Class 9 Geography Chapter 4 Notes - Climate

Advancing Monsoon (The Rainy Season)

Formation and Movement

• By early June, a low-pressure system forms over the northern plains, drawing in trade winds from the southern hemisphere.
• Southeast trade winds develop over warm subtropical regions in the southern oceans, crossing the equator and moving southwest towards the Indian peninsula.
• These winds pick up moisture as they pass over warm oceans and bring it to the subcontinent.

The onset of the South West Monsoons

Characteristics of Monsoon Winds

• The southwest monsoon winds are strong, blowing at an average speed of 30 km/h.
• The winds typically cover most of India within about a month, except for the far north-western region.
• Upon the onset of the southwest monsoon in India, a significant shift in weather occurs, with heavy rainfall on the windward side of the Western Ghats exceeding 250 cm.

Regional Rainfall Patterns

• The Deccan Plateau and parts of Madhya Pradesh receive rainfall, even though they are in rain shadow areas.
• The northeastern region of India experiences the highest rainfall, with Mawsynram in the Khasi Hills having the world's highest average rainfall.
• Rainfall in the Ganga Valley decreases from east to west, with Rajasthan and parts of Gujarat receiving very little precipitation.

Monsoon Breaks and Trough Movement

• Monsoon seasons are characterized by intermittent breaks in rainfall, alternating between wet and dry spells.
• These breaks are linked to the movement of the monsoon trough. The trough's northward shift results in substantial rainfall in the plains, while a southward shift causes extended dry spells in the plains but widespread rainfall in mountainous areas.
• Heavy rains in the Himalayan catchment areas are due to this shift in the monsoon trough.

Influence of Tropical Depressions

• Tropical depressions originating in the Bay of Bengal influence monsoon rainfall patterns, affecting both the intensity and duration of the monsoon season.
• These depressions move along the monsoon trough of low pressure, contributing to the variability in rainfall, which can lead to floods in some regions and droughts in others, disrupting agricultural activities.

Retreating/ Post Monsoons

(The Transition Season)

Monsoon Withdrawal

• In October-November, as the sun shifts southward, the monsoon trough weakens over the northern plains, leading to the development of a high-pressure system.
• The southwest monsoon winds gradually diminish, causing the monsoon to withdraw from the Northern Plains.
• October and November signify a transition from the rainy season to drier winter conditions, marked by clear skies and a shift to drier weather patterns.

October Heat

• During October, India experiences high temperatures and humidity, known as the 'October heat' phenomenon, which can make the weather oppressively hot.

Cyclonic Activity

• In late October, temperatures in northern India begin to decrease rapidly as low-pressure conditions move from northwestern India towards the Bay of Bengal.
• By early November, low-pressure conditions in the Bay of Bengal lead to the formation of cyclonic depressions over the Andaman Sea.
• These cyclones often cross the eastern coasts of India, bringing heavy rain and causing significant damage.

Impact of Cyclones

• The deltas of the Godavari, Krishna, and Kaveri rivers, as well as the coastal areas of Odisha, West Bengal, and Bangladesh, are frequently affected by cyclones, leading to substantial destruction of life and property.
• The Coromandel Coast relies significantly on depressions and cyclones for its rainfall, which plays a crucial role in shaping the region's weather patterns.

Distribution of Rainfall

High Rainfall Areas

• The western coast and northeastern India receive more than 400 cm of rainfall annually.
  

Low Rainfall Areas

• Western Rajasthan, parts of Gujarat, Haryana, and Punjab experience less than 60 cm of rainfall each year.
• The interior of the Deccan Plateau and the eastern side of the Sahyadris also receive low precipitation.
• Leh in Jammu and Kashmir is another area with very low rainfall.

• Low Rainfall Areas
  • Western Rajasthan, parts of Gujarat, Haryana, and Punjab experience less than 60 cm of rainfall each year.
  • The interior of the Deccan Plateau and the eastern side of the Sahyadris also receive low precipitation.
  • Leh in Jammu and Kashmir is another area with very low rainfall.

