Indian monsoon

A monsoon is a seasonal change in prevailing winds, not simply a rainstorm. In summer, strong heating over the Asian landmass contributes to low pressure and draws moist air from the Indian Ocean toward South Asia. The southwest monsoon generally brings India's main rainy season from June to September. In the cooler months, the large-scale circulation reverses; the retreating or northeast monsoon can bring important rains to parts of southeastern India, especially Tamil Nadu, after winds cross the Bay of Bengal. This broad seasonal contrast is shaped by large-scale atmospheric and oceanic processes, and a simple land-versus-sea heating description does not capture all the dynamics.

The India Meteorological Department does not declare the onset over Kerala from a single shower. Its criteria combine rainfall at a specified group of stations with atmospheric wind and outgoing-longwave-radiation conditions. The department describes the southwest monsoon as normally setting in over Kerala around 1 June, advancing northward in surges and covering the country around 15 July. These are climatological norms, not fixed dates: actual onset and progress vary from year to year.

After reaching the subcontinent, the southwest monsoon circulation is commonly described through an Arabian Sea branch and a Bay of Bengal branch. Their paths, together with topography, help produce strong regional contrasts. Moist air forced upward along mountain slopes can cool and condense, yielding heavy rain on windward sides, while some areas receive less rainfall. The monsoon trough and migrating weather systems also shape where and when rain falls. As a result, the seasonal total for India does not imply uniform rainfall in every state or every week.

Monsoon rainfall supports rain-fed agriculture, replenishes reservoirs and groundwater, and contributes to river flows and hydropower. It can also bring hazards: intense rain may cause floods and landslides, while delayed, weak or uneven rainfall can stress crops and water supplies. The onset, breaks, withdrawal and distribution matter alongside the seasonal national total. Climate patterns including El Niño can influence the monsoon, but no single factor determines the outcome of every season.

The IMD's published Kerala-onset guidelines specify rainfall thresholds across a group of stations and require supporting wind-field and satellite-derived outgoing-longwave-radiation conditions. This operational definition helps distinguish a sustained monsoon onset from isolated pre-monsoon rain. The Northern Limit of Monsoon records the farthest northward advance reached on a given date.