Climate

Written and reviewed by Subhra Gupta, Humanities faculty at UniDrill (M.A. Sociology, University of Delhi · 7 years teaching Humanities). Based on NCERT Geography Class XI (India: Physical Environment), Chapter 4.

How can one country be both a furnace and a freezer?

In summer the mercury occasionally touches 55°C in western Rajasthan. In winter it drops to minus 45°C around Leh. On the same December night Drass in Ladakh may fall to minus 45°C while Thiruvananthapuram or Chennai records 20 to 22°C. Yet geographers call all of India's climate by one name: monsoonal. This chapter explains how a single climate type contains so much variety, and why the monsoon sets the rhythm of Indian life.

Weather, climate and the word "monsoon"

Weather is the momentary state of the atmosphere, and it can change within a day or a week. Climate is the average of weather over a long period, and a change in it may be noticed only after 50 years or more. The elements of both are temperature, pressure, wind direction and speed, humidity and precipitation.

Monsoon means the climate associated with a seasonal reversal in the direction of winds. India has a hot monsoon climate, which is the prevailing type in South and Southeast Asia.

Unity and diversity within the monsoon

The monsoon links India with the rest of Southeast Asia, but it hides regional contrasts. These contrasts are the examples the chapter gives, and they are favourite one-line questions.

In spite of these differences, India's climate is monsoonal in rhythm and character.

What controls India's climate

Six factors decide the climate of a place. Learn each with its example.

Pause and predict: Agra and Darjiling lie on almost the same latitude. Which has the lower January temperature, and why?

Check your answer: Darjiling, at 4°C against 16°C in Agra. The cause is altitude. Air is thinner in the mountains, so it holds less heat, and temperature falls with height.

How the monsoon works

The older idea. Towards the end of the nineteenth century, scientists believed that the differential heating of land and sea in summer was the main cause. In April and May the sun shines vertically over the Tropic of Cancer, the large landmass north of the Indian Ocean heats up and an intense low pressure forms in the north-west. The Indian Ocean to the south stays cooler and has higher pressure. The low pressure draws in the south-east trade winds across the equator, and these are then deflected to become the south-west monsoon. They cross the equator between 40°E and 60°E longitude.

The modern view. Studying the monsoon at a global rather than a regional level has given a real breakthrough, and four ideas carry the explanation.

Two views. For the exam: the monsoon is explained by the shifting ITCZ, the jet streams and the ocean-atmosphere link, studied at global level. Another reading: the older land-sea heating explanation treated the monsoon as a giant sea-breeze, where a heated land pulls in cool ocean air. The textbook itself says no single theory has explained the monsoon fully, and the Indian Meteorological Department forecasts it using 16 indicators.

El Nino and the monsoon. El Nino is a complex weather system that appears once every three to seven years and brings drought, floods and other extremes. It involves a warm current off the coast of Peru in the eastern Pacific, replacing the cold Peruvian (Humboldt) current and raising the sea temperature along the Peruvian coast by about 10°C. It distorts equatorial atmospheric circulation, makes evaporation irregular and reduces plankton and fish. The word means "Child Christ", because the current appears around Christmas. It is used in India to forecast long-range monsoon rainfall. In 1990-91 a strong El Nino delayed the south-west monsoon onset by five to twelve days over most of the country.

Onset of the monsoon. The south-west monsoon sets in over the Kerala coast by 1 June and moves rapidly to reach Mumbai and Kolkata between 10 and 13 June. By mid-July it covers the whole subcontinent. Its first rains begin abruptly with thunder and lightning, which is called the burst or break of the monsoon. The day temperature drops by 5 to 8°C between mid-June and mid-July. The burst can arrive in the first week of June in coastal Kerala, Karnataka, Goa and Maharashtra, but it can be delayed to the first week of July in interior parts.

Breaks in the monsoon. If rain fails for one or more weeks after a spell of rain in the south-west monsoon period, it is a break. Reasons differ by region. In northern India, rain fails if the rain-bearing storms are not frequent along the monsoon trough. Along the west coast, dry spells are linked to days when winds blow parallel to the coast.

The four seasons

Meteorologists recognise four seasons in India: the cold weather season, the hot weather season, the south-west monsoon season and the season of the retreating monsoon.

The cold weather season

Why north India is so cold. Three reasons are given. States like Punjab, Haryana and Rajasthan are far from the sea and have a continental climate. Snowfall in the nearby Himalayan ranges brings cold-wave conditions. Around February, cold winds from the Caspian Sea and Turkmenistan bring a cold wave with frost and fog in the north-west. Night temperatures can fall below freezing in Punjab and Rajasthan.

Peninsular India. The Peninsula has no well-defined cold season. The sea moderates the temperature and the equator is near. At Thiruvananthapuram the mean maximum is 21°C in January and 29.5°C in June. Hills of the Western Ghats stay cooler.

Pressure and winds. On about 22 December the sun is overhead at the Tropic of Capricorn. Weak high pressure covers the northern plain, and pressure is slightly lower in the south: the 1019 mb isobar passes through the north-west and the 1013 mb through the far south. Winds blow outward from the north-west high towards the low pressure over the Indian Ocean, at a low speed of 3 to 5 km per hour. Relief shapes the direction: they are westerly or north-westerly down the Ganga valley, northerly in the Ganga-Brahmaputra delta and clearly north-easterly over the Bay of Bengal. Winter winds do not give rain as they blow from land to sea, because they carry little humidity and the anti-cyclonic circulation over land reduces the chance of rain.

