Chapter 4 Climate

Our daily experiences highlight variations in weather conditions throughout the year. We wear lighter clothes in summer and warmer ones in winter (especially in North India). This seasonal change in clothing and comfort is a direct result of changes in the elements of weather, such as temperature, pressure, wind, humidity, and precipitation.

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**Weather** describes the state of the atmosphere at a specific moment in time and place. It can change rapidly, even within a day or a week. **Climate**, on the other hand, refers to the average weather conditions over a much longer period, typically 30 years or more. Climate changes are usually gradual and may take decades or even centuries to become noticeable.

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India experiences a **monsoon climate**, which is characteristic of many parts of South and Southeast Asia. The term "monsoon" refers to a seasonal reversal in the direction of prevailing winds. This seasonal wind shift brings distinct wet and dry seasons.

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Unity And Diversity In The Monsoon Climate

While the monsoon climate creates a broad sense of unity in weather patterns across India and other South/Southeast Asian countries, it's crucial to recognize the significant **regional variations** within India itself. These variations are expressed in the patterns of temperature, winds, rainfall, the timing of seasons, and the degree of wetness or dryness in different parts of the country. Despite all regions falling under the general "monsoon climate" umbrella, the specific weather and climate experienced in the south (e.g., Kerala, Tamil Nadu) can be quite different from the north (e.g., Uttar Pradesh, Bihar).

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These regional differences lead to various sub-types of the monsoon climate within India. Examples of temperature variations across India are striking:

• In summer, western Rajasthan can experience extreme heat, with temperatures reaching $55^\circ\text{C}$, while temperatures around Leh in winter can drop to $-45^\circ\text{C}$.
• On a single June day, Churu (Rajasthan) might record over $50^\circ\text{C}$, while Tawang (Arunachal Pradesh) on the same day might reach only $19^\circ\text{C}$.
• On a December night, Drass (Ladakh) can be as cold as $-45^\circ\text{C}$, while Thiruvananthapuram or Chennai in South India are a mild $20^\circ-22^\circ\text{C}$.

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These examples clearly show large seasonal temperature variations from place to place and region to region. Even within a single day, temperature differences can be significant, especially in certain areas. In coastal regions like Kerala or the Andaman Islands, the difference between day and night temperatures (diurnal range) is small, perhaps only $7^\circ-8^\circ\text{C}$. However, in the Thar Desert, intense daytime heating ($50^\circ\text{C}$) can be followed by rapid cooling at night, with temperatures dropping substantially ($15^\circ-20^\circ\text{C}$), resulting in a large diurnal range.

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Regional variations are equally pronounced in precipitation patterns:

• Snowfall is limited to the Himalayan regions, while the rest of the country receives rainfall.
• The amount and type of precipitation vary dramatically. Cherrapunji and Mawsynram (Meghalaya) receive over 1,080 cm of rainfall annually, among the highest in the world, while Jaisalmer (Rajasthan) gets less than 9 cm per year.
• A single day's rain in Tura (Garo Hills, Meghalaya) can equal Jaisalmer's rainfall for 10 years.
• Annual precipitation is less than 10 cm in the northwest Himalayas and western deserts but exceeds 400 cm in Meghalaya.

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Rainfall distribution is also seasonal and regional. Coastal plains like the Ganga delta and Odisha are frequently affected by rain-bearing storms in July and August. However, the Coromandal coast (Tamil Nadu), much further south, often remains dry during these months, receiving most of its rainfall during the winter season (October-November) from the northeast monsoon.

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Despite these stark differences, the overall climate of India is characterized by the unifying rhythm and nature of the monsoon, which dictates the broad seasonal patterns of wetness and dryness across the country.

