Dictionaries define the doldrums as stagnation or a time when things slump, or general listlessness, and this term was in use as European sailors began to fully explore the Atlantic, Pacific and Indian Oceans. What they found, among many things, was a peculiar weather phenomenon that occurs up to five degrees north and south of the equator. This phenomenon was either extreme calm with little wind, or massive storms, and both could leave ships at sea for weeks. This area was nicknamed the doldrums in the 1700s, though it has more scientifically correct names too, such as the Intertropical Convergence Zone or the Equatorial Convergence Zone.
There are several factors that create the doldrums or this area of calm. Trade winds from North and South converge or meet in this area near the equator and the air becomes greatly warmed by the sun. This causes the air to move upward in almost a vertical direction, instead of remaining near the earth's or ocean's surface. With the air’s upward movement pattern, there can be little to no winds at the surface, a terrible thing for sailors who must rely on wind for travel. Alternately, huge hurricanes can result and create hazardous traveling conditions.
Sailors encountering the doldrums were in danger because there was little way to move the ships, except by rowing when wind power was unavailable. Calm wind conditions could last for weeks and this could result in desperation to see land, and to move the ship. A ship in the doldrums could start steadily losing supplies as days passed, and some sailors encountered cabin fever, as they remained almost anchored in place while no winds grew to carry them. In worse case scenarios, being stuck in this area near the equator could cause death to the entire crew while they waited for winds to pick up.
The area known as the doldrums is not simply dry and windless. Over half the days in a year produce significant rain due to the action of warm air rising. This may still not produce adequate wind to move a ship, or the storms can be so fierce with tremendous lightening and thunder that travel is a hazard. However, a rainy day didn’t necessarily mean great storms occurred.
Many days in equatorial weather are high in relative humidity with occasional rains throughout a day. It’s not difficult to imagine that this kind of weather could only have made things worse on a ship. The relative humidity would have made it seem even more hot and uncomfortable as sailors waited for enough wind to carry them to the next area of land they wished to reach.