Chapter 4 Climate

Weather is the state of the atmosphere at a particular moment in time, changing rapidly, perhaps within a day or a week. In contrast, climate refers to the average of weather conditions observed over a significantly longer period, typically 30 years or more. Climate changes are much slower and are usually noticeable only after decades or centuries.

The term "monsoon" originates from the Arabic word 'mausim', meaning season. It describes a climate characterized by a seasonal reversal in the direction of prevailing winds. India experiences a hot monsoonal climate, similar to the climate found in other parts of South and Southeast Asia.

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Koeppen’s Scheme Of Classification Of Climate

This section seems to be incorrectly placed based on the chapter flow in the source text provided. It details a global classification scheme which is more appropriate for the 'World Climate' chapter. However, I will include the content here as requested by the provided subheading structure.

The content for Koeppen's classification (Tropical Humid (A), Dry (B), Warm Temperate (C), Cold Snow Forest (D), Polar (E), Highland (H) climates, and their subtypes) aligns with the information provided in Chapter 12, which was also titled "World Climate And Climate Change". I will reproduce the relevant parts of that information here as per the user's structure, noting that this seems to be repeated or misplaced content relative to the main flow of the "Climate" chapter about India's climate.

Koeppen's classification is an empirical system based on observed temperature and precipitation data, linking climate types to vegetation distribution. It uses capital letters for main groups and small letters for subtypes indicating seasonality of precipitation and temperature severity.

(Please note: As the provided text for "Climate" does not contain the definitions of Koeppen's global zones, I cannot elaborate on them from this chapter's text. However, if the user intends for me to pull from the previous "World Climate" chapter based on these subheadings, I will do so. Assuming the user wants the Koeppen climate types as applied to India from a later section, I will move that table (`Table 4.1`) and reference it here, while acknowledging the structural discrepancy).

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Tropical Wet Climate (Af)

(Notes related to Af from Chapter 12 structure, if applicable) - *Not present in the provided text for this chapter.*

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Tropical Monsoon Climate (Am)

(Notes related to Am from Chapter 12 structure, if applicable) - *Not present in the provided text for this chapter.*

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Tropical Wet And Dry Climate (Aw)

(Notes related to Aw from Chapter 12 structure, if applicable) - *Not present in the provided text for this chapter.*

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Dry Climates : B

(Notes related to B from Chapter 12 structure, if applicable) - *Not present in the provided text for this chapter.*

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Subtropical Steppe (BSh) And Subtropical Desert (BWh) Climates

(Notes related to BSh/BWh from Chapter 12 structure, if applicable) - *Not present in the provided text for this chapter.*

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Warm Temperate (Mid-Latitude) Climates-C

(Notes related to C from Chapter 12 structure, if applicable) - *Not present in the provided text for this chapter.*

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Humid Subtropical Climate (Cwa)

(Notes related to Cwa from Chapter 12 structure, if applicable) - *Not present in the provided text for this chapter.*

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Mediterranean Climate (Cs)

(Notes related to Cs from Chapter 12 structure, if applicable) - *Not present in the provided text for this chapter.*

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Humid Subtropical (Cfa) Climate

(Notes related to Cfa from Chapter 12 structure, if applicable) - *Not present in the provided text for this chapter.*

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Marine West Coast Climate (Cfb)

(Notes related to Cfb from Chapter 12 structure, if applicable) - *Not present in the provided text for this chapter.*

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Cold Snow Forest Climates (D)

(Notes related to D from Chapter 12 structure, if applicable) - *Not present in the provided text for this chapter.*

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Cold Climate With Humid Winters (Df)

(Notes related to Df from Chapter 12 structure, if applicable) - *Not present in the provided text for this chapter.*

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Cold Climate With Dry Winters (Dw)

(Notes related to Dw from Chapter 12 structure, if applicable) - *Not present in the provided text for this chapter.*

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Polar Climates (E)

(Notes related to E from Chapter 12 structure, if applicable) - *Not present in the provided text for this chapter.*

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Tundra Climate (ET)

(Notes related to ET from Chapter 12 structure, if applicable) - *Not present in the provided text for this chapter.*

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Ice Cap Climate (EF)

(Notes related to EF from Chapter 12 structure, if applicable) - *Not present in the provided text for this chapter.*

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Highland Climates (H)

(Notes related to H from Chapter 12 structure, if applicable) - *Not present in the provided text for this chapter.*

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Factors Determining The Climate Of India

India's climate is influenced by a combination of factors that can be broadly categorized into two groups: those related to location and relief (topography), and those related to air pressure and winds (atmospheric circulation).

