4.1 Weather and Climate — A Quick Reset

Step out for a minute and ask yourself: is the air today hot or cool? Is the wind still or gusty? Is the sky clear, hazy or pouring rain? You are reading the elements of weather — temperature, pressure, wind direction and velocity, humidity and precipitation. These elements change quickly, sometimes within a single day. Climate^?, on the other hand, is the long-running average of weather conditions taken over many decades. Climate changes so slowly that the shift may only be noticeable after fifty years or more.

4.1.1 Unity in Diversity — One Climate, Many Faces

India shares a monsoon^? climate with the rest of south and south-east Asia. The word "monsoon" connotes the climate associated with a seasonal reversal in the direction of winds. Yet the climate of Kerala and Tamil Nadu in the south is quite different from that of Uttar Pradesh and Bihar in the north — even though all of them sit under the same monsoon umbrella. These regional variations show up in the patterns of wind, temperature and rainfall, in the rhythm of seasons, and in the degree of wetness or dryness. Geographers describe them as sub-types of the monsoon climate.

4.1.2 The Drama of Indian Temperatures

Consider these contrasts pulled straight from the meteorological record:

• In summer, the mercury occasionally touches 55°C in western Rajasthan; in winter it drops to about −45°C around Leh.
• Churu (Rajasthan) may register 50°C or more on a June day, while on the very same day Tawang in Arunachal Pradesh hardly touches 19°C.
• On a December night, Drass in Ladakh may sink to −45°C; on the same night Thiruvananthapuram or Chennai records a comfortable 20°C to 22°C.
• In Kerala and the Andamans, the difference between day and night temperature is hardly 7–8°C. In the Thar Desert, after a 50°C day, the night may plunge to 15–20°C.

4.1.3 The Drama of Indian Rainfall

Variations in precipitation are equally astonishing.

• Snowfall happens in the Himalayas; the rest of India only sees rain.
• Cherrapunji and Mawsynram in the Khasi Hills of Meghalaya receive over 1,080 cm of rainfall a year, while Jaisalmer in Rajasthan rarely gets more than 9 cm.
• Tura, in the Garo Hills of Meghalaya, may receive in a single day what Jaisalmer would gather in ten years.
• Annual precipitation is below 10 cm in the north-west Himalayas and the western deserts but exceeds 400 cm over Meghalaya.
• The Ganga delta and Odisha coast are battered by rain-bearing storms every third or fifth day in July and August, while the Coromandel coast, a thousand kilometres south, stays dry through these very months.
• Most of India receives rainfall during June–September; but coastal Tamil Nadu receives most of its rain at the start of the winter season.

And yet, despite these contrasts, the climate of India is monsoonal in rhythm and character.

Figure 4.A: Annual rainfall (cm) at selected Indian stations. The contrast between Mawsynram and Jaisalmer is so extreme that the second bar is barely visible.

4.2 Factors Determining the Climate of India

India's climate is controlled by a number of interacting factors — some related to its location and topography, others to atmospheric processes. Here is the full set.

4.2.1 Latitude in Detail

You already know India's latitudinal extent. The Tropic of Cancer (23½°N) passes through the central part of the country in an east–west direction. The land south of the Tropic falls in the tropical zone — close enough to the equator that solar radiation is high through the year, daily and annual ranges of temperature are small, and conditions are warm-and-muggy. North of the Tropic, the country lies in the sub-tropical and temperate zone, much further from the equator; here temperatures swing across the year, daily ranges are larger, and winters can be genuinely cold.

4.2.2 The Himalayan Shield

The lofty Himalayas, together with their north-western and north-eastern extensions, act as an invincible climatic shield. They protect the subcontinent from the cold winds that originate near the Arctic Circle and blow across central and eastern Asia in winter. They also trap the moisture-laden monsoon winds approaching from the south, forcing them to release their rainfall before they cross over to the cold deserts of Tibet and Central Asia.

4.2.3 Distribution of Land and Water

India is bordered by the Indian Ocean on three sides in the south and girdled by a high, continuous mountain wall in the north. Compared to the landmass, water heats and cools slowly. This differential heating creates different air-pressure zones in different seasons in and around the subcontinent — and a difference in air pressure is exactly what reverses the direction of the monsoon winds twice a year.

4.2.4 Distance from the Sea

Coastal areas enjoy an equable climate: the sea moderates both summer's heat and winter's chill. People in Mumbai and the Konkan coast hardly experience temperature extremes or sharp seasonal change. Inland places like Delhi, Kanpur and Amritsar are far from the sea's moderating influence; they feel the full seasonal swing — biting winter mornings and blistering summer afternoons.

4.2.5 Altitude

Temperature decreases with height because the upper air is thinner and absorbs less solar heat. The classic NCERT example: Agra and Darjiling lie on roughly the same latitude. In January, however, Agra sits at 16°C, while Darjiling shivers at 4°C. Same latitude, vastly different altitudes — vastly different climates.