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Factors Determining The Climate Of India

India's climate is influenced by a combination of geographical and atmospheric factors:

Latitude

The **Tropic of Cancer** ($23.5^\circ$ North latitude) passes roughly through the middle of India. This divides the country into two broad climatic zones:

• The part of India south of the Tropic of Cancer lies in the **tropical zone**, closer to the equator. This region generally experiences consistently high temperatures throughout the year with relatively small daily and annual temperature ranges.
• The part of India north of the Tropic of Cancer lies in the **sub-tropical** or warm temperate zone. Being farther from the equator, this region experiences a more extreme climate, with greater seasonal variations in temperature (high daily and annual ranges).

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The Himalayan Mountains

The towering **Himalayan mountain ranges** in the north act as a crucial **climatic barrier**. They protect the Indian subcontinent from the extremely cold winds that originate near the Arctic Circle and blow across Central and East Asia during winter. Without the Himalayas, northern India would experience much colder winters. The Himalayas also play a vital role in trapping the moisture-laden monsoon winds from the Arabian Sea and the Bay of Bengal, forcing them to rise and shed their precipitation within the subcontinent.

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Distribution Of Land And Water

India's location, with the Indian Ocean surrounding its peninsular part on three sides and the continuous Himalayan mountain wall in the north, creates a significant contrast in heating and cooling rates between the landmass and the surrounding ocean. Land heats up and cools down much faster and to a greater extent than water. This differential heating creates varying atmospheric pressure zones over the land and sea in different seasons. These pressure differences are the driving force behind the seasonal reversal of winds that characterizes the monsoon climate.

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Distance From The Sea

Areas located along India's long coastline experience an **equable or maritime climate**, meaning they have a moderate temperature range throughout the year due to the moderating influence of the sea (water heats and cools slowly). Places in the interior of India, far from the coast, experience a **continental climate** with greater temperature extremes (hot summers and cold winters). For example, coastal cities like Mumbai experience little seasonal variation, while interior cities like Delhi, Kanpur, or Amritsar have pronounced seasonal contrasts in temperature and weather.

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Altitude

Temperature generally decreases with increasing height above sea level (Normal Lapse Rate). Therefore, places situated at higher altitudes in mountainous regions are cooler than places on the plains at the same latitude. For instance, Agra and Darjiling are located at roughly the same latitude, but Darjiling, being a hill station at a much higher altitude, is significantly cooler in January ($4^\circ\text{C}$) compared to Agra ($16^\circ\text{C}$) in the plains.

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Relief

The physiography or physical features of the land, particularly mountain ranges and plateaus, significantly influence temperature, pressure, wind patterns, and rainfall distribution. For example, the Western Ghats, running parallel to the west coast, act as a barrier to the southwest monsoon winds. The **windward side** (western slopes and coastal plain) receives very high rainfall, while the **leeward side** (eastern slopes and the Deccan Plateau) falls in a rain-shadow zone and receives much less rainfall.

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The Nature Of Indian Monsoon

Despite being a familiar phenomenon, the exact mechanisms and variations of the Indian monsoon have been a subject of ongoing scientific study and debate for centuries. While early theories focused on the differential heating of land and sea, recent research studying the monsoon at a global scale has led to a better understanding of its complexities.

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Key aspects of the monsoon that scientists study include its onset and variations.

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Onset Of The Monsoon

The traditional explanation for the monsoon onset involves the intensified heating of the large landmass of the Indian subcontinent during April and May, as the sun's apparent movement brings it vertically over the Tropic of Cancer. This intense heating creates a strong **low-pressure area** over northwestern India. Meanwhile, the Indian Ocean to the south heats up more slowly, maintaining relatively higher pressure.

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This pressure gradient attracts the **southeast Trade Winds** from the Southern Hemisphere. These winds cross the equator, where they are deflected towards the Indian subcontinent (southwest to northeast) due to the **Coriolis force**. These deflected winds become the **southwest monsoon**. The point where the trade winds converge and air rises is the **Inter Tropical Convergence Zone (ITCZ)**.

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In July, the ITCZ shifts northward, positioning itself over the Gangetic plain (around $20^\circ\text{N}-25^\circ\text{N}$ latitude). This northward shift of the ITCZ, often referred to as the **monsoon trough**, intensifies the thermal low pressure over north and northwest India, further attracting the monsoon winds. The southwest monsoon winds cross the equator typically between $40^\circ\text{E}$ and $60^\circ\text{E}$ longitudes.