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Factors Related To Location And Relief

• Latitude: The Tropic of Cancer ($23.5^\circ$ N latitude) passes through the central part of India. This divides the country into two major climatic zones.
  • The region south of the Tropic of Cancer lies in the tropical zone, closer to the equator. This area experiences high temperatures throughout the year with small daily and annual temperature ranges.
  • The region north of the Tropic of Cancer lies in the sub-tropical or warm temperate zone. Being farther from the equator, this area experiences a more extreme climate with larger daily and annual temperature ranges, including distinct hot summers and cold winters.
• The Himalayan Mountains: The formidable Himalayan mountain ranges in the north, along with their extensions in the northwest and northeast, act as a major climatic barrier.
  • They protect the Indian subcontinent from the extremely cold, dry winds that blow from Central and East Asia, originating near the Arctic circle. This keeps north India warmer in winter than it would otherwise be at similar latitudes.
  • They trap the moisture-laden monsoon winds originating from the Indian Ocean, forcing them to rise and shed their precipitation over the Indian subcontinent.
• Distribution of Land and Water: India is a peninsula, surrounded by the Indian Ocean on three sides (Arabian Sea to the west, Bay of Bengal to the east, Indian Ocean to the south) and bounded by mountains in the north. The significant difference in how land and water heat up and cool down (land heats/cools faster than water) creates differential air pressure zones over the subcontinent and the surrounding oceans in different seasons. This pressure difference is the driving force behind the seasonal reversal of monsoon winds.
• Distance from the Sea (Continentality): Coastal areas experience a moderating influence from the sea, resulting in an equable climate with small daily and annual temperature variations. As one moves further inland, away from the sea's influence, the climate becomes more extreme, with larger temperature differences between day and night (diurnal range) and between summer and winter (annual range). Places like Mumbai have an equable climate, while places in the interior like Delhi or Amritsar experience much greater seasonal contrasts.
• Altitude: Temperature decreases with increasing height above sea level (normal lapse rate). Places in the mountains are cooler than places located on plains at the same latitude. For example, Darjiling (in the Himalayas) is significantly cooler in January than Agra (on the plains), despite being at a similar latitude, due to its much higher altitude.
• Relief (Physiography): The shape and elevation of the land influence temperature, pressure, wind patterns, and especially rainfall distribution. Mountain ranges acting as barriers can force moist winds to rise, causing high rainfall on the windward side and creating rain-shadow areas on the leeward side. For instance, the Western Ghats cause high rainfall on the west coast but create a dry leeward plateau to the east.

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Factors Related To Air Pressure And Wind

The distribution of air pressure and winds, both at the surface and in the upper atmosphere, plays a critical role in determining India's climate, particularly the monsoonal characteristics and the rhythm of seasons.

Understanding the mechanism involves considering:

• The surface distribution of pressure and wind belts.
• Upper air circulation patterns, influenced by global weather phenomena like jet streams.
• The inflow of weather disturbances, such as western cyclones in winter and tropical depressions during the monsoon.

These factors' interplay can be analyzed by looking at the dominant weather mechanisms in the winter and summer seasons.

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Mechanism Of Weather In The Winter Season

During winter months, the weather over India is largely influenced by pressure conditions over Central and Western Asia.

• Surface Pressure and Winds: A large high-pressure center develops north of the Himalayas in winter due to intense cold. This drives dry, cold continental air masses southwards towards the Indian subcontinent at low levels. These winds reach northern India, influencing the surface wind patterns. Over northwestern India, these continental winds meet the trade winds. The boundary between these air masses is not fixed but can shift.
• Jet Stream and Upper Air Circulation: High above the surface (9-13 km altitude), the atmospheric circulation is dominated by the westerly jet stream, a fast-flowing current of air that flows west to east across Asia. The Tibetan Plateau acts as a barrier, splitting the jet stream into two branches.
  • A northern branch flows north of the Himalayas.
  • A southern branch flows eastwards, south of the Himalayas, with its mean position around $25^\circ$ N latitude in February. This southern branch of the westerly jet stream is considered important in influencing winter weather conditions in India.

Diagram illustrating the direction of upper-level westerly winds (jet stream) in winter over India, showing the branch flowing south of the Himalayas.

