moesson

A moesson, or monsoon, is a seasonal reversal of winds and a corresponding pattern of heavy rainfall that affects certain tropical and subtropical regions. It is not a single weather event but a persistent climatic system linked to the differential heating of land and sea and the annual migration of the ITCZ, the intertropical convergence zone.

In the summer, intense heating of the continental interior lowers surface pressure, drawing in moist air from the ocean. This produces widespread, often heavy rainfall across the affected areas and a southwest or southwesterly wind regime. In winter, cooler air over the continent creates a high-pressure system that shifts the winds to the northeast or north, bringing a dry season as air moves from land to sea. The onset, duration, and intensity vary by region and year.

Regional forms of the moesson include the Indian monsoon, the East Asian monsoon, the West African monsoon, and the North American (Sonoran) monsoon. While monsoons deliver substantial rainfall that supports agriculture and livelihoods, they can also cause disastrous floods and landslides when rainfall is excessive or poorly timed. Variability is influenced by large-scale climate patterns such as ENSO, with El Niño often reducing rainfall in South Asia and Africa, and La Niña enhancing it.

Etymology-wise, moesson is the Dutch term for the phenomenon, derived from the same roots as the English “monsoon,” ultimately tracing to a word meaning season used in Arabic and related languages.