Class 9 Social Science Notes Chapter 4 (Climate) – Contemporary India Book

Alright class, let's focus on Chapter 4, 'Climate', from your 'Contemporary India' textbook. This is a crucial chapter, not just for your Class 9 understanding, but also forms a significant part of the Geography syllabus for various government exams. Pay close attention as we break down the concepts.

Chapter 4: Climate - Detailed Notes for Exam Preparation

1. Introduction: Climate vs. Weather

• Weather: Refers to the state of the atmosphere over an area at any point in time. It includes elements like temperature, humidity, precipitation, wind, atmospheric pressure. Weather is highly variable and can change within a day or even hours.
• Climate: Refers to the sum total of weather conditions and variations over a large area for a long period of time (generally more than 30 years). It represents the average weather pattern.
• Elements of Weather and Climate: Temperature, Atmospheric Pressure, Wind, Humidity, and Precipitation.
• India's Climate: Described as the 'Monsoon' type. This type of climate is primarily found in South and Southeast Asia. Despite an overall unity in the general pattern, there are perceptible regional variations in climatic conditions within India.
  • Example Variations: Temperature differences (e.g., 50°C in Rajasthan desert vs. -45°C in Drass, J&K), Precipitation differences (e.g., >400 cm in Meghalaya vs. <10 cm in Ladakh/western Rajasthan).

2. Climatic Controls

There are six major factors that control the climate of any place:

• (i) Latitude: Due to the Earth's curvature, solar energy received varies with latitude. Temperature generally decreases from the equator towards the poles. India is divided by the Tropic of Cancer (23°30' N), placing the southern part in the tropical zone and the northern part in the sub-tropical zone.
• (ii) Altitude: As we go higher from the Earth's surface, the atmosphere becomes less dense, and temperature decreases. Hills are therefore cooler during summers. The Himalayas prevent cold winds from Central Asia from entering India, giving it a milder winter compared to other areas on the same latitude.
• (iii) Pressure and Wind System: This depends on latitude and altitude. It influences temperature and rainfall patterns. Key aspects for India include:
  • Pressure and Surface Winds
  • Upper Air Circulation (Jet Streams)
  • Western Cyclonic Disturbances and Tropical Cyclones
• (iv) Distance from the Sea (Continentality): The sea exerts a moderating influence. Coastal areas experience less contrast in temperature conditions (equable climate). Areas far from the sea experience extreme weather conditions (continental climate - very hot summers, very cold winters).
• (v) Ocean Currents: Along with onshore winds, ocean currents affect the climate of coastal areas. Warm currents make coasts warmer, cold currents make them cooler. (Less direct impact on India compared to other factors, but relevant globally).
• (vi) Relief Features: High mountains act as barriers for cold or hot winds and can also cause precipitation if they lie in the path of rain-bearing winds (orographic rain). The leeward side (rain shadow area) remains relatively dry. The Himalayas are a prime example for India.

3. Factors Affecting India's Climate

• Latitude: The Tropic of Cancer significantly influences temperature zones.
• Altitude: The Himalayas act as a climatic divide and barrier.
• Pressure and Winds: This is the most complex and defining factor for India:
  • Surface Conditions (Winter): High pressure over Central Asia/Himalayas leads to dry, cold offshore winds blowing from land to sea over most of India.
  • Surface Conditions (Summer): Intense heating of landmass (especially NW India and Tibetan Plateau) creates low pressure. High pressure builds over the ocean south of the Indian subcontinent. This pressure difference causes a complete reversal of wind direction – winds blow from the high-pressure area over the southern Indian Ocean, cross the equator (deflected right due to Coriolis force), and blow towards the Indian landmass as the South-West Monsoon. These winds gather moisture over the warm oceans and bring widespread rain.
  • Upper Air Circulation - Jet Streams: These are narrow belts of high-altitude (above 12,000 m) westerly winds in the troposphere.
    • Sub-Tropical Westerly Jet Stream: Blows south of the Himalayas all year except summer. Responsible for bringing Western Cyclonic Disturbances during winter.
    • Tropical Easterly Jet Stream: Blows over peninsular India approximately during the summer months (around 14°N latitude). Plays a role in the monsoon mechanism.
  • Western Cyclonic Disturbances: Weather phenomena of the winter months brought in by the westerly flow (jet stream) from the Mediterranean region. They cause the much-needed winter rains over the plains (Mahawat - crucial for Rabi crops) and snowfall in the mountains.
  • Tropical Cyclones: Occur during the monsoon as well as in October-November (Retreating Monsoon). Originate over the Bay of Bengal and Arabian Sea, affecting coastal regions, particularly the eastern coast (Odisha, Andhra Pradesh, Tamil Nadu).

