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The Doldrums or the Intertropical Convergence Zone
The Doldrums is the colloquial term for the Intertropical Convergence Zone (ITCZ). The Doldrums is the area encircling the earth near the equator where winds originating in the northern and southern hemispheres come together.



The ITCZ was originally identified from the 1920s to the 1940s as the Intertropical Front (ITF), but after the recognition in the 1940s and 1950s of the significance of wind field convergence in tropical weather production, the term ITCZ was then applied. When it lies near the equator, it is called the near-equatorial trough. Where the ITCZ is drawn into and merges with a monsoonal circulation, it is sometimes referred to as a monsoon trough, a usage more common in Australia and parts of Asia. In the seamen's speech, the zone is referred as The Doldrums because of its erratic weather patterns with stagnant calms and violent thunderstorms.

The ITCZ appears as a band of clouds, usually thunderstorms, that circle the globe near the equator. In the Northern Hemisphere, the trade winds move in a southwestern direction from the northeast, while in the Southern Hemisphere, they move northwestward from the southeast. When the ITCZ is positioned north or south of the equator, these directions change according to the Coriolis effect imparted by the rotation of the earth. For instance, when the ITCZ is situated north of the equator, the southeast trade wind changes to a southwest wind as it crosses the equator. The ITCZ is formed by vertical motion largely appearing as convective activity of thunderstorms driven by solar heating, which effectively draw air in; these are the trade winds.

Its Effects

Over the oceans, where the convergence zone is better defined, the seasonal cycle is more subtle, as the convection is constrained by the distribution of ocean temperatures. Sometimes, a double ITCZ forms, with one located north and another south of the equator. When this occurs, a narrow ridge of high pressure forms between the two convergence zones, one of which is usually stronger than the other.

Within the ITCZ the average winds are slight, unlike the zones north and south of the equator where the trade winds feed. Early sailors named this belt of calm The Doldrums because of the inactivity and stagnation they found themselves in after days of no wind. To find oneself becalmed in this region in a hot and muggy climate could mean death in an era when wind was the only effective way to propel ships across the ocean. Even today the leisure and competitive sailors attempt to cross the zone as quickly as possible as the erratic weather and wind patterns may cause unexpected delays.

Visual Explanations


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