Horse latitudes are the latitudes near 30° North or 30° South at sea. These regions of calms and baffling winds along with high barometric pressure in the oceans may have earned this name, as some historians suggest, by the crews of Spanish sailing ships carrying horses from Europe to America and the West Indies who had to jettison their live cargo as fodder ran out. Ships would repeatedly become becalmed in mid-ocean in this latitude, thus severely prolonging the passage and resulting in water shortages. Many times horses died from lack of fresh water and were cast overboard.
These latitudes correspond to the normal axes of the subtropical high-pressure belts. The term ‘subtropical high pressure areas’ is now frequently used in preference to ‘horse latitude’. No one is really sure how this term originated some say it’s possibly from around the 1760’s Spanish phrase golfo de las yeguas meaning mares’ sea. Another theory as to the source of this phrase relates that the name may come from 'dead horse', sailor’s slang for advance pay, which they expected to work off by the time they reached this part of the voyage.
The calms range north and south of the Equator where conditions of high atmospheric pressure exist almost permanently. The surface air, moving from the horse latitude towards the low pressure equatorial belt are called Trade Winds which continuously blow from these belts towards the Doldrums. The Columbia Encyclopedia: Sixth Edition explains:
Two belts of latitude where winds are light and the weather is hot and dry. They are located mostly over the oceans, at about 30° lat. in each hemisphere, and have a north-south range of about 5° as they follow the seasonal migration of the sun. The horse latitudes are associated with the subtropical anticyclone and the large-scale descent of air from high-altitude currents moving toward the poles. After reaching the earth's surface, this air spreads toward the equator as part of the prevailing trade winds or toward the poles as part of the westerlies.
A sea of mirror-like calmness where the wind-speed is less than 1 knot characterizes these calms and are the conditions characteristic of the horse latitudes labeled as 0 on the Beaufort Wind Scale. Used by mariners and meteorologists to indicate wind velocity this scale was devised by Francis Beaufort (1774-1857). Even though it has been subsequently modified it is still in use at sea.
In the Northern Hemisphere the belt is sometimes identified as the calms of Cancer and the belt in the Southern Hemisphere the calms of Capricorn . You might be interested to note that the Sargasso Sea is located with this listless area of the sea. Between the Azores and the West Indies in the Atlantic Ocean the sea is within the horse latitudes. Positioned in the center of the slow clockwise-moving warm surface water currents is an ideal environment for a great deal marine biological activity with its profusion of "surface gulfweed; breeding ground for eels which migrate to Europe." The island of Bermuda is located in the northwestern part of the sea.
The Columbia Encyclopedia: Sixth Edition
Bram, Robert Philips, Norma H. Dicky, "Wind," Funk & Wagnalls New Encyclopedia , 1988.