Monsoon

A monsoon is a recurring seasonal circulation pattern, not a single rainstorm. In many regions, prevailing winds change direction or strength between seasons, shifting moisture transport and producing a marked contrast between wetter and drier periods. Monsoon systems operate over broad areas and can contain many individual weather events. Their regional forms are not identical: prominent systems occur across South and East Asia, Africa, Australia and parts of the Americas.

A key influence is that land generally heats and cools more quickly than the ocean. These seasonal temperature differences help create pressure contrasts that alter the flow of air between sea and land. At the same time, the tropical belt of rising, moist air and cloud formation—the Intertropical Convergence Zone (ITCZ)—shifts north or south over the year. Winds carrying ocean moisture toward a region can then support a rainy season; when the circulation shifts away, conditions are often drier. The exact pattern also depends on the arrangement of land and sea, mountains, ocean conditions and larger atmospheric circulation, so land–sea heating alone is not a complete explanation.

Monsoon seasons occur at different times in the two hemispheres and vary by region. In South Asia, the southwest or summer monsoon is generally the principal rainy season. India's southwest monsoon normally spans June to September, when moisture-bearing winds arrive from the Indian Ocean; a northeast or retreating monsoon brings important rainfall to parts of southern India, especially the southeast, later in the year. Monsoon circulations also influence rainfall in places such as West Africa, northern Australia and northwestern Mexico and the southwestern United States, but their winds, timing and rain patterns differ.

Seasonal monsoon rains replenish rivers, reservoirs and groundwater and are important to farming and ecosystems in many affected regions. In India, the southwest monsoon provides about three-quarters of the country's annual rainfall, according to the Ministry of Earth Sciences' 2025 overview. Rainfall is uneven across space and varies from year to year; ocean conditions such as El Niño can influence some monsoon systems, but do not determine the outcome of a particular season on their own. Too little or delayed rain can stress crops and water supplies, while concentrated or intense rain can contribute to flooding and landslides. Meteorological agencies use observations and forecasts to help communities prepare for these changing conditions.