Monsoon

A seasonal wind reversal that brings heavy rainfall to South and Southeast Asia, driven by temperature differences between land and ocean.

phenomena

What is the Monsoon?

The monsoon is a seasonal reversal of wind direction that brings dramatic changes in rainfall to South Asia, Southeast Asia, East Africa, and parts of Australia. Derived from the Arabic word mausim meaning “season,” the monsoon is the single largest seasonal weather pattern on Earth and shapes the lives of more than three billion people.

In South Asia, the monsoon delivers between 70% and 90% of the total annual rainfall to India, Pakistan, Bangladesh, Nepal and Sri Lanka. Agriculture, drinking water supplies, river systems, hydroelectric power and even regional economies all hinge on the timing and strength of the monsoon each year. A weak monsoon means drought and food insecurity; an excessive one brings flooding and landslides.

How the monsoon works

The monsoon is driven by a basic physical principle: land heats and cools faster than the ocean. From March to May, the Indian subcontinent warms intensely, with surface temperatures often exceeding 45°C in the northern plains. The hot land creates a low-pressure zone over the Indo-Gangetic plain, while the cooler Indian Ocean retains a relatively higher pressure.

This pressure differential pulls moisture-laden air from the southwest toward the continent — the southwest monsoon. The moist air is forced upward by the Western Ghats and the Himalayas, where it cools, condenses and releases enormous amounts of rain. The Inter-Tropical Convergence Zone (ITCZ) also migrates northward during this season, reinforcing the rainfall pattern.

In winter (November–February), the cycle reverses. The land cools faster than the ocean, creating a high-pressure system over Asia. Cooler, drier air flows from the northeast — the northeast monsoon — which brings rain to Tamil Nadu, southeastern Andhra Pradesh and Sri Lanka while the rest of South Asia remains dry.

The monsoon in South Asia

The Indian monsoon arrives in Kerala around 1 June each year (the official “onset”), then sweeps northward across the country over the following 30 to 40 days. By mid-July it reaches Delhi and Punjab; by early August it covers the entire subcontinent. Withdrawal begins in mid-September from Rajasthan and is complete by mid-October.

Key regional patterns:

Western Ghats and the Konkan coast receive the heaviest rain on the windward side, often above 3,000 mm per year.
Cherrapunji and Mawsynram in Meghalaya record some of the world’s highest rainfall — over 11,000 mm annually.
Northeast India — Assam, Meghalaya, Arunachal Pradesh — sees prolonged, intense rain and frequent Brahmaputra basin flooding.
Bangladesh receives heavy rain in July and August; over 70% of its annual rainfall comes during the monsoon.
Pakistan’s Sindh and Punjab see July–September monsoon rain, while Balochistan stays largely dry.
Rajasthan and the Thar Desert receive limited rainfall, mostly in July.

Measuring and forecasting the monsoon

The India Meteorological Department (IMD) issues an annual long-range forecast in April for the upcoming monsoon, expressed as a percentage of the Long Period Average (LPA) — roughly 880 mm of rainfall. Anything within 96–104% of LPA is considered “normal.”

Important monsoon indicators include:

Indian Ocean Dipole (IOD) — a positive IOD generally favours a strong Indian monsoon
El Niño / La Niña — El Niño tends to weaken the monsoon; La Niña often strengthens it
Madden-Julian Oscillation (MJO) — drives short-term variability within the season
Eurasian snow cover — heavier winter snow can delay summer monsoon onset

Monsoon and daily life

For South Asian residents, the monsoon is part of cultural identity as much as weather. Mango showers in April–May ripen fruit and prepare soil. The first heavy rain (called pehli baarish in Hindi or prothom bristi in Bengali) is celebrated with snacks like pakoras and hot tea. Schools sometimes declare rain holidays when streets flood.

But the monsoon also brings risks: urban flooding in Mumbai, Chennai, Bengaluru and Dhaka is now an almost annual event. Mosquito-borne diseases like dengue and malaria peak in the weeks after the rains. Landslides strike hill stations in Uttarakhand, Himachal Pradesh and the Western Ghats.

Travellers should plan around the monsoon: hill stations like Mussoorie and Munnar are dramatically beautiful but landslide-prone; coastal Goa is largely closed for tourism in July and August; the Western Ghats trails are slippery and dangerous.

Frequently asked questions

When does the monsoon start in India? The southwest monsoon arrives in Kerala around 1 June and reaches Delhi by 30 June or early July. Full coverage of the country is normally achieved by 15 July.

What is the difference between southwest and northeast monsoon? The southwest monsoon (June–September) brings the bulk of rain to most of India. The northeast monsoon (October–December) is much smaller and primarily affects Tamil Nadu, southern Andhra Pradesh and Sri Lanka.

Why is the monsoon so important? It supplies 70–90% of India’s annual rainfall, sustains agriculture for over 600 million farmers, fills reservoirs that power hydroelectric dams, and recharges groundwater for the dry months ahead.

Can the monsoon be predicted accurately? Long-range seasonal forecasts (issued in April) have improved significantly but still have a margin of ±4–5%. Short-range (3–7 day) forecasts from models like ECMWF and the IMD’s GFS are highly reliable for individual storms and rain spells.

Where can I see monsoon weather for my city? Check the live forecasts on Mausam Online — every city page shows hourly rain probability, daily totals, and the 7-day outlook. Major monsoon-affected cities include Mumbai, Kolkata, Chennai, Dhaka and Karachi.