Rainfall in winter. There are exceptions.

Beyond the textbook. For the exam: winter rain in north-west India comes from temperate cyclones that originate over the eastern Mediterranean and travel east. In reality: meteorologists call these western disturbances, and the westerly jet stream steers them across West Asia. Why it matters: the term is used in news weather reports and in the exercise question about the role of the westerly jet, so it helps to know both names.

The hot weather season

As the sun moves north to the Tropic of Cancer in March, temperatures rise in north India. In March the highest day temperature, about 38°C, occurs on the Deccan Plateau. In April, 38 to 43°C is found in Gujarat and Madhya Pradesh. In May the heat belt moves further north, and around 48°C is not uncommon in the north-west. South India is milder, at 26 to 32°C, because of the sea. Temperatures in the Western Ghats stay below 25°C because of altitude, and the mean daily minimum in summer rarely goes below 26°C.

Pressure and winds. The ITCZ moves north and is centred at 25°N in July. This elongated low-pressure monsoon trough stretches from the Thar desert in the north-west to Patna and the Chotanagpur plateau in the east-south-east. Winds are south-westerly on the west coast and along the coast of West Bengal and Bangladesh, and easterly or south-easterly over north Bengal and Bihar.

Within this region, hot dry winds called Loo blow in the afternoon and often well into midnight. Evening dust storms are common in May in Punjab, Haryana, eastern Rajasthan and Uttar Pradesh, bringing respite with light rain and a cool breeze. When moist winds meet dry air at the edge of the trough, violent local storms with torrential rain and hail may follow.

The south-west monsoon season

In May the north-western plains heat rapidly, and by early June the low pressure is strong enough to attract the south-east trade winds of the southern hemisphere. They cross the equator, pass over warm equatorial currents and gather large amounts of moisture. After crossing the equator, they turn south-westerly and are called the south-west monsoon. The monsoon approaches India in two branches.

Why Tamil Nadu is dry in the south-west monsoon. Two reasons. The coast lies parallel to the Bay of Bengal branch, so the winds blow along it rather than onto it. It also lies in the rain shadow of the Arabian Sea branch.

Pause and predict: rain-bearing winds blow in from the sea. Why does Pune, east of the Western Ghats, get far less rain than Mangalore on the coast?

Check your answer: Mangalore is on the windward side, where air rises up the slope, cools and drops its water. After crossing the Ghats the air descends and warms, which reduces humidity, so Pune sits in the rain shadow.

The retreating monsoon

By the end of September the south-west monsoon weakens as the low-pressure trough over the Ganga plain moves south with the sun. The monsoon leaves western Rajasthan by the first week of September and withdraws from Rajasthan, Gujarat, the western Ganga plain and the Central Highlands by the end of the month. By early October the low pressure covers the north of the Bay of Bengal, by early November it is over Karnataka and Tamil Nadu, and by mid-December it has left the Peninsula.

The season brings clear skies and rising temperature. The land is still moist, and the high temperature and humidity make the weather oppressive. This is called the October heat. In the second half of October the temperature falls rapidly in the north. The weather is dry in north India but rainy in the east of the Peninsula, where October and November are the rainiest months. Cyclonic depressions that form over the Andaman Sea and cross the east coast give the rain, and the thickly populated deltas of the Godavari, Krishna and Kaveri are their favourite targets. Much of the Coromandel coast's rainfall comes this way. Such cyclones are less frequent in the Arabian Sea.

Traditional Indian seasons

In the Indian tradition a year is divided into six two-month seasons, followed by people in north and central India. They do not suit south India, where seasonal variation is small.

Where the rain falls

The average annual rainfall in India is about 125 cm, with large variation. Snowfall is limited to the Himalayan region.

The monsoon and the economy

About 64 per cent of people depend on agriculture, which rests on the south-west monsoon, so the monsoon is the axis of the agricultural cycle. Except in the Himalayas, all parts of the country have temperatures above the threshold level for growing crops through the year. Regional variations help grow many kinds of crops. Variability of rainfall brings drought or flood to some part every year, and where irrigation is poor, a failed monsoon hurts agriculture. A sudden burst causes soil erosion. Winter rain from temperate cyclones is beneficial for rabi crops. Regional climate differences show up in the variety of food, clothes and house types.

Global warming

Climate has changed in the past, and it is changing now. Besides natural causes, human activities such as industrialisation and polluting gases are responsible for global warming. Carbon dioxide from burning fossil fuels is a major concern. Methane, chlorofluorocarbons and nitrous oxide are present in much smaller amounts, but they absorb long-wave radiation better, so they are more effective at strengthening the greenhouse effect.

Exam traps

60-second revision

Where this chapter connects

• Previous: Drainage System depends on the rainfall pattern and the Himalayan snow described here.
• Next: Natural Vegetation shows how temperature and rainfall decide forest types.
• Background: Atmospheric Circulation and Weather Systems, Solar Radiation, Heat Balance and Temperature and World Climate and Climate Change explain jet streams, heating and global warming at world level.