4.2.6 Relief

The physiography or relief of India also shapes temperature, pressure, wind direction, wind speed and the amount and distribution of rainfall. The windward sides of the Western Ghats and Assam receive heavy rainfall during June–September because moist winds are forced to rise and cool. The southern plateau, sitting on the leeward side of the Western Ghats, stays dry — a textbook rain-shadow.

4.3 The Inter-Tropical Convergence Zone (ITCZ)

The single most important atmospheric feature that "switches on" the Indian monsoon is the Inter-Tropical Convergence Zone^? or ITCZ — a broad low-pressure belt near the equator where the trade winds from both hemispheres converge and the air rises.

This shifting low-pressure belt is the engine of the monsoon. When it sits over north India in summer, it sucks in the moisture-laden south-east trade winds across the equator and converts them into the south-west monsoon. When it retreats to the southern hemisphere in winter, the trade winds resume their easterly direction and India dries out.

4.4 The Nature of the Indian Monsoon

The monsoon is a familiar phenomenon, yet it remains one of the least-understood weather systems in the world. Despite centuries of observation, no single theory has fully explained it. A real breakthrough came when scientists began to study it on a global rather than a regional scale.

Systematic study of rainfall in South Asia has helped clarify two crucial features of the monsoon:

1. The onset of the monsoon.
2. The break in the monsoon.

4.4.1 The Classical (Differential Heating) Theory of Onset

Towards the end of the nineteenth century, geographers believed that the differential heating of land and sea during the summer months was the mechanism that "set the stage" for the monsoon winds. The reasoning runs like this:

1. During April and May the sun shines vertically over the Tropic of Cancer.
2. The large landmass north of the Indian Ocean gets intensely heated.
3. An intense low-pressure cell develops over the north-western part of the subcontinent.
4. Air pressure over the Indian Ocean to the south of the landmass remains high, because water heats up much more slowly than land.
5. The low-pressure cell over north-west India attracts the south-east trade winds across the equator.
6. This northward shift of the ITCZ converts the south-east trades — after they cross the equator and turn — into the south-west monsoon.
7. These winds cross the equator between 40°E and 60°E longitudes.

4.4.2 The Jet-Stream Connection

The classical "land–sea" theory is correct as far as it goes, but it cannot explain the sudden burst of rain that marks the arrival of the monsoon. For this we have to look upward. The shift in the position of the ITCZ is closely linked to the withdrawal of the westerly jet stream from its winter position over the north Indian plain (south of the Himalayas). The easterly jet stream sets in along about 15°N latitude only after the westerly jet has moved away. This easterly jet is held responsible for the explosive burst of the monsoon over India.

4.4.3 Entry of the Monsoon into India

The south-west monsoon ordinarily reaches the Kerala coast by 1 June. From there it sweeps swiftly northward, arriving at Mumbai and Kolkata between 10–13 June. By mid-July the south-west monsoon has engulfed the entire subcontinent.

4.4.4 Break in the Monsoon

During the south-west monsoon period, after raining for several days, sometimes the rain disappears for one or more weeks. This dry interlude is called "break in the monsoon". Such breaks are quite common during the rainy season and they have different causes in different regions:

• In northern India, breaks happen when the rain-bearing storms fail to be frequent along the monsoon trough or the ITCZ.
• On the west coast, the dry spells coincide with days when the winds blow parallel to the coast (instead of striking it head-on).

4.5 El Niño, La Niña and the Southern Oscillation

India's monsoon is not a purely Asian phenomenon — it is plugged into a planet-wide weather network. Two coupled ocean–atmosphere phenomena influence it strongly: El Niño^? and the Southern Oscillation.

The Spanish phrase El Niño means "Child Christ" because the warm current first appears around Christmas (December) — and December, remember, is summer in Peru, which lies in the Southern Hemisphere.

4.5.1 What El Niño Does

• It distorts the equatorial atmospheric circulation.
• It causes irregularities in the evaporation of sea-water.
• It reduces the amount of plankton, which in turn reduces the number of fish in the ocean.

Indian meteorologists use El Niño data for forecasting long-range monsoon rainfall. In the wild El Niño event of 1990–91, the onset of the south-west monsoon was delayed across most of the country by five to twelve days.

4.5.2 La Niña — The Opposite Phase

La Niña is the opposite phase of El Niño: cooler-than-normal sea-surface temperatures across the eastern Pacific. La Niña years are generally associated with stronger Indian monsoon rainfall, but flood-related disasters can intensify in eastern India and Bangladesh.

4.5.3 The Southern Oscillation

The intensity of the south-west monsoon is also linked to the Southern Oscillation — a see-saw of surface air pressure between the eastern and western Pacific. It is measured by the difference in pressure between Tahiti (about 20°S, 140°W in French Polynesia, eastern Pacific) and Port Darwin (12°30′S, 131°E in northern Australia, western Pacific). When pressure at Tahiti is unusually low and at Darwin unusually high, El Niño conditions prevail — the Indian monsoon tends to weaken. When the pattern reverses (high at Tahiti, low at Darwin), La Niña conditions prevail and the monsoon tends to strengthen. The Indian Meteorological Department (IMD) forecasts the behaviour of the monsoon on the basis of 16 different indicators, of which the Southern Oscillation Index is one of the most important.