• Western Cyclonic Disturbances: These weather systems originate over the Mediterranean Sea during winter. They travel eastward across West Asia and Pakistan, picking up some moisture from the Caspian Sea and Persian Gulf on their way. The westerly jet stream helps steer these disturbances into the northwestern parts of India. Their arrival is often indicated by a rise in nighttime temperatures. These disturbances bring the occasional winter rainfall to the northwestern plains and snowfall to the Himalayas, which is beneficial for winter crops and sustains Himalayan rivers.
• Tropical Cyclones: Some tropical cyclones can form over the Bay of Bengal and southern Indian Ocean during transitional seasons (late post-monsoon) or rarely in winter. They move towards the east coast (Tamil Nadu, Andhra Pradesh, Odisha), bringing high winds and heavy rain, and can be very destructive.

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Mechanism Of Weather In The Summer Season

As summer begins, the sun's apparent northward movement causes temperatures to rise significantly over the Indian subcontinent, especially the northwestern plains. This leads to a complete reversal of atmospheric circulation patterns from winter.

• Surface Pressure and Winds: The intense heating over northwestern India creates a strong low-pressure area. By mid-July, this low-pressure belt, known as the Inter Tropical Convergence Zone (ITCZ) or monsoon trough, shifts northwards and becomes positioned roughly parallel to the Himalayas, centered between $20^\circ$N and $25^\circ$N latitude (over the Gangetic plain). This monsoon trough is a zone of rising air.

Diagram illustrating the direction of surface winds over India in July, showing the general southwesterly flow of the monsoon.

The ITCZ's position attracts winds from different directions. The southeast trade winds from the Southern Hemisphere, after crossing the equator between $40^\circ$E and $60^\circ$E longitudes, are deflected to the right (towards the northeast) by the Coriolis force and flow towards the low-pressure ITCZ over India. These moisture-laden winds from the Indian Ocean are the southwest monsoon (Figure 4.2).

• Jet Streams and Upper Air Circulation: Corresponding to the northward shift of the ITCZ and the development of the monsoon, the westerly jet stream withdraws from over the North Indian Plain by the middle of June. An easterly jet stream develops and flows over the southern part of the Peninsula in June (maximum speed 90 km/h), moving northwards to about $15^\circ$N in August and $22^\circ$N in September (Figure 4.3). This easterly jet stream typically does not extend north of $30^\circ$N in the upper atmosphere. Some meteorologists believe this jet stream is linked to the onset and burst of the monsoon.

Diagram illustrating the direction of upper-level easterly winds (jet stream) in summer over India, flowing over the southern Peninsula.

• Easterly Jet Stream and Tropical Cyclones: The easterly jet stream plays a role in steering tropical depressions (low-pressure systems originating in the Bay of Bengal) towards India during the monsoon season. These depressions contribute significantly to the distribution of monsoon rainfall, bringing rain along their tracks, which are often the areas of highest rainfall. The frequency, direction, and intensity of these depressions influence the monsoon rainfall pattern.

In the core of the summer ITCZ over northwestern India, hot, dry local winds called 'Loo' are common in the afternoons and evenings. Dust storms are also frequent in May, bringing temporary relief from the heat with light rain and cool breezes. Local storms of high intensity, with violent winds, heavy rain, and hail, can occur when moist air meets dry air.

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Inter Tropical Convergence Zone (ITCZ)

The Inter Tropical Convergence Zone (ITCZ) is a zone of low pressure located near the equator. It is characterized by the convergence of the trade winds from both the Northern and Southern Hemispheres, and the consequent ascent of air. In July, the ITCZ shifts northward over the Gangetic plain (around $20^\circ$N-$25^\circ$N latitude) and is referred to as the monsoon trough. This thermal low encourages rising air and attracts the southwest monsoon winds. In winter, the ITCZ shifts southwards, leading to the reversal of winds and the occurrence of the northeast monsoon.

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Some Famous Local Storms Of Hot Weather Season

During the hot weather season, severe local storms can occur, bringing sudden changes in weather:

• Mango Shower: Pre-monsoon showers common in Kerala and coastal Karnataka towards the end of summer. They are beneficial for the early ripening of mangoes.
• Blossom Shower: Pre-monsoon showers in Kerala and nearby areas that help in the blossoming of coffee flowers.
• Nor'westers (Kalbaisakhi): Severe evening thunderstorms occurring mainly in West Bengal and Assam. Known for their intensity ("calamity of Baisakh" month), they bring strong winds, heavy rain, and sometimes hail. They are useful for tea, jute, and rice cultivation. In Assam, they are also called "Bardoli Chheerha".
• Loo: Hot, dry, and oppressive winds that blow during the afternoon in the northern plains, particularly between Punjab and Bihar. They are characterized by very high temperatures and can be detrimental to health.