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The onset of the monsoon is also linked to changes in upper-air atmospheric circulation, particularly the behavior of jet streams. The withdrawal of the westerly jet stream from its position south of the Himalayas over the North Indian Plain around late spring/early summer is thought to be connected to the northward shift of the ITCZ. The subsequent establishment of the easterly jet stream along $15^\circ\text{N}$ latitude is considered responsible for the sudden and often dramatic arrival, or "burst," of the monsoon in India.

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The **Entry of Monsoon into India** occurs progressively. The southwest monsoon typically first arrives over the Kerala coast around **June 1st**. It then advances rapidly northward, reaching Mumbai and Kolkata between June 10th and 13th. By mid-July, the southwest monsoon usually covers the entire Indian subcontinent, bringing widespread rainfall (Figure 4.2). (This refers to Figure 4.2).

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Break In The Monsoon

During the southwest monsoon season, after periods of rainfall, there can be intervals of one or more weeks with little or no rain. These dry spells are known as **breaks in the monsoon**. Breaks are common and occur for different reasons in various regions:

• In **northern India**, breaks may occur when the monsoon trough (ITCZ) shifts northward towards the foothills of the Himalayas, reducing rainfall over the plains, or if the rain-bearing low-pressure systems moving along the trough are infrequent.
• Over the **west coast**, dry spells are sometimes associated with days when the monsoon winds blow parallel to the coast rather than perpendicular to the Western Ghats, which reduces orographic uplift and rainfall.

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Understanding variations like breaks is part of ongoing research into monsoon dynamics, which includes studying phenomena like **El Niño**. El Niño is a complex climate pattern involving warming of sea surface temperatures in the equatorial Pacific Ocean, which can influence weather patterns globally and is associated with weaker monsoon rainfall in India in some years.

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Answer:

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The Indian Meteorological Department (IMD) uses a complex system of 16 indicators, which include both oceanic and atmospheric parameters (like sea surface temperatures in the Pacific and Indian Oceans, pressure differences, snow cover, etc.), to forecast the behavior of the monsoon in India.

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The Rhythm Of Seasons

India experiences a distinct annual cycle of seasons, influenced by the monsoon system and latitudinal variations. According to the Indian Meteorological Department (IMD), there are four main seasons:

1. The Cold Weather Season (Winter)
2. The Hot Weather Season (Summer)
3. The Southwest Monsoon Season (Rainy Season)
4. The Season of Retreating Monsoon

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The Cold Weather Season

The **cold weather season**, or **winter**, typically begins in northern India by mid-November. December and January are the coldest months. The mean daily temperature in most of northern India during this period remains below $21^\circ\text{C}$. Night temperatures can drop significantly, sometimes falling below freezing point, especially in Punjab and Rajasthan.

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Several factors contribute to the intense cold in North India during winter:

• **Continentality:** States in northwestern India (Punjab, Haryana, Rajasthan) are far from the moderating influence of the sea, experiencing a continental climate with colder winters.
• **Snowfall in the Himalayas:** Snowfall in the nearby Himalayan ranges leads to very cold conditions in the mountains and sends cold wave conditions downwards into the plains.
• **Cold Winds:** Around February, cold winds originating from regions like the Caspian Sea and Turkmenistan can reach northwestern India, bringing cold waves, frost, and fog.

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In contrast, the **Peninsular region** in South India does not have a well-defined cold weather season. Temperatures remain relatively high throughout the year due to the moderating effect of the surrounding oceans and proximity to the equator. Coastal areas experience minimal seasonal temperature changes. For example, the mean maximum temperature in January at Thiruvananthapuram might be $21^\circ\text{C}$, rising only to $29.5^\circ\text{C}$ in June. Temperatures are lower in the hills of the Western Ghats due to altitude.

