The monsoons: how they form and why they matter

What the word means

In the monsoon months rivers swell, the soil gets soaked, crops grow and life flourishes, so this season lies at the heart of life in India. The word comes from the Arabic mausim, meaning season. Strictly, it names the winds that change with the seasons across a wide stretch of the Indian Ocean and the lands bordering it, from South Asia to Africa and Australia. Because the summer winds bring heavy rain, the word is commonly used for the rains rather than the winds.

How the monsoon forms

The mechanism is complex, but it rests on a simple fact: land heats up and cools down faster than the ocean. When summer starts, the huge landmass of Asia warms up and a strong low-pressure system develops over it. Air moves from where pressure is high to where it is low, so winds are pulled in from the cooler ocean, where pressure is high, towards the hot land. Laden with moisture, they drop it over the land as heavy rain once it condenses. In winter the pattern reverses: the land cools down faster than the ocean, so the land has high pressure and the warmer ocean lower pressure. The winds now blow from the land to the ocean and bring dry conditions to much of Asia.

The southwest monsoon

In India the monsoon rains usually reach the southern tip in early June and move north over several weeks, until they cover the whole subcontinent by mid-July. The progress is not smooth. The Western Ghats stand in the way like a wall: their western slopes get heavy rain, while the Deccan Plateau to the east gets less, often with breaks. Because these winds come from the southwest, this is called the summer or southwest monsoon.

The northeast monsoon

When winter comes near, the winds turn around and blow out from the land towards the ocean. These dry winds bring cold weather to south India, but the part of them that crosses the Bay of Bengal picks up some moisture and carries rain to some parts of eastern and southern India. We call this the winter or northeast monsoon.

Record rain and monsoon music

Mawsynram, in Meghalaya, gets more rain on average than any other place on Earth: about 11,000 mm a year, or 11 metres of water. The monsoon has also inspired many ragas in both Carnatic and Hindustani classical music, such as Meghamalhar and Amruthavarshini.

Reading nature’s signs

Predicting the monsoon has always mattered in India, so people watched nature closely and built up traditional knowledge, a heritage worth preserving. Fishermen on the Konkan coast expect the monsoon when fish that usually keep to deeper water start showing up at the surface. In parts of south India, people say that once the Golden Shower tree (Cassia fistula) blooms, the rains follow within 50 days. Some communities believe that crows building their nests high on tree-tops means less rain, while lower nests mean heavy rain.

When the monsoon fails

The climate shapes culture, with many festivals tied to the seasons and the harvest, and it shapes the economy too. A monsoon failure means poor rainfall in the monsoon season. Agriculture suffers, people (generally women) have to walk long distances for water, and farm labourers are likely to move to the cities. Grains, vegetables and fruit become costlier, which fuels inflation, and industries that depend on predictable weather and water also suffer.

Why it matters

The monsoon fills reservoirs, feeds crops and sets the rhythm of the year for millions. The land and sea effects behind it are explained in factors that shape climate, and the floods and landslides that heavy rain can bring in climate disasters and climate change. Answers to every question are in the NCERT solutions for Chapter 3; see all of the Chapter 3 notes.