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The Nature Of Indian Monsoon

The Indian monsoon is a complex and significant climatic phenomenon. Despite extensive research, a single theory fully explaining its intricate nature and causation remains elusive. Recent studies approach the monsoon at a global level, considering interactions with other weather systems. Key aspects studied include the onset of the monsoon, the rain-bearing systems within it, and breaks in rainfall.

Factors like the difference in pressure between Tahiti (East Pacific) and Port Darwin (Australia) are used to measure the intensity of the Southern Oscillation, related to monsoon strength. The Indian Meteorological Department (IMD) uses various indicators to forecast monsoon behavior.

The El Niño-Southern Oscillation (ENSO) is a complex weather system appearing every 3-7 years, affecting global weather extremes, including in India. El Niño, a warming of sea surface temperatures off the coast of Peru, distorts atmospheric circulation and can reduce plankton. While El Niño brings rain to some arid regions, it is often associated with reduced monsoon rainfall and drought conditions in India. It is used as an indicator for long-range monsoon forecasting.

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Onset Of The Monsoon

The traditional theory attributes monsoon onset to the differential heating of land and sea. In April and May, intense heating of the north Indian landmass under the vertically shining sun creates a strong low-pressure area in the northwest. Simultaneously, the Indian Ocean to the south heats more slowly, maintaining relatively higher pressure. This pressure difference attracts the southeast trade winds from the Southern Hemisphere. As these winds cross the equator, they are deflected by the Coriolis force and turn towards the Indian subcontinent, blowing from a southwesterly direction, bringing abundant moisture from the Indian Ocean. This marks the onset of the southwest monsoon.

The northward shift of the ITCZ to its summer position over India is also closely linked to the monsoon onset. The withdrawal of the westerly jet stream from over the North Indian Plain is believed by some meteorologists to be related to this shift and the subsequent establishment of the easterly jet stream, which may play a role in the monsoon burst.

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Entry Of Monsoon Into India

The southwest monsoon typically arrives over the Kerala coast around June 1st. It then rapidly advances northwards, reaching Mumbai and Kolkata by June 10th-13th. By mid-July, the entire Indian subcontinent is usually covered by the southwest monsoon (Figure 4.5 illustrates the normal dates of onset).

Map illustrating the average dates when the Southwest Monsoon typically arrives in different parts of India.

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Rain-Bearing Systems And Rainfall Distribution

The southwest monsoon approaches the Indian landmass in two main branches:

• The Arabian Sea Branch: Originates over the Arabian Sea and splits into three sub-branches upon reaching the Indian coast.
  • One branch is obstructed by the Western Ghats. As moist winds ascend the windward slopes (900-1200 m), they cool and cause very heavy orographic rainfall (250-400 cm) on the windward side (Western Coastal Plain, windward slopes of Sahyadris). After crossing the Ghats, the air descends, warms, and loses moisture, creating a rain-shadow area with little rainfall on the leeward side (eastern slopes of Western Ghats, parts of Deccan Plateau).
  • Another branch strikes the coast north of Mumbai and travels through the Narmada and Tapi river valleys, bringing rainfall to central India. It eventually merges with the Bay of Bengal branch in the Ganga plains.
  • A third branch hits the Saurashtra Peninsula and Kachchh, then passes over west Rajasthan and along the Aravalis, causing scanty rainfall due to limited moisture and the rain-shadow effect of the Aravalis. This branch also joins the Bay of Bengal branch in Punjab and Haryana, contributing to rainfall in the western Himalayas.
• The Bay of Bengal Branch: Strikes the coast of Myanmar and Bangladesh. The Arakan Hills in Myanmar deflect a large part of this branch towards the Indian subcontinent, entering West Bengal and Bangladesh from the south and southeast. This branch then divides into two under the influence of the Himalayas and the low pressure in northwest India.
  • One branch moves westward along the Ganga plains, causing rainfall as it progresses towards the Punjab plains.
  • The other branch moves up the Brahmaputra valley in the north and northeast, bringing widespread rains. A sub-branch hits the Garo and Khasi hills of Meghalaya, where Mawsynram on the crest of the Khasi hills receives the world's highest average annual rainfall due to extreme orographic uplift.

The Tamil Nadu coast remains largely dry during the southwest monsoon season because it lies parallel to the Bay of Bengal branch (not obstructing it) and is situated in the rain-shadow area of the Arabian Sea branch (Figure 4.10 shows monsoon rainfall distribution).

Map illustrating the spatial pattern of rainfall across India during the Southwest Monsoon season (June to September), showing areas of high, medium, and low precipitation.

The frequency of tropical depressions originating from the Bay of Bengal influences rainfall distribution. Their paths, mainly determined by the position of the monsoon trough (ITCZ), cause rainfall along their tracks. Oscillations in the monsoon trough affect the track and intensity of these depressions, leading to variations in rainfall amount and distribution year to year.