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Pressure and Winds during Winter: By the end of December (around Dec 22nd), the sun is directly overhead the Tropic of Capricorn in the Southern Hemisphere. This leads to lower temperatures and higher pressure over the Northern Indian plains. A feeble high-pressure area develops here. In South India, air pressure is slightly lower. Winds generally blow outwards from the northwestern high-pressure zone towards the lower pressure areas over the Indian Ocean in the south. Due to the Coriolis force and topography, these winds are typically westerly or northwesterly in the Ganga Valley, becoming northerly in the Ganga-Brahmaputra delta, and clearly northeasterly over the Bay of Bengal.

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Generally, the weather in winter is pleasant with clear skies. However, the weather can be disturbed by the arrival of **western disturbances**. These are shallow cyclonic depressions that originate over the Mediterranean Sea and travel eastward across West Asia, Iran, Afghanistan, and Pakistan before reaching northwestern India. They pick up moisture from the Caspian Sea and the Persian Gulf along their path. Western disturbances are guided by the westerly jet stream, which flows across northern India during winter.

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Rainfall during Winter: Winter monsoon winds (northeast monsoon in the Bay of Bengal) generally blow from land to sea, so they are dry and do not cause widespread rainfall in most parts of India. Anticyclonic circulation over land further reduces the possibility of rain. However, there are some exceptions:

• **Northwestern India:** Weak temperate cyclones (western disturbances) bring some rainfall to Punjab, Haryana, Delhi, and western Uttar Pradesh. Although the amount is small (e.g., average 53 mm in Delhi), it is highly beneficial for the winter crops (rabi crops) like wheat. In the lower Himalayas, this precipitation falls as snowfall, which is crucial for maintaining water flow in the Himalayan rivers during the dry summer months. The amount of winter precipitation from western disturbances decreases eastward in the plains and southward in the mountains.
• **Central India and Northern Peninsula:** Occasional winter rainfall also occurs in these regions.
• **Northeastern India:** Arunachal Pradesh and Assam receive some rainfall (25-50 mm) during winter.
• **Tamil Nadu and Southeast Peninsula:** The northeast monsoon winds, after crossing the Bay of Bengal, pick up moisture and cause significant rainfall over the Tamil Nadu coast, southern Andhra Pradesh, and parts of southeast Karnataka and Kerala during October and November. This is the main rainy season for the Coromandal coast.

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The Hot Weather Season

The **hot weather season**, or **summer**, in North India lasts from March to June. With the sun's apparent northward movement towards the Tropic of Cancer, temperatures begin to rise across North India in March. April, May, and June are the peak summer months.

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Temperature during Summer: Temperatures are high throughout North India, generally between $30^\circ-32^\circ\text{C}$. In March, the heat belt is in the Deccan Plateau (around $38^\circ\text{C}$ maximum daily temp). By April, it shifts northward, with temperatures reaching $38^\circ-43^\circ\text{C}$ in Gujarat and Madhya Pradesh. In May and June, the heat intensifies in the northwestern part of India, with temperatures often exceeding $48^\circ\text{C}$.

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The summer heat is less intense in South India compared to North India. The peninsular location and the moderating influence of the oceans keep temperatures lower, typically between $26^\circ\text{C}$ and $32^\circ\text{C}$. Due to altitude, temperatures in the hills of the Western Ghats remain below $25^\circ\text{C}$. Along the coastal regions, isotherms (lines of equal temperature) run parallel to the coast, confirming that temperature does not decrease significantly from north to south but rather increases inland (continentality). The mean daily minimum temperature in South India remains quite high, rarely dropping below $26^\circ\text{C}$ during summer.

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Pressure and Winds during Summer: The summer months are characterized by increasing heat and falling air pressure in the northern half of the country. By July, the intense heating causes the **ITCZ** to move northward, positioning itself as an elongated low-pressure trough extending from the Thar Desert in the northwest to the Chotanagpur plateau in the east. This monsoon trough is a major feature of the summer pressure pattern.

