The Intertropical Convergence Zone

The Intertropical Convergence Zone, universally abbreviated ITCZ, is the band encircling the globe near the equator where the northeast trade winds of the Northern Hemisphere and the southeast trade winds of the Southern Hemisphere converge. As these two air masses meet, the warm, moisture-laden air has nowhere to go but up, producing a persistent zone of low surface pressure, intense convection, and towering cumulonimbus clouds that generates some of the heaviest and most reliable rainfall on Earth. The physical mechanism was first correctly explained in general terms by the English astronomer and meteorologist George Hadley in a 1735 paper, "Concerning the Cause of the General Trade-Winds," which described the rising, poleward-flowing, and sinking limbs of what is now called the Hadley cell — the ITCZ marks the rising branch of this circulation in both hemispheres simultaneously. Long before the physics was understood, mariners crossing this belt under sail knew it simply as the doldrums, a region notorious for calms, light and variable winds, and sudden violent squalls that could leave becalmed ships drifting for days or weeks, a hazard vividly recorded in ships' logs throughout the age of sail and later immortalized in Samuel Taylor Coleridge's The Rime of the Ancient Mariner (1798).