Class VII Geography Chapter 4 “Air”

Class VII Geography Chapter 4 "Air"

Written By Avinash Sharan

22nd May 2021

Class VII Geography Chapter 4 “Air” NCERT Explanation and Solutions

Today we will learn about Class VII Geography Chapter 4 “Air”. This chapter “Air” is very important not only for the students of class VII but also for teachers and competitive examinations. Many questions are asked from NCERT Class VII Geography Chapter 4 “Air”.

So let’s try to understand this chapter. If you want to read first chapter,  ENVIRONMENT second chapter “INSIDE OUR EARTH” and third chapter   OUR CHANGING EARTH , then you can read it by clicking on the link.

Class VII Geography Chapter 4 “Air”

Air is present around our earth.

Our earth is round (spherical) and many kilometers thick layer of air surrounds it.

Suppose you are standing alone in an open field.

So what’s around you? Air.

Similarly, the earth also lies in the space, and what is there around it? Air.

It means, our earth is surrounded by air all around what we call the atmosphere.

This atmosphere not only helps us to breathe, but also prevents us from the harmful rays coming from the sun.

Without atmosphere, the earth will become so hot during the day and it will be so cold at night that it will be difficult to survive on it.

how? (We will read about it further)

Therefore, it is the atmosphere that makes the earth fit for our living

Composition of the atmosphere (texture):

The atmosphere is composed of many types of gases like Nitrogen, Oxygen, Carbon di oxide, Argon etc..

When you breathe, many gases enter the body.

Our body takes oxygen from it and releases the remaining gas.

Class VII Geography Chapter 4 “Air”


Structure of the atmosphere

The atmosphere is divided into five layers.

Just like in winter our body is covered with many layers of clothes (vest, shirt, half sweater, jacket, shawl, etc.)

Similarly, the atmosphere is also made up of many layers.

These are Troposphere, Stratosphere, Mesosphere, Thermosphere, and Exosphere.

Troposphere

This layer is the lowest and most important layer of the atmosphere.

It is about 15 km from the equator and 10 km from the north and south pole.

We live and breathe in this layer.

All types of weather changes (thunderstorms, rain fog, hail) occur in this layer.

Stratosphere

The stratosphere is above the troposphere.

Its height is about 50 km from the earth.

Neither there is a thunderstorm nor there is rain, so the airplane flies in this layer easily.

Where this layer ends, a thin layer is found at the borders called ozone.

This layer absorbs the harmful rays coming from the sun and does not allow it to reach the earth.

Two holes created in this layer got healed due to  lockdown (less pollution) worldwide.

Mesosphere

This layer is about 60 km above the earth.

When Meteors coming from space enter this layer at a high speed, they get burnt due to friction with air.

These are sometimes seen as broken stars.

Kalpana Chawla’s an Indian astronaut’s Space Shuttle  also got burnt as soon as it entered Mesosphere, due to which she died.

Food for thought:

Why spaceships do not burn up while leaving the atmosphere if they can burn up while entering?

Thermosphere

Thermosphere extends up to 40 km to 400 km from the earth.

As we move up in this layer, the temperature starts increasing rapidly.

In this layer, there is also a thin layer of ion.

The radio signal that we send from the earth goes up to this layer and gets reflected and comes back to the earth.

Signal in radio, TV or mobile comes from this layer.

Exosphere

The uppermost and outermost layer of the atmosphere is called the exosphere.

The lightest gases in the atmosphere, such as helium and hydrogen, are found in this layer.

Weather and Climate (Class 7 Chapter 4 “Air”)

Weather is the condition of the atmosphere for a short period of time.

Many times it happens that there is rain in one part of the city and there is sunshine on the other part.

IIt means, the weather on one side is clear and the weather on the other side is wet.

After a while the rain stops and  the sun appears again. 

It is possible that  the day is  hot with rain, the evening windy and night pleasant.

Therefore, weather

  •  can change within minutes. 
  • It affects a small area and
  • It is difficult to predict.

The long-term average weather, on the other hand, tells the climate of that place.

Like, for example, the equator and its surroundings are warm for long years of the year.

Similarly, the poles are cold for most of the year.

This is called climate.

It means- The climate

  • is for a longer time and larger area.
  • It can also be predicted easily.

Anyone sitting at home can tell that it will be hot tomorrow at  the equator and cold at poles.

Temperature

Atmospheric temperature simply means air temperature.

The hotness and the coldness (AVERAGE) present in the air is called temperature.

For example,

if today’s maximum temperature is 30 degrees and the minimum temperature is 12 degrees,

then its average temperature will be

30 + 12 = 42,

42/2 = 21 degrees.

The formula to find the mean is maximum + minimum temperature divided by 2.

Do you know how the atmosphere gets cold or heated ?

