4.6 The Rhythm of Seasons

The climatic conditions of India can best be described as an annual cycle of seasons. The Indian Meteorological Department (IMD) recognises four seasons:

1. The Cold Weather Season (December–February)
2. The Hot Weather Season (March–May)
3. The Southwest Monsoon Season (June–September)
4. The Retreating Monsoon Season (October–November)

4.7 The Cold Weather Season (December–February)

4.7.1 Temperature

The cold weather season usually sets in by mid-November in northern India. December and January are the coldest months in the northern plain. The mean daily temperature stays below 21°C over most of the north. Night temperatures can be very low, sometimes dropping below freezing in Punjab and Rajasthan.

There are three main reasons for this excessive cold:

1. Punjab, Haryana and Rajasthan lie far from the moderating influence of the sea — they have a continental climate.
2. Snowfall on the nearby Himalayan ranges creates a cold-wave situation.
3. Around February, cold winds from the Caspian Sea and Turkmenistan bring frost and fog over the north-western parts of India.

The Peninsular region, in contrast, has no well-defined cold weather season. Coastal areas show hardly any change in temperature pattern across the year because of the moderating effect of the sea and proximity to the equator. Thiruvananthapuram, for instance, has a mean maximum temperature of 21°C in January and 29.5°C in June — barely 8°C apart. Temperatures in the hills of the Western Ghats remain comparatively low because of altitude.

4.7.2 Pressure and Winds

By 22 December, the sun shines vertically over the Tropic of Capricorn in the Southern Hemisphere. The weather in this season is characterised by a feeble high-pressure cell over the northern plain. Air pressure is slightly lower in south India. The isobars of 1019 mb and 1013 mb pass through north-west India and the far south respectively. As a result, winds blow outward from the north-western high-pressure zone to the low-pressure zone over the Indian Ocean.

Because the pressure gradient is gentle, the winds are light — about 3–5 km per hour. Topography modifies their 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 where the land no longer constrains them.

During winter, the weather over India is usually pleasant. But this calm gets disturbed by shallow cyclonic depressions^? that originate over the eastern Mediterranean Sea and travel eastwards across West Asia, Iran, Afghanistan and Pakistan before reaching north-west India. On the way, they pick up extra moisture from the Caspian Sea (north) and the Persian Gulf (south). The westerly jet stream is the steering current that delivers them.

4.7.3 Rainfall

Winter monsoons do not generally cause rainfall, because they blow from land to sea, carry little humidity, and are part of an anti-cyclonic circulation. Yet there are notable exceptions:

• North-west India: weak temperate cyclones from the Mediterranean cause rainfall in Punjab, Haryana, Delhi and western Uttar Pradesh. The amount is meagre but extremely valuable for the rabi crops. Precipitation falls as snow in the lower Himalayas — and that snow sustains river flow during the dry summer months. The amount of winter rainfall decreases from west to east in the plains and from north to south in the mountains. Average winter rainfall at Delhi is about 53 mm; in Punjab and Bihar it is between 25 mm and 18 mm.
• Central India and northern peninsular south get occasional winter rainfall.
• Arunachal Pradesh and Assam receive 25–50 mm in the winter months.
• October–November: the north-east monsoon, while crossing the Bay of Bengal, picks up moisture and dumps torrential rain on the Tamil Nadu coast, southern Andhra Pradesh, south-east Karnataka and south-east Kerala.

4.8 The Hot Weather Season (March–May)

4.8.1 Temperature

With the apparent northward movement of the sun towards the Tropic of Cancer in March, temperatures begin to climb in the north. April, May and June are the months of summer in north India. In most parts of the country, temperatures lie between 30–32°C.

• In March the highest day temperature of about 38°C occurs on the Deccan Plateau.
• In April, temperatures of 38–43°C are common in Gujarat and Madhya Pradesh.
• In May, the heat belt moves further north; in north-west India, temperatures around 48°C are not uncommon.

