Class 11 Geography Notes Chapter 4 (Climate) – Geographia me Amli Kamm (Urdu) Book
Alright students, let's delve into Chapter 4: Climate, specifically focusing on the Indian context as detailed in your NCERT syllabus. Understanding India's climate is absolutely essential, not just for your exams, but for comprehending the country's agriculture, economy, and way of life. We will cover the key factors, mechanisms, seasons, and rainfall patterns. Pay close attention to the concepts and terminology.
Chapter 4: Climate (India - Physical Environment)
1. Introduction: Weather vs. Climate
- Weather: Refers to the state of the atmosphere over an area at any specific point in time. It includes elements like temperature, humidity, precipitation, wind, and atmospheric pressure. Weather is highly variable and changes frequently (day-to-day, even hour-to-hour).
- Climate: Refers to the sum total of weather conditions and variations over a large area for a long period of time (typically over 30 years). It represents the average weather pattern.
- India's Climate: Broadly described as 'Monsoon' type. This type is primarily found in South and Southeast Asia. However, within India, there are significant regional variations due to the vast size and diverse relief.
2. Factors Affecting India's Climate
These factors determine the variations in temperature, pressure, wind, and rainfall across the country:
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A. Factors Related to Location and Relief:
- Latitude: The Tropic of Cancer (23°30' N) passes through the middle of India, dividing it into tropical (south) and sub-tropical (north) zones. Southern parts experience higher temperatures and less annual range compared to the north.
- The Himalayan Mountains: Act as a formidable climatic divide.
- Protect the subcontinent from extremely cold winds from Central Asia during winter.
- Trap the monsoon winds, forcing them to shed moisture within the subcontinent.
- Distribution of Land and Water: India is a peninsula surrounded by the Indian Ocean, Arabian Sea, and Bay of Bengal. This influences temperature (moderating effect near coasts) and pressure conditions, crucial for monsoon development.
- Distance from the Sea (Continentality): Areas far from the sea experience extreme climate (hot summers, cold winters - e.g., Delhi), while coastal areas have an equable or maritime climate (less temperature range - e.g., Mumbai, Chennai).
- Altitude: Temperature decreases with increasing height (normal lapse rate approx. 6.5°C per 1000m). Places in the mountains are cooler than places on the plains (e.g., Shimla vs. Ludhiana at similar latitudes).
- Relief: Physiography plays a vital role in rainfall distribution. The windward side of mountains (like the Western Ghats) receives heavy rainfall, while the leeward side (rain shadow area like Deccan Plateau interior) remains relatively dry.
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B. Factors Related to Air Pressure and Wind:
- Surface Pressure and Winds: During winter, high pressure develops over Central Asia and the Indian subcontinent's north, leading to offshore winds (land to sea). In summer, intense low pressure forms over NW India (Thermal Low) and high pressure develops over the ocean south of India, causing a complete reversal of winds (onshore Southwest Monsoon).
- Upper Air Circulation (Jet Streams):
- Subtropical Westerly Jet Stream: Blows south of the Himalayas during winter. It steers Western Disturbances towards India. In summer, it shifts north of the Himalayas with the apparent northward movement of the sun.
- Tropical Easterly Jet Stream: Develops during summer over peninsular India (around 14°N). It is believed to play a significant role in the burst and intensity of the Southwest Monsoon.
- Western Cyclonic Disturbances and Tropical Cyclones:
- Western Disturbances: Weather phenomena of the winter months brought in by the westerly flow (jet stream) from the Mediterranean region. They cause moderate to light rainfall in North and Northwestern India (crucial for Rabi crops like wheat) and snowfall in the Himalayas.
- Tropical Cyclones: Originate over the Bay of Bengal and Arabian Sea during the monsoon season (especially October-November, post-monsoon). They cause heavy rainfall, strong winds, and storm surges in coastal areas (e.g., Odisha, Andhra Pradesh, Tamil Nadu, West Bengal, Gujarat).
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C. El Niño Southern Oscillation (ENSO):
- A complex weather pattern resulting from variations in ocean temperatures in the Equatorial Pacific.
- El Niño: Warming of the Pacific Ocean water off the coast of Peru. Often associated with weaker monsoons and drought conditions in India.
- La Niña: Cooling of the same Pacific waters. Generally associated with stronger monsoons and wetter conditions in India.
