Climate of India and Monsoon

India has a tropical monsoon climate. The word monsoon comes from the Arabic mausim, meaning season, because the winds here reverse their direction with the seasons. Half the country lies south of the Tropic of Cancer, so it is warm all year, yet the Himalaya in the north, the sea on three sides and the seasonal reversal of winds together give it a rhythm that few other countries at this latitude have.

The Tropic of Cancer (23.5° N) passes through eight states: Gujarat, Rajasthan, Madhya Pradesh, Chhattisgarh, Jharkhand, West Bengal, Tripura and Mizoram. Because the country is split by it, we say India is partly tropical and partly subtropical. Yet the monsoon winds blend the two halves into one climate, which is why geographers say India has "unity in diversity" in climate as well.

The Himalaya as a climate wall. It does two jobs. In winter it stops the freezing Central Asian winds from entering the plains, so Delhi is far milder than places of the same latitude in China or the Middle East. In summer it stops the moist monsoon winds from escaping to the north and forces them to rise and rain over the plains. Without it, north India would be a desert. Note: the Himalaya does not create convection currents.

The monsoon is not just "rain". It is a giant seasonal wind system. In summer the huge Asian landmass heats up, a low-pressure area forms over north-west India, and air is sucked in from the cool Indian Ocean. The south-east trade winds of the southern hemisphere cross the equator, are turned to the right by the Coriolis force and become the south-west monsoon. That is why the monsoon is called an extension of the south-east trade winds.

Jet streams are narrow, very fast winds high in the troposphere. In winter the Subtropical Westerly Jet blows from west to east south of the Himalaya and brings western disturbances. In early June it moves to the north of the Tibetan Plateau, and a Tropical Easterly Jet (east to west, over the peninsula near 15° N) takes over. This switch is linked to the sudden onset of the monsoon, and the easterly jet also steers the tropical depressions of the monsoon season.

A good way to learn the monsoon is as a journey with a start, a middle and a retreat. It begins at the southern tip, rushes up both coasts, covers the whole country in about five weeks, stalls now and then, and finally slips away from the north-west. The dates below are the normal dates fixed by the IMD (revised in recent years), and the actual date changes a little every year.

Burst of the monsoon is the sudden arrival of the first heavy rain, often with thunder and lightning, after the hot dry weeks. Break in the monsoon is a dry spell of several days in the middle of the rainy season. It happens when the monsoon trough moves close to the Himalayan foothills, so the plains stay dry while the Himalayan foothills, north-east India and the south-east peninsula get heavy rain. Floods in the foothills and drought-like conditions in the plains can occur at the same time.

India does not have only summer, winter and rain. The IMD divides the year into four seasons, and traditional Indian literature divides it into six ritus. Each season has its own pressure pattern and its own special weather, and the examiners love to ask which storm or wind belongs to which season and which region.

Western disturbances (WD) are low-pressure systems, called extra-tropical cyclones, that start in the Mediterranean region. They are carried east across Iran, Afghanistan and Pakistan by the Subtropical Westerly Jet and reach north-west India in winter (mainly December to March). They bring rain to Punjab, Haryana, Delhi and west Uttar Pradesh, and snow to the western Himalaya. This light winter rain is a boon for the rabi crops, especially wheat. They can also bring cold waves, fog and cloudy skies.

Rain in India is like a see-saw: very heavy at some places and scanty at others, and it depends on one question, where does the moist wind get forced to rise? Wherever a mountain blocks the wind (the Western Ghats, the Khasi hills, the Himalaya), the windward side drenches and the leeward side stays dry. That one idea explains almost every rainfall question.

Why the Tamil Nadu coast is dry in June to September. It lies parallel to the Bay of Bengal branch of the south-west monsoon, so those winds skim along the coast without being forced to rise. It also sits in the rain shadow of the Western Ghats against the Arabian Sea branch. Its rain comes in October to December from the retreating and north-east monsoon. In the same way, west Rajasthan gets little rain because the Arabian Sea branch runs parallel to the Aravalli.

Climate, unlike weather, is the average pattern of a place over about 30 years or more. To study a whole country, geographers group places that share the same temperature and rainfall pattern into a climatic region. For India the most quoted scheme is the Koppen scheme, which uses letter codes. Learn how to read the code once, and every region becomes easy.

Other ways India's climate is divided. The Planning Commission of India divided the country into 15 agro-climatic regions (1989) on the basis of soil, climate and cropping pattern. The IMD also gives seasons (winter, pre-monsoon, monsoon, post-monsoon). When a question just says "climatic regions of India" and gives codes like Amw, Aw, BShw, Cwg, think Koppen.