Indian Subcontinent: Climate and Rainfall Chapter Notes | Footprints Class 7: Book Solutions, Notes & Worksheets PDF Download

Introduction

Experts have observed that the monsoon season in India, which spans from June to September, has become more unpredictable in the past decade due to climate change. This has led to varying patterns of rainfall across different regions, with some areas experiencing heavy rains while others face drought-like conditions. In 2023, Himachal Pradesh and Sikkim suffered significant devastation due to the erratic monsoon, causing loss of lives and destruction of infrastructure.

Weather and Climate on the Indian Subcontinent

• Weather refers to the current atmospheric conditions, like temperature and rainfall.
• Climate, on the other hand, describes the typical weather patterns in a region over a long period.
• Climate elements include temperature, rainfall, winds, and more.

Mechanism of Monsoon

• The monsoon is driven by differential heating of land and water surfaces.
• It involves a seasonal reversal of wind patterns, leading to the south-west and north-east monsoons.

Weather and Climate

When we talk about weather and climate, we are looking at how the atmosphere behaves in a specific place over different periods of time. Let's break down these concepts into simpler terms.

What is Weather?

• Weather is how the air and sky are behaving in a certain place for a short amount of time.
• It includes things like how hot or cold it is, how wet or dry it is, how windy it is, if it's raining, how sunny it is, and how cloudy it is.
• Weather can change quickly, sometimes even within hours. For example, you might hear someone say, "The weather was nice at the beach today," or "It was bad in the morning but got better by noon."

What is Climate?

• Climate is the overall pattern of weather conditions in a larger area over a long period of time, usually about 30 to 40 years.
• It looks at things like average temperature, how much pressure the air has, how humid it is, how fast the wind blows, how much rain falls, how sunny it is, and special events like fog or frost.
• For instance, Japan has a climate that ranges from not too hot to kind of tropical.

Factors Influencing a Region's Climate

Understanding what influences the climate of a region is crucial. Various factors come into play, shaping the temperature, air pressure, wind patterns, humidity, and the movement of moisture in the atmosphere.
Let's delve into these factors in simpler terms:

Temperature

• Sun's Influence: The Sun provides heat, known as solar radiation. Earth receives this as insolation, affecting temperatures across latitudes.
• Measuring Temperature: Thermometers gauge temperature in Celsius or Fahrenheit.

Factors Affecting Temperature
Various factors influence the temperature of a place on Earth's surface:

• Latitude: Due to the Earth's curved surface and the varying angles at which the Sun's rays strike, temperature differences arise from the Equator to the Poles. The latitude of a location impacts the angle of sunlight, the duration of daylight, and the length of the path through the atmosphere that the sun's rays must travel. Consequently, variations in latitude lead to differences in temperature.
• Altitude: As altitude or height increases, temperature decreases. The standard rate of this decrease, known as the lapse rate, is approximately 1°C for every 165 meters of ascent.
• Distance from the Sea: Water takes longer to absorb solar energy and also takes more time to release the same heat. Therefore, coastal areas experience a moderate and stable climate, with less intense summer heat and milder winter temperatures. In contrast, regions far from the sea have an extreme or continental climate, with very cold winters and very hot summers.
• Prevailing Winds: Winds carry heat from their place of origin to the areas over which they blow. For instance, winds coming from hot deserts are warm, while those from polar regions are cold.
• Ocean Currents: Warm and cold ocean currents affect the temperature of the areas they flow past. These currents also alter the temperatures of winds blowing over them.
• Slope of Land: In mountainous regions of the Northern Hemisphere, southern slopes receive direct sunlight and are warmer compared to northern slopes, which are less suitable for agriculture.

Atmospheric Pressure

• Definition: Pressure from the air above, highest at sea level.
• Measuring Air Pressure: Barometers quantify air pressure in millibars.

Factors Affecting Air Pressure
Several factors influence atmospheric pressure on Earth's surface:

• Altitude: As altitude increases, the density of air decreases, leading to a reduction in air pressure. The pressure drops by 1 mb for every 10 meters of ascent.
• Temperature: When temperature rises, air expands, becomes lighter, and ascends, resulting in low pressure on the Earth's surface. Conversely, when the surface temperature is very low, the air above cools, contracts, becomes denser and heavier, and creates high pressure.
• Moisture: Air containing water vapor is lighter than dry air, so an increase in moisture content in the atmosphere causes a decrease in air pressure.
• Rotation of the Earth: The Earth's rotation can sometimes displace air masses, altering the pressure conditions in a given area.

