The Inter-Tropical Convergence Zone is the belt of low pressure near the equator where the north-east and south-east trade winds meet, air rises and heavy rain falls; it shifts north and south with the seasons, and its northward move over South Asia in summer is closely tied to the Indian monsoon.
Near the equator, intense heating makes air rise, creating a band of low pressure. Trade winds from the subtropical high-pressure belts on either side blow towards this band, and where they meet, the air is forced upward, cools and releases heavy rain. This convergence zone circles the globe and is visible in satellite images as a line of thunderstorm clouds. Sailors in the age of sail called it the doldrums because winds were often weak and ships could be becalmed for days.
The ITCZ does not stay at the equator. It follows the zone of maximum heating, moving towards the summer hemisphere, and its movement is much larger over land than over oceans. Over South Asia, the strong heating of the subcontinent and the Tibetan Plateau in summer pulls the ITCZ far north, to the Indo-Gangetic plains, where it is called the monsoon trough. In winter it retreats south of the equator. This seasonal migration explains the reversal of winds and the rainy and dry seasons of the tropics, including the Indian monsoon.
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