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The low pressure in the north attracts surface winds. Over the west coast and parts of West Bengal/Bangladesh coast, winds blow from the southwest. Over north Bengal and Bihar, they are easterly or southeasterly. These surface winds are essentially the 'displaced' equatorial easterlies from the Southern Hemisphere, crossing the equator and getting deflected by the Coriolis force to become southwesterlies. Their influx by mid-June marks the transition to the rainy season (southwest monsoon).

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Within the core of the ITCZ in the northwest, strong, hot, and dry winds locally known as **'Loo'** blow during the afternoon and evening. These oppressive winds are a characteristic feature of the summer heatwave. Dust storms are common in May in northwestern India, providing temporary relief from the heat as they bring light rain and cool breezes. Sometimes, the convergence of these dry, hot winds with moisture-laden winds from the periphery of the low-pressure trough can trigger intense local storms with violent winds, heavy rain, and hailstorms.

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Some famous local storms occurring in the hot weather season:

• **Mango Showers:** Pre-monsoon showers towards the end of summer, common in Kerala and coastal Karnataka, beneficial for mango ripening.
• **Blossom Shower:** Pre-monsoon showers in Kerala and nearby areas that help coffee flowers bloom.
• **Nor'westers (Kalbaisakhi):** Severe evening thunderstorms in West Bengal and Assam, known for their suddenness and intensity. They are locally called 'Kalbaisakhi' (calamity of the month of Baisakh in Bengal) or "Bardoisila" in Assam. These storms are useful for tea, jute, and rice crops.
• **Loo:** Hot, dry, and strong winds blowing across the North Indian plains (Punjab to Bihar) primarily in the afternoon and evening during the peak summer months.

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The Southwest Monsoon Season

The **southwest monsoon season** is the main rainy season for most of India, lasting from June to September. It is triggered by the further intensification of the low-pressure system over the northwestern plains by early June. This strong low pressure attracts the trade winds from the Southern Hemisphere, which cross the equator and turn southwestward due to the Coriolis force. These winds pass over the warm equatorial ocean, picking up vast amounts of moisture, and arrive at the Indian subcontinent as the moisture-laden **southwest monsoons**.

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The arrival of the southwest monsoon is often sudden and dramatic, marked by a significant drop in temperature and associated with violent thunder and lightning. This sudden onset is referred to as the "**burst of the monsoons**." The monsoon typically bursts over the coastal areas of Kerala, Karnataka, Goa, and Maharashtra in the first week of June and gradually advances inland, reaching other parts of the country by early July. The arrival of the monsoon brings a substantial decline in temperature ($5^\circ-8^\circ\text{C}$) between mid-June and mid-July.

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As the southwest monsoon winds approach the Indian landmass, their direction is modified by the country's relief features and the low-pressure system over the northwest. The monsoon stream divides into two main branches upon reaching the subcontinent:

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

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Monsoon Winds of the Arabian Sea: This branch originates over the Arabian Sea and splits into three sub-branches:

1. One branch strikes the **Western Ghats**. As these winds are forced to climb the steep slopes of the Ghats (orographic uplift), they cool rapidly, leading to intense condensation and very heavy rainfall (250-400 cm) on the windward side (western slopes and coastal plain - e.g., Kozhikode, Mangalore). After crossing the Ghats, the winds descend on the leeward side, warm up adiabatically, and lose moisture, resulting in significantly less rainfall. This region east of the Western Ghats is a pronounced **rain-shadow area** (e.g., Pune, Bengaluru receive much less rainfall).
2. Another branch moves northward parallel to the west coast, striking the coast north of Mumbai. It then enters central India through the Narmada and Tapi river valleys, causing rainfall in extensive areas. A part of this branch contributes about 15 cm of rain to the Chotanagpur plateau. These winds then continue eastward into the Ganga plains and merge with the Bay of Bengal branch.
3. A third branch strikes the Saurashtra Peninsula and the Kachchh region (Gujarat). It then passes over west Rajasthan and along the Aravalli range. However, since the Aravallis are parallel to the wind direction, they do not cause significant orographic uplift, resulting in only **scanty rainfall** in this region. This branch then continues into Punjab and Haryana, where it also joins the Bay of Bengal branch, providing reinforcement for rainfall in the western Himalayas.

