The seasonal reversal of winds over the Indian Ocean and southern Asia that divides the year into a dry half and a wet one — the rains on which South Asian farming, Indian Ocean trade, and East African life have depended for millennia. Because its arrival and its failure decide harvests and livelihoods, the monsoon is a prime case of a natural rhythm that becomes the scaffolding of whole societies.
Every year, the wind reverses. From May to September it blows northeast across the Indian Ocean, carrying moisture from the sea. From October to April it reverses and blows southwest. For two thousand years, merchants, farmers, and empires organized their entire existence around this calendar. The monsoon was not a backdrop to Indian Ocean civilization. It was the mechanism.