4. The Indian Monsoon

• Mechanism: The monsoon is understood as a seasonal reversal in wind direction. Key factors contributing to its mechanism:
  • (a) Differential heating and cooling of land and water creates low pressure on land and high pressure over the sea in summer (and vice-versa in winter).
  • (b) Shift of the Inter Tropical Convergence Zone (ITCZ). In summer, it shifts over the Ganga plain (often called the 'monsoon trough').
  • (c) Presence of high-pressure area east of Madagascar (approx. 20°S over the Indian Ocean) affects the monsoon's intensity.
  • (d) Intense heating of the Tibetan plateau during summer creates strong vertical air currents and low pressure over the plateau.
  • (e) Movement of the westerly jet stream north of the Himalayas and the presence of the tropical easterly jet stream during summer.
  • (f) Southern Oscillation (SO): Changes in pressure conditions over the Southern Oceans also affect monsoons. Linked with El Niño (warming of Peru current) which often leads to weaker monsoons in India. La Niña (cooler phase) is often associated with good monsoons. ENSO = El Niño Southern Oscillation.
• Onset and Withdrawal:
  • Onset: Usually arrives at the southern tip around the first week of June. Progresses rapidly, covering the country by mid-July. It bifurcates into two branches:
    • Arabian Sea Branch: Hits the Western Ghats first. Moves northwards along the coast and also over central India.
    • Bay of Bengal Branch: Arrives in Assam first. Deflected westwards by the Himalayas, moves up the Ganga plains.
    • The two branches often merge over the northwestern part of the Ganga plains.
  • Withdrawal (Retreating Monsoon): A more gradual process. Begins in northwestern states by early September. Withdraws completely from the northern half by mid-October, and from the entire peninsula by early December.

5. The Seasons (Based on Monsoon Cycle)

India experiences four main seasons:

• (i) The Cold Weather Season (Winter):
  • Duration: Mid-November to February.
  • Temperature: Decreases from south to north. Average temp in Chennai (24°-25°C), Northern plains (10°-15°C). Frost is common in the north.
  • Winds: Northeast trade winds prevail (land to sea - mostly dry).
  • Precipitation: Most of India is dry. Some rainfall occurs on the Tamil Nadu coast from these winds picking moisture over the Bay of Bengal. Crucial rainfall in NW India from Western Cyclonic Disturbances ('Mahawat').
  • Pressure: High pressure over the north, clear sky, low temperatures, low humidity, feeble variable winds.
• (ii) The Hot Weather Season (Summer):
  • Duration: March to May.
  • Temperature: Increases due to the apparent northward movement of the sun. Highest temperatures in March (Deccan ~38°C), April (Gujarat/MP ~42°C), May (NW India ~45°C). Temperature remains lower in Peninsular India due to moderating influence of the ocean.
  • Pressure: Low-pressure area develops over NW India, extending to the east and southeast (ITCZ shifts).
  • Winds: 'Loo' - Strong, gusty, hot, dry winds blowing during the day over North and NW India. Dust storms are common.
  • Pre-Monsoon Showers: Towards the end of summer. Help in the ripening of mangoes (Mango Showers in Kerala/Karnataka), good for coffee flowers (Blossom Shower in Kerala), associated with thunderstorms/hail in West Bengal (Kal Baisakhi - Calamity of the month of Baisakh).
• (iii) Advancing Monsoon (The Rainy Season):
  • Duration: Early June to Mid-September.
  • Mechanism: Low-pressure trough intensifies, attracting trade winds from the Southern Hemisphere (South-West Monsoon).
  • Characteristics: Sudden onset often accompanied by violent thunderstorms (Burst of the Monsoon). Alternating wet and dry spells (Breaks in Monsoon) related to the movement of the monsoon trough. Heavy rainfall causes floods, while breaks lead to dry spells. Rainfall is highly variable year to year.
  • Distribution: Heavy rainfall in Western Ghats windward side, NE India (Mawsynram in Meghalaya receives the highest rainfall in the world). Moderate in Ganga valley, decreases east to west. Low rainfall in rain shadow areas (Deccan plateau, parts of MP, Rajasthan, Gujarat, Leh).
• (iv) Retreating/Post Monsoon (The Transition Season):
  • Duration: October-November.
  • Mechanism: Apparent movement of the sun towards the south. Monsoon trough weakens and is replaced by high pressure. SW monsoon winds weaken and withdraw.
  • Temperature: Transition from hot rainy season to dry winter. Clear skies, rise in temperature during the day (October Heat - due to high temp and humidity). Temperatures start falling rapidly in North India by late October.
  • Pressure/Winds: Low pressure shifts from NW India to the Bay of Bengal by early November.
  • Precipitation: This period is associated with the occurrence of cyclonic depressions originating over the Andaman Sea. These cyclones often cross the eastern coasts causing heavy and widespread rain, especially on the Coromandel Coast (Tamil Nadu, Southern Andhra Pradesh). These cyclones can be very destructive.

6. Distribution of Rainfall

• Highly uneven across India.
• Areas of Heavy Rainfall (>200 cm): Western coast, Western Ghats, Sub-Himalayan areas in Northeast, Hills of Meghalaya (Mawsynram).
• Areas of Moderate Rainfall (100-200 cm): Eastern slopes of Western Ghats, Northern plain (eastern parts), Odisha, MP, parts of Tamil Nadu & Andhra Pradesh.
• Areas of Low Rainfall (60-100 cm): Upper Ganga valley, parts of Punjab, Haryana, Eastern Rajasthan, Gujarat, Deccan Plateau interior.
• Areas of Scanty Rainfall (<60 cm): Parts of the Peninsula (especially rain shadow areas), Ladakh, Western Rajasthan.
• Variability: High variability in areas of low rainfall (like Rajasthan, Gujarat) makes them drought-prone. High rainfall areas are prone to floods.

7. Monsoon as a Unifying Bond

Despite variations, the monsoon acts as a unifying factor for India:

• Seasonal Cycle: The entire country follows a distinct rhythm of seasons dictated by the monsoon.
• Agriculture: Indian agriculture is heavily dependent on monsoon rains. Arrival of monsoon is awaited eagerly. Agricultural cycle (Kharif/Rabi crops) revolves around it.
• Water Availability: Rivers, fed by monsoon rains, unite different regions. Water availability for drinking, irrigation, and hydro-power depends on it.
• Flora and Fauna: The landscape, plant life, and animal life are heavily influenced by the monsoon.
• Festivals and Culture: Many Indian festivals are linked to the agricultural cycle and seasons, hence indirectly to the monsoon (e.g., Teej, Onam, Pongal, Baisakhi).
• Economic Life: The entire economy, especially the rural agrarian economy, is tied to the success or failure of the monsoon.