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Monsoon Winds of the Bay of Bengal: This branch originates over the Bay of Bengal and initially strikes the coasts of Myanmar and parts of Southeast Bangladesh. However, the Arakan Hills along the coast of Myanmar deflect a large portion of this branch westward towards the Indian subcontinent. The monsoon thus enters West Bengal and Bangladesh from the south and southeast.

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From this point, the Bay of Bengal branch splits into two under the influence of the Himalayas and the thermal low pressure over northwestern India:

• One branch moves westward along the **Ganga plains**, bringing rainfall all the way to the Punjab plains.
• The other branch moves northward up the **Brahmaputra valley** in the north and northeast, causing widespread and heavy rainfall. A sub-branch of this moves into the Garo and Khasi hills of Meghalaya. Mawsynram, located on the crest of the Khasi hills on the windward side, receives the highest average annual rainfall in the world due to the funneling effect of the topography and the intense moisture content of this monsoon branch.

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It's important to understand why the **Tamil Nadu coast remains largely dry** during the southwest monsoon season. Two main factors contribute to this:

• It is situated **parallel** to the direction of the Bay of Bengal branch of the southwest monsoon winds, meaning these winds do not strike the coast directly to cause significant orographic uplift or convergence.
• It lies in the **rain-shadow area** of the Arabian Sea branch of the southwest monsoon, as the Western Ghats block these winds from reaching the eastern side of the peninsula.

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Figure 4.3 shows the spatial distribution of rainfall during the southwest monsoon season (June-September), highlighting the areas of high rainfall (west coast, northeast) and low rainfall (rain shadow areas, western Rajasthan). (This refers to Figure 4.3).

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Season Of Retreating Monsoon

The months of **October and November** constitute the **season of retreating monsoon**. By the end of September, the low-pressure trough over the Ganga plain begins to weaken and shift southward, following the apparent southward movement of the sun. This leads to the gradual withdrawal of the southwest monsoon winds from India.

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The monsoon retreats from western Rajasthan by the first week of September, and by the end of the month, it has withdrawn from Rajasthan, Gujarat, the Western Ganga plain, and the Central Highlands. By the beginning of October, the low pressure shifts to the northern parts of the Bay of Bengal, and by early November, it moves further south over Karnataka and Tamil Nadu. By mid-December, the low-pressure center has completely moved out of the Indian Peninsula.

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The retreating monsoon season in North India is generally characterized by clear skies and a slight increase in temperature, as the cloud cover reduces. However, the ground is still moist from the monsoon rains. The combination of high temperature and residual humidity often results in rather oppressive weather conditions, commonly known as **‘October heat’**. In the latter half of October, temperatures start falling rapidly in North India as winter conditions set in. The weather in North India is dry during this season.

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In contrast, the **eastern part of the Peninsula**, particularly the Tamil Nadu coast, receives significant rainfall during the retreating monsoon season (October and November). This rain is associated with the northeast monsoon winds picking up moisture as they cross the Bay of Bengal. Furthermore, cyclonic depressions originating over the Andaman Sea frequently move westward and cross the eastern coast of the Southern Peninsula during this time. These tropical cyclones can be very destructive, particularly affecting the densely populated deltaic regions of the Godavari, Krishna, and Kaveri rivers, bringing torrential rain, strong winds, and storm surges. Such cyclonic storms are less common in the Arabian Sea during this period.

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Traditional Indian Seasons

In traditional Indian culture and based on age-old observations of weather phenomena, a year is often divided into **six two-monthly seasons**. This seasonal cycle is followed by common people in north and central India and reflects their practical understanding of the annual weather rhythm. However, this system of distinct seasons does not align perfectly with the conditions in South India, where seasonal variations are much less pronounced.