- Southern Oscillation: The see-saw pattern of atmospheric pressure between the eastern Pacific and Indian Ocean. Linked with El Niño/La Niña.
3. The Indian Monsoon Mechanism
The monsoon is a complex phenomenon influenced by multiple factors acting together:
- Differential Heating and Cooling: Land heats up and cools down faster than water. This creates low pressure over the Indian landmass and high pressure over the surrounding seas during summer, driving the moisture-laden winds from sea to land. The reverse happens in winter.
- Shift of the Inter Tropical Convergence Zone (ITCZ): The ITCZ (a low-pressure belt near the equator where trade winds converge) shifts northwards over the Ganga plain during summer. This 'monsoon trough' is crucial for the monsoon rains.
- High-Pressure Area East of Madagascar (Mascarene High): The intensity and position of this high-pressure area (approx. 20°S over the Indian Ocean) affect the strength of the Southwest Monsoon winds blowing towards India.
- Tibetan Plateau: Intense heating of the Tibetan plateau during summer creates strong vertical air currents and low pressure over the plateau at high altitudes, strengthening the monsoon circulation.
- Jet Streams: The movement of the Westerly Jet north of the Himalayas and the presence of the Tropical Easterly Jet over India during summer are crucial triggers and intensifiers.
- ENSO: As mentioned above, El Niño and La Niña significantly impact monsoon strength.
4. The Onset and Advance of the Monsoon
- Onset: Typically arrives at the southern tip of the Indian peninsula (Kerala coast) around the first week of June.
- Branches: Divides into two branches:
- Arabian Sea Branch: Strikes the Western Ghats, causing heavy rainfall on the windward side. Moves northwards, reaching Mumbai around 10th June and eventually covering Saurashtra-Kachchh and central India.
- Bay of Bengal Branch: Advances rapidly, arriving in Assam in the first week of June. Gets deflected westward by the Himalayan ranges, moving up the Ganga plains.
- Merger: The two branches merge over the northwestern part of the Ganga plains.
- Withdrawal/Retreat: Begins from northwestern states by early September. Withdraws completely from the northern half by mid-October and from the peninsula by early December. The retreat is more gradual than the onset.
5. The Rhythm of Seasons (Based on Monsoon Cycle)
India experiences four distinct seasons:
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a) The Cold Weather Season (Winter):
- Duration: Mid-November to February.
- Characteristics: Clear sky, low temperatures, low humidity, feeble variable winds. Temperature decreases from south to north. Frost is common in the north.
- Pressure & Winds: High pressure over northern India; offshore winds prevail.
- Precipitation: Most of India remains dry. However, rainfall occurs in:
- Northwestern plains due to Western Disturbances.
- Tamil Nadu coast receives rainfall from Northeast Monsoon winds picking moisture over the Bay of Bengal.
- Snowfall in the Himalayas.
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b) The Hot Weather Season (Summer):
- Duration: March to May.
- Characteristics: Apparent northward movement of the sun leads to rising temperatures and falling air pressure across India. Intense heating, especially in the northwest.
- 'Loo': Strong, gusty, hot, dry winds blowing during the day over North and Northwestern India.
- Pre-Monsoon Showers: Towards the end of summer, localized thunderstorms occur, bringing welcome showers:
- 'Mango Showers': Help in the ripening of mangoes (Kerala, Karnataka).
- 'Blossom Showers': Help coffee flowers blossom (Kerala and nearby areas).
- 'Kalbaisakhi' (Nor'westers): Thunderstorms in West Bengal and Assam, often destructive but useful for tea, jute, and rice cultivation.
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c) The Advancing Monsoon Season (Rainy Season):
- Duration: Early June to mid-September.
- Characteristics: Southwest Monsoon winds blow across the country, bringing widespread rainfall. Marked by the 'burst' of the monsoon (sudden onset with violent thunder and lightning).
- Rainfall Distribution: Heavy rainfall in Western Ghats (windward side) and Northeast India (Mawsynram receives the highest rainfall in the world). Moderate rainfall in Ganga valley and central India. Low rainfall in parts of Deccan Plateau (rain shadow), Rajasthan, Gujarat, Ladakh.
- 'Breaks' in Monsoon: Periods of dry spells within the rainy season, associated with the shifting position of the monsoon trough.