The hot weather season in south India is mild compared to the north. The peninsular situation, surrounded by oceans, keeps temperatures in the range 26–32°C. In the hills of the Western Ghats, altitude keeps temperatures below 25°C. In coastal regions, the north–south orientation of isotherms parallel to the coast confirms that temperature does not decrease from north to south but rather increases from the coast inland. The mean daily minimum temperature in summer rarely goes below 26°C.

4.8.2 Pressure and Winds

Summer is a period of excessive heat and falling air pressure in the northern half of the country. The intense heating of the subcontinent makes the ITCZ migrate north, settling at about 25°N in July. This elongated low-pressure trough — the monsoon trough — runs from the Thar Desert in the north-west to Patna and the Chotanagpur plateau in the east-southeast.

The location of the ITCZ pulls in surface winds: south-westerly on the west coast and along the West-Bengal–Bangladesh coast; easterly or south-easterly over north Bengal and Bihar. These south-westerlies are in fact the displaced equatorial easterlies we met in Part 1. Their arrival by mid-June flips the country into the rainy season.

In the heart of the ITCZ in the north-west, dry hot winds called Loo^? blow in the afternoon and often continue well into midnight. Dust storms in the evening are very common during May in Punjab, Haryana, eastern Rajasthan and Uttar Pradesh. They are short-lived but bring welcome relief from the heat — light rain, cool breeze, and sometimes hail. Occasionally, moisture-laden winds get drawn into the trough's periphery; the sudden contact between dry and moist air masses produces local storms of great intensity, with violent winds, torrential rain and even hailstorms.

4.8.3 Famous Local Storms of the Hot Weather Season

4.9 The Southwest Monsoon Season (June–September)

4.9.1 The Burst of the Monsoon

The rapid rise of temperature in May over the north-western plains intensifies the low pressure even further. By early June, it is powerful enough to attract the trade winds of the Southern Hemisphere coming from the Indian Ocean. These south-east trades cross the equator, enter the Bay of Bengal and the Arabian Sea, get caught in the air circulation over India, and after passing over the warm equatorial ocean carry abundant moisture. After crossing the equator they curve into a south-westerly flow — hence the name south-west monsoon.

The arrival of the rain is abrupt. The first downpour drops temperatures sharply. This sudden onset of moisture-laden winds, accompanied by violent thunder and lightning, is called the "break" or "burst" of the monsoon. The monsoon may burst in the first week of June over the coastal areas of Kerala, Karnataka, Goa and Maharashtra, while in the country's interior it may be delayed to the first week of July. Day temperatures decline by 5°C to 8°C between mid-June and mid-July.

As these winds approach the land, their south-westerly direction is modified by relief and by the thermal low over north-west India. The monsoon then approaches the landmass in two branches:

1. The Arabian Sea branch
2. The Bay of Bengal branch

4.9.2 The Arabian Sea Branch — Three Sub-Branches

The monsoon winds originating over the Arabian Sea further split into three sub-branches:

4.9.3 The Bay of Bengal Branch

The Bay of Bengal branch first strikes the coast of Myanmar and part of south-east Bangladesh. But the Arakan Hills along the Myanmar coast deflect a big portion of this branch towards the Indian subcontinent. The monsoon therefore enters West Bengal and Bangladesh from the south and south-east instead of the south-west.

From here this branch splits into two under the joint influence of the Himalayas and the thermal low of north-west India:

• One branch moves westward along the Ganga plains, reaching as far as the Punjab plains.
• The other moves up the Brahmaputra valley in the north and north-east, causing widespread rains; its sub-branch strikes the Garo and Khasi hills of Meghalaya. Mawsynram, located on the crest of the Khasi hills, receives the highest average annual rainfall in the world.

4.9.4 Why the Tamil Nadu Coast Stays Dry

Two factors explain why the Tamil Nadu coast remains dry during the south-west monsoon:

1. The Tamil Nadu coast lies parallel to the Bay of Bengal branch — winds skim along the coast instead of striking it.
2. It lies in the rain-shadow area of the Arabian Sea branch.