Winds

• Definition: Horizontal movement of air from high to low pressure areas.
• Measurement: Anemometers track wind speed and wind vanes indicate direction.

Humidity

• Humidity refers to the moisture present in the air around us. When temperatures are high, humidity increases due to a higher rate of evaporation. The amount of moisture in the atmosphere varies depending on location and time. Humidity is measured using a hygrometer.
• Moisture moves in a cyclic manner between the Earth's surface and the atmosphere, known as the hydrological cycle. This cycle involves the movement of moisture through processes called evaporation, condensation, and precipitation.
• Evaporation is the process where water turns into water vapor due to heat. Condensation is the reverse of evaporation, where water vapor in the air transforms into tiny droplets of water or ice at low temperatures. Evaporation occurs when the air is dry, but condensation only happens when the air is saturated and cannot hold any more moisture. When these tiny droplets combine to form larger droplets that are too heavy to stay in the air, they fall to the Earth's surface as rain or snow, which are the most common forms of precipitation. The amount of rainfall is measured using a rain gauge.

Factors Affecting India's Climate

Let's see how different things affect India's weather. India mostly has a tropical monsoon climate. Let's break down how each of these factors influences India's climate.

Key Points:

• Latitude: India's position on Earth's map makes it part of the warm zones. Places closer to the equator are hotter than those further away. Higher latitudes can be warm in summer and cold in winter.
• Altitude: Higher places like mountains and plateaus are cooler with varied rainfall compared to lower areas like plains.
• Direction of mountain ranges: How mountains are aligned affects wind flow and rain patterns. The Himalayas, running east-west, block cold winds and trap monsoon rains.
• Distance from the sea: Coastal areas feel the influence of sea and land breezes due to the differing temperatures of the water and land. Coastal regions have more stable climates compared to far inland areas with extreme weather.
• Seasonal winds: Wind patterns mixed with the landscape create the unique monsoon weather of India, varying temperatures and rainfall.

Seasonal Cycle of the Indian Monsoon

In India, there are four main seasons: summer, monsoon, retreating monsoon, and winter.
Let's break down each of them in a simple way:

Summer

• During summer, the Northern Hemisphere begins to receive direct sunlight, causing temperatures to rise in India. 
• Temperatures increase steadily from 30°C in March to over 40°C in May. By mid-June, the Sun is directly over the Tropic of Cancer, leading to hot summers in most parts of north-western and central India. 
• The Great Indian Desert even records temperatures exceeding 50°C. Coastal regions experience slightly lower temperatures, around 30°C, due to the cooling effect of sea breezes, which moderate the temperature.
• In hilly areas, temperatures remain lower because temperature decreases with altitude. As the winds become hotter, they rise, leading to low air pressure over the land. Meanwhile, the surrounding seas, being cooler, develop higher pressure.

Rainy Season or Monsoons

The rainy season in India, also known as the monsoon season, is a period of significant rainfall that occurs from mid-June to September. During this time, the weather undergoes changes due to the movement of winds and moisture-laden air.
Key Points:

• Definition: The word 'monsoon' comes from the Arabic word 'mausim,' which means 'season.'
• Duration: The rainy season in India typically lasts from mid-June to September.
• Wind Movement: Winds travel from high-pressure regions to low-pressure regions, drawing in moisture-laden winds from the Indian Ocean.
• Origin of Monsoon Winds: The monsoon winds, also known as the south-west monsoons, originate in the cooler Indian Ocean in the Southern Hemisphere.
• Rain Formation: These winds absorb moisture as they travel over water bodies, leading to rain formation when they reach the hot mainland of India.
• Impact: Coastal areas experience heavy rainfall, thunderstorms, cloudy skies, and intermittent sunshine during the monsoon season.