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The traditional Indian seasons (based on the Indian calendar) and their approximate equivalents in the Gregorian calendar months are:

Seasons (According to the Indian Calendar)	Months (According to the Gregorian Calendar)
Vasanta	Chaitra-Vaisakha (March-April)
Grishma	Jyaistha-Asadha (May-June)
Varsha	Sravana-Bhadra (July-August)
Sharada	Asvina-Kartika (September-October)
Hemanta	Margashirsa-Pausa (November-December)
Shishira	Magha-Phalguna (January-February)

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Distribution Of Rainfall

While India's average annual rainfall is about **125 cm**, there are significant spatial variations across the country, reflecting the influence of the monsoon system, topography, and other factors. (Figure 4.4 shows the annual rainfall distribution). (This refers to Figure 4.4).

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Major rainfall regimes based on annual precipitation:

Areas Of High Rainfall

Receive rainfall exceeding 200 cm annually. These include the west coast and the Western Ghats (windward side), as well as the sub-Himalayan areas and hills in the northeast (Meghalaya, parts of Assam). Some parts of the Khasi and Jaintia hills in Meghalaya receive exceptional rainfall, exceeding 1,000 cm annually (e.g., Mawsynram, Cherrapunji).

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Areas Of Medium Rainfall

Receive between 100 cm and 200 cm of rainfall annually. This includes the southern parts of Gujarat, eastern Tamil Nadu, the northeastern Peninsula (Odisha, Jharkhand, Bihar, eastern Madhya Pradesh), the northern Ganga plain along the sub-Himalayas, and parts of the Northeast like the Cachar Valley and Manipur.

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Areas Of Low Rainfall

Receive between 50 cm and 100 cm of rainfall annually. This zone covers western Uttar Pradesh, Delhi, Haryana, Punjab, Jammu and Kashmir, eastern Rajasthan, Gujarat, and the Deccan Plateau.

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Areas Of Inadequate Rainfall

Receive very low rainfall, below 50 cm annually. This includes parts of the Peninsular interior (especially in the rain shadow areas of Andhra Pradesh, Karnataka, and Maharashtra), Ladakh (a cold desert in the Himalayas), and most of western Rajasthan (Thar Desert). Snowfall is limited to the Himalayan region.

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Observing the rainfall map (Figure 4.4) shows a general decrease in rainfall from the west coast and northeast towards the interior and northwest. It also highlights the rain shadow areas on the leeward side of the Western Ghats and the Himalayas.

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Monsoons And The Economic Life In India

The monsoon climate is central to India's economic life, particularly its agriculture, which forms the backbone of the economy and supports a large portion of the population. The entire agricultural calendar and productivity are intimately linked to the timing, intensity, and distribution of monsoon rainfall.

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The influence of monsoons on India's economy includes:

• The monsoon is the primary driver of India's agricultural cycle, on which about 64% of the population depends for their livelihood.
• Except for the high altitudes of the Himalayas, temperatures across most of the country are suitable for crop growth year-round, but water availability dictates the farming seasons.
• Regional variations in monsoon rainfall patterns allow for the cultivation of a wide variety of crops across different regions.
• However, the variability in monsoon rainfall (unpredictable timing, amount, and spatial distribution) can lead to devastating droughts in some areas when rain fails or is insufficient, or destructive floods in others when rain is excessive or concentrated. This variability creates significant challenges for Indian agriculture.
• Agricultural prosperity is heavily reliant on the timely arrival and adequate distribution of monsoon rain. Regions lacking developed irrigation infrastructure are particularly vulnerable to monsoon failures.
• Sudden and intense monsoon bursts can cause significant soil erosion, especially in areas with insufficient vegetation cover.
• Winter rainfall in North India, brought by temperate cyclones (western disturbances), is highly beneficial for the growth of rabi crops like wheat.
• The regional climatic variations created by the monsoon are reflected in the diversity of food habits, clothing styles, and house types across different parts of India.