4.10 Season of Retreating Monsoon (October–November)

The months of October and November are known as the retreating monsoon. By the end of September, the south-west monsoon weakens as the low-pressure trough of the Ganga plain begins to move south in response to the southward march of the sun.

• The monsoon retreats from the western Rajasthan by the first week of September.
• By the end of the month it has withdrawn from Rajasthan, Gujarat, Western Ganga plain and Central Highlands.
• By the beginning of October, the low pressure covers the northern parts of the Bay of Bengal.
• By early November, it moves over Karnataka and Tamil Nadu.
• By the middle of December, the low-pressure centre is completely removed from the Peninsula.

The retreating south-west monsoon is marked by clear skies and rising temperatures. The land is still moist; high temperature plus high humidity makes the weather oppressive — this is the famous "October heat". In the second half of October, the mercury begins to fall rapidly, especially in north India.

The retreating monsoon weather is dry in north India but wet in the eastern peninsula. October and November are the rainiest months of the year for the Coromandel coast. The widespread rain of this season is associated with the passage of cyclonic depressions originating over the Andaman Sea and crossing the eastern coast of the southern peninsula. These tropical cyclones are very destructive. The thickly populated deltas of the Godavari, Krishna and Kaveri are their preferred targets — every year cyclones bring disaster here. A few cyclonic storms also strike the coasts of West Bengal, Bangladesh and Myanmar. Such storms are less frequent in the Arabian Sea.

4.11 Traditional Indian Seasons

In the Indian tradition, a year is divided into six two-monthly seasons. This cycle, followed by common people in north and central India, is based on practical experience and age-old observation. It does not match the seasons of south India, where seasonal variation is small.

4.12 Distribution of Rainfall

The average annual rainfall in India is about 125 cm, but the spatial variation is enormous. Geographers describe the country in four broad rainfall categories.

Snowfall is restricted to the Himalayan region.

Figure 4.B: Approximate annual rainfall (cm) at representative stations across India's rainfall belts.

4.12.1 Variability of Rainfall

India's rainfall is highly variable from year to year — and that variability is the single biggest source of insecurity in Indian agriculture. Areas of high rainfall show low variability, while areas of low rainfall show high variability. A failure of just a few centimetres of expected rainfall in Rajasthan or Marathwada is a drought; the same shortfall in Cherrapunji is barely noticed.

4.13 Climatic Regions of India — The Köppen System

The German climatologist Wladimir Köppen^? proposed a world-famous scheme of climatic classification using letter codes. The first capital letter denotes the broad climatic group (A — tropical, B — dry, C — warm temperate, D — cold snow forest, E — polar). The second small letter describes the seasonal pattern of precipitation (w — winter dry, s — summer dry, f — no dry season, m — monsoonal). A third small letter denotes temperature severity. India contains a remarkable range of these climates.

4.14 Monsoons and the Economic Life of India

The monsoon is more than a weather pattern — it is the axis around which the entire agricultural cycle of India revolves. Its impact on the country's economy and culture is enormous.

1. The monsoon is the axis around which India's agricultural cycle turns. About 64 per cent of India's people depend on agriculture for their livelihood, and that agriculture rests on the south-west monsoon.
2. Except for the Himalayas, all parts of the country have temperatures above the threshold needed to grow crops or plants throughout the year.
3. Regional variations in monsoon climate help in growing many different types of crops.
4. Variability of rainfall produces droughts or floods every year somewhere in the country.
5. India's agricultural prosperity depends very much on timely and adequately distributed rainfall. If the monsoon fails, agriculture is hit hardest in regions where irrigation is poorly developed.
6. Sudden monsoon bursts cause soil erosion over large areas in India.
7. Winter rainfall by temperate cyclones in north India is highly beneficial for the rabi crops.
8. Regional climatic variation is reflected in the great variety of food, clothes and house types across the country.