Monsoon rain often comes in spells interspersed with dry intervals ("breaks"). Breaks in rainfall are associated with the tracks of cyclonic depressions or, on the west coast, with winds blowing parallel to the coast (not bringing moisture onshore).

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Ei-Nino And The Indian Monsoon

El Niño is a complex climate pattern involving oceanic and atmospheric phenomena, occurring typically every 3 to 7 years. It is characterized by the warming of sea surface temperatures in the central and eastern tropical Pacific Ocean off the coast of Peru, replacing the normal cool Peruvian (Humboldt) current. This warming can significantly impact global weather patterns, often associated with droughts in some regions and floods in others.

In relation to the Indian monsoon, El Niño events are often correlated with reduced southwest monsoon rainfall and drought conditions in India. The warming in the Pacific disrupts the normal atmospheric circulation patterns that are favorable for the development and maintenance of the Indian monsoon. El Niño is therefore used as an indicator for forecasting the long-range performance of the monsoon in India.

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Break In The Monsoon

A break in the monsoon refers to dry spells occurring during the southwest monsoon season, where rainfall ceases for one or more weeks after a period of rain. These breaks are common and their causes vary regionally:

• In northern India, breaks can occur if the rain-bearing cyclonic depressions are infrequent or if the monsoon trough (ITCZ) shifts closer to the Himalayas, reducing rainfall over the plains.
• On the west coast, dry spells are associated with periods when winds blow parallel to the coast, rather than perpendicular to it, reducing the orographic uplift and moisture delivery.

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The Rhythm Of Seasons

India's climate is best understood through its annual cycle of distinct seasons. The Indian Meteorological Department (IMD) recognizes 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 (Post-Monsoon/Autumn)

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The Cold Weather Season

This season generally lasts from mid-November to February in northern India. December and January are the coldest months. Mean daily temperatures are typically below $21^\circ C$ in north India (Figure 4.6 shows January temperatures). Night temperatures can drop significantly, sometimes below freezing in Punjab and Rajasthan.

Map illustrating the average temperature distribution across India during the month of January, reflecting the cold weather season in the north and milder conditions in the south.

Factors contributing to the cold in north India include:

• Continentality: Areas far from the coast experience a continental climate with greater temperature extremes.
• Snowfall in Himalayas: Snowfall in the nearby mountains contributes to lower temperatures and cold waves in the plains.
• Cold Winds: Cold winds originating from Central Asia and over the Caspian Sea region bring cold wave conditions, frost, and fog, particularly in northwestern India around February.

Peninsular India does not experience a severe cold season due to the moderating influence of the surrounding oceans and proximity to the equator. Coastal areas have little seasonal temperature change (e.g., Thiruvananthapuram's maximum temperature is around $31^\circ C$ in January and $29.5^\circ C$ in June). Temperatures are lower in the hills of the Western Ghats due to altitude.

Pressure and Winds: In winter, high-pressure conditions develop over northern India (Figure 4.7 shows January pressure/winds). Lower pressure prevails over southern India and the Indian Ocean. Winds blow outwards from the northern high pressure, generally from the northwest in the Ganga valley, north in the Ganga-Brahmaputra delta, and northeast over the Bay of Bengal. These are light, dry winds with low velocity, as the pressure gradient is weak.

Map illustrating the atmospheric pressure patterns (isobars) and direction of surface winds over India during January.

Winter weather is generally pleasant but is occasionally disturbed by the arrival of western cyclonic disturbances, which bring winter rainfall to northwestern India and snowfall to the Himalayas. These disturbances are steered by the westerly jet stream.

Rainfall: Most parts of India do not receive rainfall in winter as dry continental winds move from land to sea. Exceptions:

• Winter rainfall in northwestern India (Punjab, Haryana, Delhi, western UP) caused by western cyclonic disturbances from the Mediterranean Sea. This meager rainfall is crucial for rabi crops (winter crops). Precipitation is snowfall in the lower Himalayas, essential for sustaining river flow in summer. Rainfall decreases eastward in the plains and southward in the mountains.
• Occasional winter rainfall in central India and northern parts of the southern Peninsula.
• Rain (25-50 mm) in Arunachal Pradesh and Assam.
• Torrential rainfall on the Tamil Nadu coast, southern Andhra Pradesh, southeast Karnataka, and southeast Kerala in October-November, caused by the northeast monsoon winds picking up moisture over the Bay of Bengal.

