The intertropical convergence zone is a belt of low pressure that circles the Earth with calm air, especially over the oceans. Scientists call the ITCZ various names, including the doldrums or the equatorial calms. When sail-powered ships reached these dreaded zones, they faced the potential of stalling for days or weeks without enough wind to proceed, according to Reference.com.
During the eighth century, large sailing ships grew large enough to reach the tropics. These subtropical regions, beginning at about 30 degrees, are the horse latitudes. The strong trade winds move ships quickly toward their destination. Near the equator, the trade winds from the northern and southern hemispheres converge, resulting in a zone where there is often a lack of wind.
The economic effect of the doldrums caused them to be of great interest and a topic of scientific study. It was not until World War II that real-time observations of surface weather showed a lack of temperature differential, allowing scientists to understand the cause of the relative calmness on the seas as caused by convergence of the trade winds, according to Weatherwise.
The convergence zone affects the weather on continents as well. It is the reason many equatorial land masses experience seasons of monsoons followed by seasons of drought. The relative calmness of the convergence zone is also a factor in the formation of cyclones over the water, which affects the weather patterns on landforms they strike.