Many people would consider Sun responsible for this?

But Sun is innocent.

If the sun is responsible, then the maximum temperature of the day would be at 12 o’clock in the afternoon.

That is the time when the sun is right on our head and the rays are straight at that time.

Why

Insolation is responsible for this.is the maximum  temperature between 2;30 to 3;30 p. m?

Now, what is Insolation?

Insolation

It is true that the earth gets heated from the sun.

But it is also true that the Earth also returns the full heat of the Sun every day.

Even after sunset, the earth keeps returning the heat of the sun slowly.

At 10 o’clock night, 12 o’clock, 2 o’clock, and by 4 – 5 o’clock in the morning, the earth returns full heat of the sun.

At this time, the earth gets cold. The air over it  also becomes cold.

Therefore, the minimum temperature of a day is just before the sun rise.

So, Insolation means incoming of the solar radiation.

When the earth absorbs the heat of the sun. it is called insolation.

Now, the places like equator, where there will be bright sunshine,

the earth will absorb more heat and it will take more time to return.

On the other hand, polar regions will receive less sunshine and will also

take less time to return the heat back into the atmosphere.

Hence the polar regions are cold enough.

WHAT IS GREENHOUSE?

GREENHOUSE: A HOUSE MADE OF GLASS FOR GROWING CROPS CLASS VII AIR

There is a building of greenhouse made up of glass.

It is used to grow trees and plants in cool places.

When the bulb burns or the sunlight falls on this glass building, its heat gets captured in this house.

With this, such trees are grown in colder regions, which generally grows in warmer regions.

Therefore, it is called green house..

What is the Greenhouse Effect ?

The greenhouse effect is a natural phenomenon, which helps keep the Earth’s  surface warm.

This is why the earth is neither too cold nor too hot.

Because of which life on earth is possible.

On the earth, the heat coming from the Sun crosses the atmosphere and reaches the Earth.

Part of this incoming energy is absorbed by soil, trees, plants and other means.

Much of this absorbed energy is converted into heat, which keeps the Earth warm.

This is what we call greenhouse or green house effect.

When the amount of carbon dioxide increases in the atmosphere due to pollution, it absorbs more heat.

Because of this, the temperature of the earth is constantly increasing.

This is also the reason for the melting of ice in Antarctica.

It is known as Global warming.

Air pressure

Air pressure simply means pressure of air.

A pressure of about 6.5 (six and a half kg.) on one square inch is on our head exerted by the atmosphere.

This means that every person is carrying more than 500 kg pressure on his body.

But the question arises that why don’t we feel the pressure?

Because the pressure applies from all the four sides which gets balanced.

For example, if you let two people pull your hand in different directions with equal strength, then you will be standing in one place.

This pressure is highest at sea level.

As we go up, the air pressure decreases.

In the summer, the air becomes light and light air rises up, which creates a low pressure below.

Then strong wind (thunderstorm) moves from all directions to fill this low pressure area.

In contrast, in colder regions the air is colder.

Cold air is heavy. Due to being heavy, it cannot rise above the ground, which creates an area of ​​high air pressure.

Wind (class 7 geography chapter 4 air)

Just like water flows from higher to lower areas.

Similarly, wind also moves from high pressure to low pressure.

Movement of air from high pressure to low pressure is called wind.

The earth is round (Spherical), due to which the rays of the sun do not fall equally everywhere.

For this reason, the Earth is much warmer at the equator and colder elsewhere.

Where the Earth will be warm, the air will start rising.

Then cold air will move from the surroundings to fill that space.

In this way, the air keeps moving on the earth all the time.

Sometimes slowly and sometimes fast.

Wind movement can mainly be divided into three parts:

Permanent winds:

North-Eastern and South-Eastern Trade winds:

The rays of the sun fall straight on and around the equator (from 0 degrees to 5 degrees).

Because of this, this area becomes very hot.

This region is also known as Doldrum.  (0 degree to 5 or 10 degree)

Trade winds can be defined as the wind that flows towards the equator from the north-east in the Northern Hemisphere or from the south-east in the Southern Hemisphere.

These are also known as tropical easterlies and are known for their consistency in force and direction.

These winds are formed when the hot air rises and hits the equator where it is pulled towards the poles making them chilled.

These winds are used by the sailors.

Christopher Columbus discovered America with the help of trade winds.

Westerlies:

These winds blow from the west towards the east in the middle latitudes between 30° and 60° latitude. i

These winds get their name from their direction of origin.

These are blown from the horse latitudes towards the poles.

When the equatorial winds rise from the equator, it gets colder and heavier and descend near 30–35 degrees.

In ancient times, when a ship full of horses entered this region, the ship was deformed due to high pressure and started drowning.

In such a situation, the traders had to throw the horses into the sea to lighten the ship.