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The Hot Weather Season

This season starts in March in north India as temperatures rise due to the sun's northward movement. April, May, and June are the peak summer months. Temperatures reach $30^\circ-32^\circ C$ in most areas. In March, $38^\circ C$ is reached in the Deccan Plateau; in April, $38^\circ-43^\circ C$ in Gujarat and MP; in May, temperatures can exceed $48^\circ C$ in northwestern India (Figure 4.8 shows July temperatures, representative of summer heat). The heat belt shifts northward over time.

Map illustrating the average temperature distribution across India during the month of July, showing high temperatures over the northern plains.

Summer in south India is milder due to the moderating effect of the oceans and the Peninsular shape. Temperatures remain between $26^\circ C$ and $32^\circ C$. Hills in the Western Ghats are cooler (below $25^\circ C$). In coastal regions, temperature increases from the coast to the interior rather than north to south. Mean daily minimum temperatures are also high (rarely below $26^\circ C$).

Pressure and Winds: Summer is characterized by intense heat and falling air pressure in the northern half. The ITCZ shifts northwards to a position centered at $25^\circ N$ in July, forming an elongated low-pressure monsoon trough from the Thar desert to the Chotanagpur plateau (Figure 4.9 shows July pressure/winds). This attracts surface winds from different directions.

Map illustrating the atmospheric pressure patterns (isobars) and direction of surface winds over India during July, showing the development of the monsoon trough.

The surface circulation is southwesterly on the west coast, along the coasts of West Bengal and Bangladesh, and easterly or southeasterly over north Bengal and Bihar. These currents, essentially displaced equatorial westerlies, bring the moisture that signals the start of the rainy season by mid-June.

Hot, dry local winds called 'Loo' blow in the afternoon in the heart of the ITCZ (northwest India). Dust storms are common in May in northwestern regions, providing temporary relief with light rain and cool breezes. Local storms of high intensity can form when dry and moist air masses meet.

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The Southwest Monsoon Season

By early June, the intensifying low pressure over the northwestern plains becomes strong enough to attract the trade winds from the Southern Hemisphere. These winds cross the equator, are deflected, and enter the Bay of Bengal and the Arabian Sea, flowing in a southwesterly direction towards India. These moisture-laden winds from the Indian Ocean are the Southwest Monsoons.

The onset of the southwest monsoon is often sudden and associated with violent thunderstorms and lightning, known as the “burst” or “break” of the monsoons. This initial rain brings a significant drop in temperature ($5^\circ C$ to $8^\circ C$). The monsoon typically bursts over the Kerala coast in the first week of June and progresses inland, covering the entire subcontinent by mid-July.

The southwest monsoon divides into two main branches as it approaches the landmass:

• The Arabian Sea Branch: Splits into sub-branches. One hits the Western Ghats, causing heavy orographic rainfall on the windward side and creating a rain-shadow area on the leeward side. Another branch moves north of Mumbai, flowing through river valleys in central India and joining the Bay of Bengal branch in the Ganga plains. A third branch affects Saurashtra and Kachchh, then brings limited rain along the Aravalis before joining the Bay of Bengal branch in Punjab/Haryana, contributing to rain in the western Himalayas.
• The Bay of Bengal Branch: Strikes the Myanmar coast and Bangladesh. The Arakan Hills deflect a portion towards India, entering West Bengal and Bangladesh from the south/southeast. It then splits: one branch moves westward along the Ganga plains (reaching Punjab), and the other moves up the Brahmaputra valley (north/northeast), causing widespread rain. A sub-branch causes extreme rainfall in Meghalaya (Mawsynram). The Tamil Nadu coast receives little rain during this season due to its parallel orientation to this branch and being in the rain-shadow of the Arabian Sea branch.

Characteristics of Monsoonal Rainfall:

• Seasonal (June-September).
• Governed by relief (orographic), with heavy rain on windward slopes.
• Declines with distance from the sea.
• Occurs in wet spells interspersed with rainless intervals ("breaks"). Breaks are often linked to the movement of cyclonic depressions.
• Heavy downpours can cause significant runoff and soil erosion.
• Crucial for India's agriculture (over three-fourths of total rain), which supports a large population.
• Spatially uneven distribution (ranging from < 12 cm to > 250 cm annually).
• Onset and retreat can be delayed or earlier than usual, impacting crops.

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Season Of Retreating Monsoon

The months of October and November mark the retreating monsoon season. By late September, the southwest monsoon weakens as the low-pressure trough over the Ganga plain moves southwards following the sun. The monsoon begins withdrawing from western Rajasthan by the first week of September, and from other parts of northwest/central India by the end of the month. By October, the low-pressure center shifts to the Bay of Bengal and eventually moves over southern India (Karnataka, Tamil Nadu) by November, completely withdrawing from the Peninsula by mid-December.

