coriolis regarding the preservation of angular momentum. A rather nice description of the Coriolis effect can be found at

As long as the earth has been a sphere, the tropics, where the sun’s rays are vertical, have been heated more than the rest of the globe, where the sun’s energy strikes at a tangent. The hot air around the equator rises and flows to higher latitudes, and cooler air flows in to replace it. And as long as the earth has rotated on its axis, the Coriolis effect has turned this wind to the right in the northern hemisphere and left in the southern. Therefore, even in earth’s primitive atmosphere that lacked free oxygen, when no animals walked on earth and no fish swam in the sea, the trade winds were already blowing. If you had the DeLorean from “Back To The Future” and you could haul a boat on a trailer, you could transport yourself three billion years back in time and experience trade winds sailing. Of course, you wouldn’t be able to breathe, but everything has a downside.

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