Air in motion – it’s a constant and unstoppable feature of life on the Earth, given that the planet is endlessly spinning on its axis and variably warmed by our Mr. Sun. Currents of wind flow in predictable patterns and you’ve all heard about the westerlies and the easterlies and the trade winds. The trade winds, so called because they facilitated the merchant trade of the sailing ship days, blow between about 30 degrees north and south of the equator. It’s hot at the equator, so as these two air masses meet, they heat up and rise, rather than blow across the surface. In other words, it’s mostly pretty calm along the equator and if you’re trying to go somewhere by sail, you’re out of luck.
Technically this belt, about 5 degrees on either side of the equator, is called the Intertropical Convergence Zone, or ITCZ. Colloquially, it’s known as the doldrums, and given that the origin of the word points to the Old English “dol”, for foolish or dull, the word apparently referred to human mental stagnation before it was applied to the slack breeze in the middle of the tropics. Bored, dispirited and overheated, sailors stuck in the doldrums could face illness and starvation. Further aggravating their plight, the heat of the equatorial region can make for a lot of evaporation and humidity, which can lead to rapidly-forming storms. Sudden 30 knot gales can be a bit too much of a good thing.
In the neighborhood of 30 to 35 degrees north and south, the air that was heated along the equator descends and creates another belt of weak cross-surface wind. Sailors becalmed in these regions were known to jettison horses and other livestock from their ships in order to conserve precious water supplies and lighten their load, and these parallels became known as the horse latitudes. It’s no wonder the world switched to steam power for ships when it could – getting stuck in the doldrums or the horse latitudes brought delay and physical and mental anguish.
Here are a couple of diagrams illustrating of all this – each lends a different bit of clarity to the concept:
Disclaimer: I am not now, nor have I ever been a meteorologist and the above is my best distillation of some fairly complex heat and air physics. No animals (except the horses and sailors) were harmed in the composition of this Geo-Joint and the author takes no responsibility for the reader’s use of this information.