The retreating monsoon season is characterized by clear skies and rising temperatures in north India. High temperature and humidity create oppressive weather, known as 'October heat'. Temperatures fall rapidly in north India from late October. While north India is dry, the eastern part of the Peninsula (primarily Tamil Nadu coast) receives significant rainfall during October and November. This rain is associated with tropical cyclonic depressions originating over the Andaman Sea that move westward and cross the eastern coast of the southern Peninsula. These cyclones are often destructive and affect densely populated delta regions (Godavari, Krishna, Kaveri) and coastal areas of West Bengal, Bangladesh, and Myanmar. Cyclonic storms are less frequent in the Arabian Sea during this period.

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Traditional Indian Seasons

In the Indian tradition, a year is divided into six two-monthly seasons based on practical experience and observed weather patterns, particularly in north and central India. This system differs from meteorological seasons and may not fully apply to south India where seasonal variations are less pronounced.

Traditional Indian Season	Indian Calendar Months	Gregorian Calendar Months
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

The average annual rainfall in India is about 125 cm, but its spatial distribution is highly varied (Figure 4.11 shows annual rainfall distribution).

Map illustrating the spatial pattern of average annual rainfall across India, showing areas receiving high, medium, low, and inadequate precipitation.

• Areas of High Rainfall (> 200 cm): West coast (Western Ghats), sub-Himalayan areas and hills of the northeast (Meghalaya). Parts of Khasi and Jaintia hills receive over 1,000 cm. Brahmaputra valley and adjoining hills receive slightly less (< 200 cm).
• Areas of Medium Rainfall (100-200 cm): Southern Gujarat, eastern Tamil Nadu, northeastern Peninsula (Odisha, Jharkhand, Bihar, eastern MP), northern Ganga plain along the sub-Himalayas, Cachar Valley, and Manipur.
• Areas of Low Rainfall (50-100 cm): Western UP, Delhi, Haryana, Punjab, Jammu and Kashmir, eastern Rajasthan, Gujarat, and parts of the Deccan Plateau.
• Areas of Inadequate Rainfall (< 50 cm): Parts of the Peninsula (Andhra Pradesh, Karnataka, Maharashtra), Ladakh, and most of western Rajasthan.

Snowfall is limited to the Himalayan region.

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Variability Of Rainfall

Indian rainfall is characterized by significant variability from year to year. Variability is often measured using the Coefficient of Variation (C.V.) calculated as: $C.V. = (\frac{\text{Standard Deviation}}{\text{Mean}}) \times 100$. Higher C.V. values indicate greater variability in rainfall amounts.

Figure 4.12 shows the variability of annual rainfall across India.

Map illustrating the percentage variability of average annual rainfall across India, showing areas with low, medium, and high year-to-year fluctuations in precipitation.

• Low Variability (< 25%): Found in areas with high annual rainfall (over 100 cm), such as the west coast, Western Ghats, northeastern Peninsula, eastern Ganga plains, northeastern India, Uttarakhand, Himachal Pradesh, and southwestern Jammu and Kashmir.
• Medium Variability (25-50%): Covers most of India, where annual rainfall is between 50 and 100 cm.
• High Variability (> 50%): Found in areas with low annual rainfall (less than 50 cm), such as western Rajasthan, northern Jammu and Kashmir, and interior parts of the Deccan plateau.

High variability means that rainfall in these regions is unreliable; they are prone to frequent droughts.

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Climatic Regions Of India

While India has a broad monsoonal climate unity, regional variations in weather elements allow for the identification of climatic sub-types or regions. Climatic regions are areas with relatively homogeneous climatic conditions resulting from the combination of various factors. Temperature and rainfall are primary criteria for classification.

Based on Koeppen's scheme (using monthly temperature and precipitation values), India is divided into eight climatic regions (Table 4.1 and Figure 4.13). Koeppen uses capital letters for major climate types (A: Tropical, B: Dry, C: Warm Temperate, D: Cool Temperate, E: Polar) and small letters for subtypes (f: no dry season, m: monsoon, w: winter dry, s: summer dry, h: hot, k: cold dry, c: cool summer, g: Gangetic plain).

