Every summer, India waits eagerly for the monsoon. The word comes from the Arabic mausim, meaning season. The southwest monsoon brings most of the country’s annual rainfall, refilling rivers, reservoirs and groundwater.

What causes the monsoon?

The monsoon is a seasonal reversal of winds driven by the difference in heating between land and sea:

• In summer, the Indian landmass heats up much faster than the Indian Ocean.
• Hot air over land rises, creating a low-pressure area.
• Cooler, moist air from the ocean, where pressure is higher, rushes in to replace it.
• As this moist air rises over land and mountains, it cools and releases heavy rain.

The Himalayas play a key role by trapping the moist winds and keeping them over the subcontinent.

Arrival and spread

The southwest monsoon usually reaches Kerala around 1 June. It then splits into two branches:

• Arabian Sea branch: brings heavy rain to the Western Ghats, Kerala, Karnataka, Goa and Mumbai.
• Bay of Bengal branch: moves into the Northeast and then turns west along the Himalayas to the Gangetic plains.

It typically covers the whole country by early to mid July and begins withdrawing from September.

Wettest and driest places

• Mawsynram and Cherrapunji in Meghalaya are among the wettest places on Earth.
• Western Rajasthan and Ladakh receive very little monsoon rain.

The northeast monsoon

From October to December, winds reverse and blow from the northeast, bringing rain to Tamil Nadu and parts of southeast India.

Why the monsoon matters

• A large share of Indian farmland depends on monsoon rain, especially for crops like rice, pulses and cotton.
• A good monsoon boosts farm incomes, food production and the wider economy.
• A weak monsoon can cause drought, while extreme rain can cause floods.

The India Meteorological Department (IMD) issues monsoon forecasts every year.