• Moderate Rainfall Areas

  • Most of the rest of the country receives moderate rainfall.
  • Snowfall is generally limited to the Himalayan region.

• Rainfall Variability and Its Impacts

  • Annual rainfall varies significantly across the country due to the nature of the monsoons.
  • High rainfall regions are prone to flooding, while low rainfall areas are susceptible to drought conditions.

Monsoon as a Unifying Bond

Role of the Himalayas

• The Himalayas act as a barrier against extremely cold winds from Central Asia, protecting the Indian subcontinent.
• This protection results in consistently higher temperatures in northern India compared to other regions at similar latitudes.

Impact on the Peninsular Plateau

• The Peninsular Plateau, surrounded by the sea on three sides, benefits from moderate temperatures due to the maritime influence.
• This influence contributes to the region’s climate stability.

Monsoon's Unifying Influence

• The monsoon creates a rhythmic cycle of seasons through seasonal shifts in wind patterns and associated weather changes.
• The unpredictability of rainfall and its uneven distribution are characteristic features of the monsoon.

Impact on Indian Life

• The monsoon affects various aspects of Indian life, including the landscape, flora and fauna, agricultural practices, and cultural events.
• The agricultural calendar and livelihoods are closely tied to the monsoon, with celebrations and activities revolving around its arrival.

Cohesion Across Regions

• The monsoon plays a crucial role in providing water for agriculture, kickstarting the farming process.
• River valleys nourished by monsoon rains function as a cohesive unit, connecting different regions of India.

Difficult Words

1. Weather vs. Climate: Weather describes the short-term atmospheric conditions in a specific place at a specific time, while climate refers to the average long-term weather patterns over a significant period, typically 30 years. This distinction helps in understanding daily weather forecasts versus long-term climate patterns.

2. Temperature Variations: The significant differences in temperature across various regions due to factors like geographical location, altitude, and proximity to water bodies. For example, Rajasthan experiences extreme heat, while Pahalgam in Jammu and Kashmir has moderate summer temperatures.

3. Precipitation Variations: Differences in the amount and type of precipitation (rain, snow, sleet) that different regions receive based on their climate zones. High variability affects agriculture, water resources, and overall weather patterns.

4. Climatic Controls: Factors that influence a region's climate, including latitude, altitude, pressure and wind systems, distance from the sea, ocean currents, and relief features. Each of these factors contributes differently to shaping the local climate by affecting temperature, humidity, and precipitation.

5. Latitude and Climate: The position of a place on the Earth’s surface in relation to the equator (latitude) affects its solar radiation intake, influencing its overall climate. Tropical regions near the equator receive more sunlight and have warmer climates compared to polar regions near the poles.

6. Altitude's Impact on Climate: Higher altitudes are characterized by thinner air, which can hold less heat, leading to cooler temperatures. This is why mountainous regions typically have colder climates than areas at sea level.

7. Pressure and Wind Systems: The movement and interaction of different air pressure systems across the Earth influence weather patterns and climatic conditions. For example, high-pressure systems are generally associated with stable and dry weather, while low-pressure systems can lead to storms and rainfall.

8. Continentality (Distance from the Sea): The effect of an area's distance from the sea on its climate, where areas closer to the sea have milder climates and smaller temperature fluctuations compared to inland areas.

9. Ocean Currents: Streams of seawater moved by various global wind systems that can significantly influence coastal climates by carrying warm or cold water across different parts of the globe.

10. Relief Features: Geographic features like mountains that affect climate by blocking wind and precipitation, creating varied weather patterns on their windward and leeward sides.

11. Monsoon: A seasonal wind system that dramatically influences the climate of regions like India by bringing heavy rains during the summer months. The monsoon is crucial for agriculture but also causes significant weather-related challenges like flooding.

12. Cyclonic Depressions: Low-pressure areas in the atmosphere that can lead to severe weather conditions, including storms and heavy rainfall, particularly impactful in shaping the seasonal weather patterns in regions like India.

13. Ferrel's Law: Describes the deflection of wind due to Earth's rotation; in the Northern Hemisphere, winds are deflected to the right, affecting ocean currents and weather systems.