Key Terms to Remember:

• Climate: Long-term average weather conditions.
• Weather: Short-term atmospheric conditions.
• Monsoon: Seasonal reversal of winds.
• Tropic of Cancer: 23°30' N latitude dividing India.
• Coriolis Force: Force caused by Earth's rotation, deflecting winds (right in NH, left in SH).
• Jet Stream: High-altitude, fast-flowing air currents.
• Western Cyclonic Disturbances: Temperate cyclones from the Mediterranean causing winter rain in NW India.
• Mahawat: Local name for winter rainfall in NW India, crucial for Rabi crops.
• ITCZ (Inter Tropical Convergence Zone): Low-pressure belt near the equator where trade winds converge.
• Loo: Hot, dry, local wind in North India during summer.
• Kal Baisakhi: Thunderstorms in West Bengal during summer.
• Mango Showers: Pre-monsoon showers in Kerala/Karnataka.
• Burst of Monsoon: Sudden onset of monsoon rains.
• Breaks in Monsoon: Dry spells during the rainy season.
• October Heat: Oppressive weather condition during the retreating monsoon due to high temperature and humidity.
• ENSO (El Niño Southern Oscillation): Oceanic-atmospheric phenomenon affecting global weather, including Indian monsoon.

Multiple Choice Questions (MCQs)

1. Which of the following factors is considered the most dominant in determining India's climate?
   (a) Latitude
   (b) Altitude
   (c) Monsoon Winds
   (d) Distance from the Sea

2. The term 'Mahawat' is associated with:
   (a) Pre-monsoon showers in Kerala
   (b) Winter rainfall in North-Western India
   (c) Cyclonic storms on the East coast
   (d) Dust storms in Rajasthan during summer

3. During the summer season, the Inter Tropical Convergence Zone (ITCZ) typically shifts over which region in India?
   (a) Deccan Plateau
   (b) Coromandel Coast
   (c) Ganga Plain
   (d) Western Ghats

4. Which of the following phenomena is responsible for bringing winter rain to the Coromandel Coast (Tamil Nadu)?
   (a) South-West Monsoon
   (b) Western Disturbances
   (c) Retreating Monsoon (North-East Monsoon winds picking moisture over Bay of Bengal)
   (d) Loo winds

5. The 'Burst of the Monsoon' refers to:
   (a) The gradual withdrawal of the monsoon
   (b) The sudden and violent onset of monsoon rains
   (c) Dry spells during the monsoon season
   (d) Pre-monsoon thunderstorms

6. Which one of the following places receives the highest rainfall in the world?
   (a) Cherrapunji
   (b) Mawsynram
   (c) Guwahati
   (d) Shillong

7. The 'Loo' is a characteristic feature of the weather in which part of India during the hot weather season?
   (a) Coastal Odisha
   (b) North and North-Western India
   (c) Kerala and Karnataka
   (d) Assam and Meghalaya

8. The Coriolis force is caused by:
   (a) Earth's revolution around the sun
   (b) Differences in atmospheric pressure
   (c) Earth's rotation on its axis
   (d) Gravitational pull of the moon

9. Which of the following is NOT a major climatic control factor?
   (a) Ocean Currents
   (b) Longitude
   (c) Altitude
   (d) Pressure and Wind System

10. The phenomenon often associated with weaker monsoons in India is:
    (a) La Niña
    (b) Jet Stream intensification
    (c) El Niño
    (d) Strong Western Disturbances

Answers to MCQs:

1. (c) Monsoon Winds
2. (b) Winter rainfall in North-Western India
3. (c) Ganga Plain
4. (c) Retreating Monsoon (North-East Monsoon winds picking moisture over Bay of Bengal)
5. (b) The sudden and violent onset of monsoon rains
6. (b) Mawsynram
7. (b) North and North-Western India
8. (c) Earth's rotation on its axis
9. (b) Longitude (Latitude is a control, not Longitude)
10. (c) El Niño

Study these notes thoroughly. Understanding the mechanisms, especially related to pressure, winds, and the monsoon cycle, is key for competitive exams. Good luck with your preparation!