NCERT Class 9 Social Science Understanding Society India and Beyond Part 1 Chapter 03 Atmosphere and Climate PDF Download

Chapter 3: Atmosphere and Climate

The Big Questions

1. What is the composition of the atmosphere?
2. How do the different layers of the atmosphere affect the planet Earth?
3. What is the mechanism of monsoon?
4. How can we reduce our carbon footprint?

What do you see when you look up in the sky? You might see the floating clouds, the sunshine, and feel a breeze as well. All of these are a result of a blanket of air surrounding the Earth, called the atmosphere. It is pulled down around the Earth by gravity, and is a mixture of gases in various proportions, vital for the survival of all living beings on the Earth. The atmosphere shields us from the harmful radiation coming from the Sun, including ultraviolet radiation. It regulates the Earth's temperature by trapping some of the Sun's energy and prevents it from escaping back into space. The atmosphere is a key component of the Earth's weather and climate systems, influencing factors such as temperature, humidity, and air pressure.

Think About It

Can you imagine what would happen if there were no atmosphere? Discuss your thoughts with your friends and teachers.

[Figure 3.1: Earth from space showing the atmosphere, See in your textbook]

Composition and Structure of the Atmosphere

Composition of the Atmosphere

The Earth's atmosphere is composed of a mixture of various gases. Nitrogen and oxygen are the two primary and most abundant gases that are essential to life on the Earth. Carbon dioxide, argon, helium, neon, krypton, xenon, ozone, and hydrogen are some other gases present in the atmosphere, but are found in lesser quantities. Besides these, the atmosphere also consists of water vapour and tiny dust particles. The amount of water vapour in the atmosphere varies, but it generally ranges from 0.1 per cent to 0.4 per cent. It plays a significant role in cloud formation and precipitation. The composition of the atmosphere also varies with altitude.

[Figure 3.2: Composition of the atmosphere - Nitrogen 78%, Oxygen 21%, Argon 0.93%, Carbon Dioxide 0.04%, Others 0.03%, See in your textbook]

Let's Recall

How is nitrogen useful for plants? Remember that you learnt this in the chapter 'The Invisible Living World: Beyond Our Naked Eyes', in your Grade 8 Science textbook.

Teacher's Note

When you see atmospheric composition percentages, remember that the first two elements - nitrogen and oxygen - make up 99% of the air. All the other gases combined are less than 1%. This matters because you need to focus on nitrogen and oxygen for most questions about how air affects life on Earth.

Structure of the Atmosphere

The atmosphere has a layered structure. These layers are defined on the basis of changes in temperature and density with increasing altitude. The density of air is highest near the Earth's surface and decreases with altitude. The major layers extending from the Earth's surface to outer space are illustrated below.

[Figure 3.3: Layers of the atmosphere showing Troposphere, Stratosphere, Mesosphere, Thermosphere and Exosphere with altitude markers, temperature variation line, and features like Aeroplane, Tropopause, Stratopause, Mesopause, Aurora, Satellites and Meteors, See in your textbook]

Troposphere

The troposphere is the most important layer of the atmosphere, with an average height of about 12 kilometres. In this layer, temperature decreases with increasing altitude. The air we breathe exists here, along with most of the water vapour and clouds. Nearly all the weather phenomena, such as rainfall, fog, and hail, occur in this layer. The troposphere is separated from the stratosphere by a transition zone known as the tropopause.

Teacher's Note

The troposphere is where you live and where all weather happens - this is the layer you need to remember most clearly. The height of 12 kilometres might seem big, but compare it to Earth's radius of about 6,400 kilometres, and you see the atmosphere is actually a very thin shell. Most exam questions about weather and breathing air are about the troposphere.

Key Points

• The atmosphere is made mostly of nitrogen (78%) and oxygen (21%), with small amounts of other gases, water vapour and dust particles.
• The atmosphere has several layers defined by how temperature and air density change with altitude, with the troposphere being the lowest and most important layer where we live.
• The troposphere extends about 12 kilometres high, temperature decreases as you go up, and nearly all weather occurs in this layer.
• Gravity pulls the atmosphere down around Earth, and the atmosphere protects us from harmful solar radiation while regulating the planet's temperature.

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