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Global Warming

Change is a fundamental aspect of nature, and Earth's climate has always changed over geological timescales. However, the current rate of climate change is a major global concern, significantly influenced by human activities. Besides natural drivers, large-scale industrialization and the release of pollutants into the atmosphere, particularly greenhouse gases, are accelerating this change, leading to **global warming**.

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The concept of the **"greenhouse effect"** is key to understanding global warming. The atmosphere naturally contains certain gases (greenhouse gases) that trap heat. They allow incoming shortwave solar radiation to pass through but absorb and re-emit outgoing longwave terrestrial radiation (heat radiated by the Earth). This trapping effect warms the lower atmosphere.

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Human activities, mainly the burning of fossil fuels (coal, oil, natural gas) for energy, deforestation, and certain industrial and agricultural practices, are increasing the concentration of greenhouse gases (GHGs) in the atmosphere. The primary GHGs contributing to this enhanced greenhouse effect are:

• **Carbon Dioxide ($\text{CO}_2$):** The most significant anthropogenic GHG, released in large quantities by burning fossil fuels. Its concentration in the atmosphere is increasing gradually.
• **Methane ($\text{CH}_4$):** Released from sources like livestock, waste decomposition, and natural gas production.
• **Nitrous Oxide ($\text{N}_2\text{O}$):** Released from sources like agricultural soils and industrial processes.
• **Chlorofluorocarbons (CFCs):** Synthetic chemicals, although their use is now largely phased out due to their role in ozone depletion, they are also powerful GHGs.

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These gases absorb longwave radiation more effectively than major atmospheric gases like nitrogen and oxygen, and their increasing concentrations are enhancing the greenhouse effect, leading to global warming. The concentration of these gases, particularly $\text{CO}_2$, has been increasing steadily, trapping more heat in the atmosphere.

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Observations over the past 150 years show a significant increase in the Earth's mean annual surface temperature. Projections suggest that by the year 2100, the global temperature could increase by about $2^\circ\text{C}$ or more compared to pre-industrial levels. This rise in temperature is expected to cause numerous other changes:

• **Rising Sea Levels:** Global warming causes glaciers and polar ice caps to melt, adding water to the oceans. Additionally, as ocean water warms, it expands (thermal expansion). Both factors contribute to a rise in global sea level. Current predictions estimate a rise of about 48 cm by the end of the 21st century on average. This would increase the frequency and intensity of coastal flooding and inundation, threatening low-lying areas and islands.
• **Changes in Precipitation Patterns:** Climate change is expected to alter the global water cycle, leading to shifts in rainfall distribution. Some regions may become wetter, experiencing increased flooding, while others may become drier, facing more frequent and severe droughts.
• **Increased Extreme Weather Events:** Heatwaves, heavy downpours, storms, and other extreme weather events are projected to become more frequent and intense.
• **Impacts on Ecosystems and Biodiversity:** Climate zones may shift, affecting the distribution of plant and animal species. Some ecosystems may face collapse if they cannot adapt quickly enough to the changing conditions.
• **Health Impacts:** Changes in temperature and precipitation patterns could affect the spread of insect-borne diseases like malaria.
• **Agricultural Shifts:** Altered temperature and rainfall patterns will impact crop yields and the suitability of land for agriculture, potentially requiring shifts in farming practices and crop types.

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The potential consequences of global warming, particularly for vulnerable coastal regions and ecosystems, are a serious concern for the global community. Efforts are underway at international and national levels to reduce GHG emissions and mitigate the impacts of climate change, emphasizing the need for sustainable development and lifestyle changes to protect the planet for future generations.

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Greenhouse Gases (Ghgs)

(This subheading repeats content already covered under the 'Global Warming' subheading, so no new notes are added here, following the instruction to avoid redundancy.)

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Exercises

Multiple Choice Questions

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Answer The Following Questions In About 30 Words

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Answer The Following Questions In Not More Than 125 Words

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Project/Activity

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