Type of Climate (Koeppen Code)	Areas
Amw – Monsoon with short dry season	West coast of India south of Goa
As – Monsoon with dry summer	Coromandel coast of Tamil Nadu
Aw – Tropical savannah	Most of the Peninsular plateaus, south of the Tropic of Cancer
BShw – Semi-arid steppe climate	North-western Gujarat, some parts of western Rajasthan and Punjab
Bwhw – Hot desert	Extreme western Rajasthan
Cwg – Monsoon with dry winter	Ganga plain, eastern Rajasthan, northern Madhya Pradesh, most of North-east India
Dfc – Cold humid winter with short summer	Arunachal Pradesh
E – Polar type (ET/EF)	Jammu and Kashmir, Himachal Pradesh and Uttarakhand (high altitude areas)

Map illustrating the division of India into different climatic regions based on the criteria of Koeppen's classification system.

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Monsoons And The Economic Life In India

The monsoon system is of paramount importance to India's economy and way of life:

• It is the central axis around which India's entire agricultural cycle revolves. Agriculture is the primary livelihood for a large percentage of the population and is heavily dependent on the southwest monsoon rains.
• Most parts of India (except the Himalayas) have temperatures above the threshold needed for crop growth throughout the year, but water availability from the monsoon makes multi-cropping possible.
• The regional variations in monsoon climate enable the cultivation of a wide variety of crops adapted to different conditions across the country.
• However, the variability of monsoon rainfall leads to the risk of droughts in areas with insufficient rain or floods in areas with excessive rain, impacting agricultural productivity and causing economic hardship.
• Timely and well-distributed monsoon rainfall is crucial for agricultural prosperity. Failure or irregularity of the monsoon negatively affects agriculture, especially in areas lacking developed irrigation facilities.
• Sudden heavy monsoon bursts can lead to significant soil erosion.
• Winter rainfall in north India from western temperate cyclones is highly beneficial for the growth of rabi (winter) crops like wheat.
• The regional diversity in climate influences India's cultural landscape, reflected in the vast variety of food habits, clothing styles, and house types across different regions.

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Global Warming

Climate change, including global warming, is a natural process that has occurred throughout Earth's history, but it is also being significantly influenced by human activities today. Evidence from past geological periods indicates that the Earth's climate has fluctuated between warmer phases and ice ages. The current increase in global temperatures, known as global warming, is a major concern.

While natural factors contribute to climate variability, human activities, particularly large-scale industrialization and pollution, are significant drivers of current global warming. The debate often centers on the role of the "greenhouse effect".

The Earth's temperature has increased over the past 150 years. Projections suggest that by the year 2100, global temperatures could rise by about $2^\circ C$ compared to pre-industrial levels. This temperature increase is expected to cause various other changes in the Earth system.

One of the most significant projected consequences is a rise in sea level. This is due to two main factors: the melting of glaciers and ice caps on land and the thermal expansion of ocean water as it warms. Current predictions estimate an average sea level rise of about 48 cm by the end of the 21st century. Rising sea levels increase the risk of coastal flooding, erosion, and inundation of low-lying areas and small island nations, leading to environmental and social problems.

Climate change can also shift climatic boundaries, making some regions wetter and others drier, altering agricultural patterns. It may also increase the spread of insect-borne diseases like malaria and impact ecosystems and human populations globally.

The potential impact of a 50 cm sea level rise on India's sea coasts would be significant, threatening coastal communities, infrastructure, agricultural land in deltaic regions, and coastal ecosystems like mangroves.

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Greenhouse Gases (GHGs)

Certain gases in the atmosphere act like the glass in a greenhouse, trapping heat. These are called greenhouse gases (GHGs). They allow incoming shortwave solar radiation to pass through but absorb and re-emit outgoing longwave infrared radiation from the Earth's surface. This absorption warms the atmosphere.

While water vapour is the most abundant natural GHG, the primary GHGs whose concentrations are increasing due to human activities are:

• Carbon Dioxide ($CO_2$): The main contributor to global warming, primarily from burning fossil fuels (coal, oil, gas) and deforestation. Its concentration is increasing annually.
• Methane ($CH_4$): From sources like livestock, rice paddies, landfills, and fossil fuel production. More potent per molecule than $CO_2$.
• Chlorofluorocarbons (CFCs): Synthetic chemicals formerly used in refrigerants and aerosols. Very powerful GHGs, also deplete the ozone layer.
• Nitrous Oxide ($N_2O$): From agriculture, fossil fuel combustion, and industrial processes.
• Ozone ($O_3$): While stratospheric ozone is beneficial, tropospheric ozone is a pollutant and a GHG.

These gases are better absorbers of longwave radiation than carbon dioxide (per molecule) and contribute significantly to the enhanced greenhouse effect and global warming. Their increasing concentrations are leading to a significant rise in the Earth's average temperature, with potential severe consequences for the climate system and life on Earth.

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