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d) The Retreating Monsoon Season (Transition Season/Autumn):
- Duration: October to November.
- Characteristics: Apparent southward movement of the sun. Monsoon trough weakens and shifts southwards. Gradual withdrawal of the monsoon.
- Weather: Skies become clear. Temperature rises again ('October Heat' - due to high temperature and humidity). Land is still moist.
- Cyclones: Low-pressure conditions shift to the Bay of Bengal, leading to the formation of tropical cyclones which often strike the eastern coast (Tamil Nadu, Andhra Pradesh, Odisha). These cyclones also cause rainfall on the Coromandel coast.
6. Distribution of Rainfall
- Highly uneven distribution across India.
- Areas of Heavy Rainfall (Over 200 cm): Western slopes of Western Ghats, Northeastern states (especially Meghalaya hills).
- Areas of Moderate Rainfall (100-200 cm): Most of the Ganga valley, Eastern coastal plains, parts of central India.
- Areas of Low Rainfall (Less than 60-100 cm): Interior Deccan plateau, parts of Punjab, Haryana, Gujarat, Rajasthan.
- Areas of Scanty Rainfall (Less than 50 cm): Western Rajasthan, adjoining parts of Gujarat, Haryana, Punjab, and Ladakh region.
- Variability: Rainfall is highly variable from year to year, especially in areas of low rainfall, leading to drought or flood situations.
7. Monsoons as a Unifying Bond
Despite regional variations, the monsoon regime influences the entire Indian subcontinent and its rhythm of life:
- Agriculture heavily depends on monsoon rains.
- River flows are augmented during monsoons.
- Cultural festivals are often tied to the monsoon cycle.
- The landscape, flora, and fauna are shaped by the monsoon.
Multiple Choice Questions (MCQs) for Exam Preparation:
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Which of the following factors is most responsible for the winter rainfall in Northwestern India?
(a) Southwest Monsoon
(b) Northeast Monsoon
(c) Western Disturbances
(d) Tropical Cyclones -
The term 'Monsoon' is derived from the Arabic word 'Mausim', which means:
(a) Wind
(b) Season
(c) Rain
(d) Climate -
Which part of India receives rainfall from both the Southwest Monsoon and the Northeast Monsoon?
(a) Kerala Coast
(b) Ganga Plains
(c) Tamil Nadu Coast
(d) Northeast India -
'Loo' is a characteristic feature of the weather in India during which season?
(a) Cold Weather Season (Winter)
(b) Hot Weather Season (Summer)
(c) Advancing Monsoon Season (Rainy)
(d) Retreating Monsoon Season (Autumn) -
The shift of the Inter Tropical Convergence Zone (ITCZ) over the Ganga Plain is associated with which phenomenon?
(a) Western Disturbances
(b) Onset of Southwest Monsoon
(c) Retreat of Monsoon
(d) Formation of Tropical Cyclones -
Which of the following locations receives the highest average annual rainfall in the world?
(a) Cherrapunji
(b) Mawsynram
(c) Mumbai
(d) Shillong -
The Subtropical Westerly Jet Stream influences India's climate primarily during:
(a) Summer Season
(b) Winter Season
(c) Rainy Season
(d) Autumn Season -
Which phenomenon is generally associated with weaker monsoon rains in India?
(a) La Niña
(b) Strong Mascarene High
(c) El Niño
(d) Active Tropical Easterly Jet -
The term 'October Heat' is characterized by:
(a) Low temperature and low humidity
(b) High temperature and low humidity
(c) Low temperature and high humidity
(d) High temperature and high humidity -
The Arabian Sea branch of the Southwest Monsoon causes heavy rainfall primarily on the:
(a) Eastern slopes of the Western Ghats
(b) Windward (Western) slopes of the Western Ghats
(c) Coromandel Coast
(d) Northern Plains
Answer Key:
- (c) Western Disturbances
- (b) Season
- (c) Tamil Nadu Coast
- (b) Hot Weather Season (Summer)
- (b) Onset of Southwest Monsoon
- (b) Mawsynram
- (b) Winter Season
- (c) El Niño
- (d) High temperature and high humidity
- (b) Windward (Western) slopes of the Western Ghats
Make sure you revise these notes thoroughly. Focus on understanding the interplay of different factors that shape India's unique monsoon climate. Good luck with your preparation!