Known to seafarers worldwide as the 'doldrums', the Inter-Tropical Convergence Zone, (ITCZ), is a belt around the Earth that extends about five degrees north and south of the equator.

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In this area, the prevailing trade winds of the northern hemisphere blow to the southwest. This leads them to collide with the southern hemisphere’s driving northeast trade winds.

Because of intense solar heating near the equator, the warm, moist air is forced up into the atmosphere much like a hot air balloon. With the air rising, it cools, causing persistent bands of showers and storms around the Earth’s midsection.

Then, the rising air mass subsides in what is known as the 'horse latitudes', where the air moves downward toward Earth's surface.

Because the air circulates in an upward direction, there is often little surface wind in the ITCZ. That is why sailors well know that the area can becalm sailing ships for weeks. And that’s why they call it the doldrums

NOAA explains.