Chapter 4 — Climate

Weather is the momentary state of the atmosphere and can change within a day or a week. Climate is the average of weather conditions over a longer period and changes imperceptibly — it may take 50 years or more to be noticed.

Monsoon connotes the climate associated with a seasonal reversal in the direction of winds. India has a hot monsoonal climate, which is the prevalent climate in south and southeast Asia.

Latitude, the Himalayan mountains, distribution of land and water, distance from the sea, altitude, and relief. Together these factors explain why India's climate shows vast regional variations within the broad monsoon framework.

The Inter-Tropical Convergence Zone (ITCZ) is a low-pressure zone at the equator where trade winds converge and air tends to ascend. In July it shifts northward to around 20°N–25°N over the Gangetic plain, forming the monsoon trough that drives the onset of the southwest monsoon.

The southwest monsoon sets in over the Kerala coast by 1 June. It reaches Mumbai and Kolkata between 10 and 13 June, and by mid-July it engulfs the entire subcontinent.

The Arabian Sea branch and the Bay of Bengal branch. The Arabian Sea branch splits further into three sub-branches covering the Western Ghats, central India via the Narmada–Tapi valleys, and Saurashtra–Rajasthan. The Bay of Bengal branch enters West Bengal from the south/southeast after being deflected by the Arakan Hills, then splits westward along the Ganga plains and northward up the Brahmaputra valley.

The Tamil Nadu coast is situated parallel to the Bay of Bengal branch of the southwest monsoon, and it also lies in the rain-shadow area of the Arabian Sea branch. As a result, the Coromandal coast remains largely dry during June–September and receives most of its rainfall from cyclonic depressions during October–November.

During the southwest monsoon period, if rain fails to occur for one or more weeks after a few rainy days, it is called a break in the monsoon. In northern India this happens when rain-bearing storms are infrequent along the monsoon trough; on the west coast it is associated with winds blowing parallel to the coast.

During the retreating monsoon season in October–November, the southwest monsoon withdraws and skies clear. The land is still moist, and the combination of high temperature and high humidity creates oppressive conditions known as 'October heat'. In the second half of October, temperatures in northern India begin to fall rapidly.

El Niño is a complex weather system that appears once every 3–7 years, involving warm currents off the coast of Peru in the Eastern Pacific. It is used in India for forecasting long-range monsoon rainfall. In 1990–91, a wild El Niño event delayed the onset of the southwest monsoon by 5 to 12 days across most parts of the country.

Temperature decreases with height because air becomes thinner at higher altitudes. Agra and Darjiling are on the same latitude, but Agra's temperature in January is 16°C while Darjiling's is only 4°C because of its greater altitude.

Mango Shower — pre-monsoon showers in Kerala and coastal Karnataka that help ripen mangoes. Blossom Shower — helps coffee flowers bloom in Kerala. Nor Westers (Kalbaisakhi) — violent evening thunderstorms in Bengal and Assam useful for tea, jute, and rice; called 'Bardoisila' in Assam. Loo — hot, dry, oppressive winds blowing across the northern plains from Punjab to Bihar, most intense between Delhi and Patna.

Vasanta (March–April), Grishma (May–June), Varsha (July–August), Sharada (September–October), Hemanta (November–December), and Shishira (January–February). This six-season system is followed in north and central India but does not match the seasons of south India where there is little seasonal variation.

India's average annual rainfall is about 125 cm. The west coast, Western Ghats, sub-Himalayan northeast, and Meghalaya hills receive over 200 cm (Khasi and Jaintia hills exceed 1,000 cm). Western UP, Delhi, Haryana, Punjab, and parts of Rajasthan receive 50–100 cm. Ladakh and most of western Rajasthan receive below 50 cm, while the northwest Himalayas and western deserts receive less than 10 cm annually.

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