Seasons and the monsoon

India's climate is called tropical monsoon. The India Meteorological Department (IMD) divides our year into four seasons:

The Himalayan states also have two more seasons: autumn and spring.

Traditionally, India counts six seasons (ṛtu), each about two months long (Table 3.2):

Even Hindustani classical music links certain rāgas to certain seasons.

"Where does the word 'monsoon' come from?" Sailors who came to India long ago noticed that the winds turn round with the seasons, and they used this to sail here and back. Arab traders named it from the Arabic word *mausim*, which means season. So a monsoon is a seasonal reversal of the wind.

To understand the monsoon, remember one rule from the last lesson: land heats up and cools down faster than the sea, and wind blows from high pressure to low pressure.

The south-west (summer) monsoon, June to September.

• In summer, India's land heats up faster than the oceans around it.
• The hot land makes low pressure over India; the cooler Indian Ocean has high pressure.
• So moist winds blow from the sea to the land, across the Indian Ocean, the Arabian Sea and the Bay of Bengal, and bring rain.

This monsoon gives India most of its rain for the whole year.

Look at Fig. 3.10. The monsoon usually reaches Kerala around 1 June and spreads north and west, reaching the far north-west by about 8 July. Fig. 3.11 shows it retreating: it starts leaving the north-west around mid-September and has left most of the country by mid-October.

The north-east (winter) monsoon, October to February. Now everything reverses.

• The land cools faster than the sea, so there is high pressure over the land and low pressure over the sea.
• Cold, dry winds blow from the land to the sea, so most of India gets no rain.
• But winds that cross the Bay of Bengal pick up moisture and drop it on the east coast: Tamil Nadu, Andhra Pradesh and parts of Karnataka.

"Why does it matter so much?" Most of India's farming depends on monsoon rain for sowing and growing crops. A good monsoon fills rivers, reservoirs and wells and means enough food. It shapes daily life, transport, festivals and jobs, especially in villages. But too much rain brings floods, and a weak monsoon brings drought.

India has tried to forecast the monsoon for a very long time:

• The Kṛiṣhiparāśhara used the positions of the Moon and the Sun.
• Varāhamihira's Bṛihatsaṁhitā used the nakṣhatras (lunar mansions): the sky along the Moon's path split into 27 equal parts; the Moon spends about a day in each.
• Kauṭilya's Arthaśhāstra records measurements of rainfall, used for collecting revenue and giving relief.
• Kālidāsa's Meghadūtam (around the 5th century CE) mentions when the monsoon arrives over central India and traces the path of the clouds.

Today, the National Monsoon Mission and Mission Mausam build modern models to make forecasts better.