Pattern of Rainfall Distribution Across India

In India, the way rainfall is spread across different regions follows a specific pattern based on the movement of the monsoon winds. These winds, particularly the south-west monsoon winds, play a crucial role in determining where and how much rain falls in different parts of the country.
Key Points:

• South-West Monsoon Branches: The south-west monsoon winds split into two branches as they approach India: the Arabian Sea branch and the Bay of Bengal branch.
• Arabian Sea Branch: This branch travels towards the west coast of India, bringing heavy rainfall to regions like Kerala, Karnataka, Goa, and parts of Maharashtra.
• Rainfall Distribution: Areas along the windward slopes of the Western Ghats receive the heaviest rainfall, while the leeward side experiences a rain shadow effect with significantly less rainfall.
• Rain-Shadow Regions: Regions like the Great Indian Desert and areas to the east of the Western Ghats are considered rain-shadow regions due to receiving minimal rainfall.
• Bay of Bengal Branch: Divides into two further branches, with one moving towards eastern India, Myanmar, and Bangladesh, leading to heavy rainfall in regions like Meghalaya and Assam.
• Rainfall Intensity: Rainfall intensity varies across different regions, with some areas receiving over 350 cm of rainfall while others, like western Rajasthan, receive as little as 25 cm.
• Extreme Rainfall Records: Notable places like Cherrapunji and Mawsynram hold records for the highest recorded rainfall globally.
• Seasonal Variations: Rainfall patterns change over the monsoon season, with areas like Punjab experiencing reduced rainfall towards the end of September.
• Impact on Different Regions: Different regions in India receive varying amounts of rainfall based on their location and proximity to the monsoon winds.

Retreating Monsoon

By the end of September, when the Sun is right over the equator, India experiences changes due to the retreat of monsoon winds.
Here's what happens during this time:

• October: The weather becomes clearer and cooler. Dry winds start blowing from the land to the sea.
• Temperature Change: In the Northern Plains, it starts getting cooler as the Sun's rays move away from directly hitting the Tropic of Cancer and start falling on the Equator.
• Retreating Monsoon: The south-west monsoon winds gradually withdraw during this period, which is why it's called the retreating monsoon season. By November, most of India is free from monsoons.

Winter

As December arrives, the Sun is directly over the Tropic of Capricorn, marking the beginning of winter in India that lasts from late November to the end of February.
Here's what you can expect:

• General Weather: It's cold and dry across the country.
• Temperature Range: Temperatures vary between 10°C and 22°C in most areas. The Northern Plains and the Great Indian Desert get very cold, while the south remains warmer due to the nearby seas. Coastal regions enjoy moderate temperatures.
• Wind Patterns: Winds now move from the cool land to the warmer sea as high pressure builds over land and low pressure over seas. These dry land winds, known as the north-east monsoons, do not bring rain to most parts of India but pick up moisture when passing over the Bay of Bengal, bringing rainfall to places like Tamil Nadu.
• Western Disturbances: Some disturbances from the Mediterranean region bring scattered showers to north-western India and snowfall to areas like Jammu and Kashmir, Himachal Pradesh, and Uttarakhand.

India's climate is greatly influenced by these seasonal wind shifts.

Effects of Monsoons on People's Lives

The monsoon season in India plays a crucial role in the daily lives of people, affecting agriculture, water supply, economy, and various aspects of livelihood. Let's explore how the monsoon impacts the lives of people in India.

Key Impacts of Monsoons

• Water for Agriculture: The monsoons bring most of the rainfall to India, essential for agriculture. South-west monsoons support crops like rice, pulses, and oilseeds.
• Cropping Patterns: The distribution of rainfall determines which crops can be grown in different regions, influencing the cropping patterns.
• Food Security: Adequate monsoon rains ensure food supply, preventing famine and food shortages. Insufficient rainfall leads to food shortages and high food prices.
• Economic Impact: Good monsoons lead to increased agricultural output, higher farmer incomes, and stimulate demand for products, benefiting the economy.
• Water Recharge: Monsoons replenish water sources like rivers, lakes, and wells, ensuring a steady water supply for various needs.
• Hydroelectric Power: Regular rainfall is vital for generating hydroelectric power, which relies on consistent monsoon rains.
• Natural Vegetation: The type of vegetation in a region depends on rainfall distribution. For instance, tropical forests thrive in areas with high rainfall.
• Fishing and Livestock: Fishing seasons and livestock raising are linked to monsoon patterns, affecting the availability of fish and water for livestock.
• Negative Impacts: Excessive rainfall can lead to floods, causing damage to crops, property, and lives. Late or failed monsoons result in droughts, crop failures, and water scarcity, impacting many people.