For this reason, this area is known as Horse latitudes.

After coming down, these winds get divided into two parts

In the Northern Hemisphere, these are predominantly from the south-west and in the Southern Hemisphere,

they are from the north-west. The weather patterns in the United States and Canada are due to westerlies.

Polar easterly winds

The temperature at the poles (90 degrees) is always low, due to which the air present is cold.

Cold air is heavy and covers all around the surface.

The air at 66 1/2 degrees will be comparatively less cold than the air at 90 degrees.

If we compare the  air then can we say that the air at 90 degree is cold and the air found at 66 and1/2 degrees is comparatively warm?

Now due to temperature difference, air at 66 and 1/2 degrees will rise creating a low pressure area.

and the cold winds from the poles will move to fill the region.

These winds are called Polar easterlies.

Seasonal winds

It is understood from the name itself that seasonal winds change their direction according to the season.

The monsoon wind that follows the summer season in India is an example of seasonal wind.

Generally, these winds blow from India (Tropic of cancer) towards equator throughout the year.

At the end of summer season, these winds reverse and start moving towards India from the equator.

Due to coming from the Indian Ocean, these winds are moisture laden due to which there is rain in India.

These seasonal winds are monsoon winds.

Local winds

These winds continue to move in a small area at any time of the day.

For example, the Loo in summer (hot, dusty and dry wind) is an excellent example of local wind.

Even at the sea shore, the wind blows for 24 hours.

Either from the sea to land or from the land to the sea.

Winds running in such small areas are called local winds.

Moisture

The amount of water vapor present in the air is called moisture.

The ability to absorb water in hot air is high.

So often you will see that many people blow warm air from their mouth to clean their spectacles.

When you blow warm air, small particles of water droplets appear on the glass of the spectacles, which are cleaned from the cloth.

During rainy days, the amount of humidity in the air increases due to which our clothes do not dry quickly.

Remember that there is moisture in hot air and cold air is always dry.

When hot air becomes dry, we call it Loo.

This air is so dry (thirsty) that it pulls water from your body and creates shortage of water.

On the contrary, we see that the air on the sea side is full of moisture.

This is why people living on the seashore sweat all day long.

Conclusion:

Class 7 Social Science Geography Chapter 4 “Air” is a difficult chapter for the students.

This chapter is full of important concepts.

 Lot of questions are asked from this chapter in competitive examinations.

First, read and understand the Chapter carefully before attempting the question and answers. 

It is better to go through the question and answer and try to write it in your own words after understanding.

Some important questions like doldrums, horse latitudes, why the plane flies in the stratosphere,

Ooty which is close to the equator and Delhi is far from the equator.

Still why Ooty os colder than Delhi? What is insolation?

Why are villages colder than cities, etc.

A question for all readers:

Mount Everest is about 9 km above the sea level.

Even the direct rays of the sun fall over it, yet it is always covered with snow. Why?


Class VII Geography Chapter 4 “Air” NCERT Explanation and Solutions

Exercise Questions

1. Answer the following questions.

(i) What is atmosphere?

(Ans) Our earth is surrounded by a huge blanket of air called atmosphere.

(ii) Which two gases make the bulk of the atmosphere?

Nitrogen (78%) and Oxygen (21%)

(iii) Which gas creates green house effect in the atmosphere?

Carbon di Oxide.

(iv) What is weather?

It is the condition of the atmosphere which includes temperature, rainfall, humidity) for a short period of time,

(v) Name three types of rainfall?

Three types of rainfall are:

i) Convectional rainfall

ii) Relief rainfall

iii) Cyclonic rainfall.

(vi) What is air pressure?

(Ans)  Air pressure is the pressure exerted by the weight of air on the earth’s surface.

2. Answer the following questions in one word:

(i) Which of the following gases protects us from harmful sun rays –  Ozone

(ii) The most important layer of the atmosphere is – Troposphere

(iii) Which of the following layers of the atmosphere is free from clouds? – Stratosphere

(iv) As we go up the layers of the atmosphere, the pressure – Decreases

(v) When precipitation comes down to the earth in the liquid form, it is called – Rain

3. Match the following.

(i) Trade Winds – Permanent wind  

(ii) Loo               – Local wind  

(iii) Monsoon   – Seasonal wind 

(iv) Wind           – Horizontal movement of Air   

4. Give reasons:

  1. Wet clothes take longer time to dry on a humid day.

Reason:

On a humid day the air is hot and humid (full of water vapor).

Hence, evaporation is very slow.

This is the reason why wet clothes take longer time to dry on a humid day.

  1. Amount of insolation decreases from equator towards poles?

Reason:

Our earth is spherical in shape.

As the direct rays of sun falls on the equator therefore, insolation is more.

As we move to the north, the rays of the sun get slanting so insolation is less.

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