A monsoon is a seasonal shift in wind direction that occurs because land and ocean surfaces heat and cool at different rates, according to the U.S. National Oceanic and Atmospheric Administration's NESDIS education office. Monsoon winds "blow from cold to warm," the agency says, which is why the direction of the prevailing wind reverses between seasons rather than staying constant like a normal trade wind.
What makes land-sea heating turn into wind — and rain
In summer, land warms faster than the nearby ocean. Warm air rises over the heated continent, and air from the cooler ocean flows in to replace it, carrying moisture that condenses into the heavy rainstorms associated with a wet monsoon season, NOAA explains. In winter the pattern reverses: the ocean stays relatively warm while the land cools, so the wind — and the rain it carries — shifts offshore, leaving a dry season.
NASA's Earthdata program adds a second layer to the mechanism: the intertropical convergence zone, or ITCZ, a band near the equator where air rising from the tropics draws in air from the subtropics. As that air converges, rises and cools, it forms the cloud bands that produce monsoon downpours. Seasonal changes in sunlight intensity and the trade winds push the ITCZ north or south of the equator, which is why monsoon rain belts move with the seasons rather than sitting in one place year-round.
Not just South Asia: monsoons occur on several continents
NASA Earthdata lists monsoon systems on Asia, the Indian subcontinent, Australia, Africa, South America and North America. The North American monsoon, for example, begins in Mexico in June, reaches the southwestern United States in July and ends by mid-September, according to NOAA NESDIS; more than half of Arizona and New Mexico's annual rainfall falls during that July-to-September window.
Kerala's current monsoon warning, explained
India's southwestern state of Kerala is where interest in monsoon mechanics has spiked, and it sits at the meeting point of two separate monsoon systems in the same year. The India Meteorological Department (IMD) said conditions were favourable for the onset of the northeast monsoon over the southern peninsula within two to three days of October 12, 2026, according to The Tribune. The IMD forecast isolated heavy rainfall between October 15 and 18 over Kerala and Mahe, coastal and south interior Karnataka, Tamil Nadu, Puducherry and Karaikal, and coastal Andhra Pradesh, followed by a wet spell lasting five to six more days.
Earlier in the year, Kerala had already experienced the region's other rainy season, the southwest monsoon. That system reached Kerala on June 4, 2026 — three days after the conventional June 1 onset date and nine days later than the IMD's own May 26 forecast, according to Down To Earth.
How El Niño and La Niña reshape monsoon strength
Monsoon rainfall is not identical every year. NASA Earthdata notes that ocean conditions, including El Niño and La Niña cycles, affect how strong or weak a given monsoon season turns out, with knock-on effects for agriculture, infrastructure and public health. For the 2026 southwest monsoon, the IMD put the probability of El Niño conditions developing during the season at 92%, Down To Earth reported, and revised its seasonal rainfall forecast to 90% of the long-period average of 868.6 millimetres for June–September, with a 60% chance of deficient or below-normal rainfall.
Major monsoon systems compared| System | Typical season | Key mechanism | Notable 2026 detail |
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| Indian southwest monsoon | June–September | Land heats faster than the Indian Ocean, reversing winds | Reached Kerala on June 4, 2026, under a forecast 92% chance of El Niño (Down To Earth) |
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| Indian northeast monsoon | October–December | Retreating southwest monsoon hands off to northeasterly flow over the Bay of Bengal | IMD forecast onset over the southern peninsula within 2–3 days of October 12, 2026 (The Tribune) |
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| North American monsoon | June (Mexico) to mid-September (U.S. Southwest) | Summer land heating draws moisture from the Pacific and Gulf of California | Supplies over 50% of Arizona and New Mexico's annual rainfall (NOAA NESDIS) |
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Why it matters beyond weather forecasts
Because monsoons deliver most of a region's annual rainfall in a concentrated window, small shifts in onset date, intensity or El Niño-linked strength can affect water supply, crop planting and flood risk across tropical Asia, parts of Africa, Australia and the Americas, as described by NASA Earthdata's overview of monsoon variability.
Questions
- What exactly causes a monsoon?
- A monsoon forms because land heats up and cools down faster than the ocean beside it. The resulting pressure difference reverses the wind direction with the seasons, pulling in moist ocean air during the warm half of the year, according to NOAA NESDIS.
- Why does Kerala have two separate monsoon periods?
- Kerala sits where two different systems arrive in the same year: the southwest monsoon, which reached it on June 4, 2026, and the northeast monsoon, whose onset over the southern peninsula the IMD forecast for mid-October 2026, per Down To Earth and The Tribune.
- What is the ITCZ and how does it relate to monsoons?
- The intertropical convergence zone is a band near the equator where tropical and subtropical air converge, rise and cool, producing the cloud bands behind monsoon rainstorms. It shifts north or south of the equator with the seasons, according to NASA Earthdata.
- How does El Niño affect monsoon rainfall?
- NASA Earthdata notes that ocean conditions such as El Niño and La Niña cycles affect how strong or weak a monsoon season turns out. For 2026, the IMD put the odds of El Niño developing during the southwest monsoon at 92% and lowered its seasonal rainfall forecast to 90% of the long-period average, with a 60% chance of deficient rain, Down To Earth reported.
- Does the North American monsoon work the same way as the Indian monsoon?
- Yes, the same land-ocean heating principle applies: the North American monsoon forms when summer heating over Mexico and the U.S. Southwest draws moist air from the Pacific Ocean and Gulf of California, supplying more than half of Arizona and New Mexico's annual rain between July and mid-September, per NOAA NESDIS.
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