Class 7 Social Science Air Exam Notes Set 01

Class 7 Social Science Air Exam Notes Set A. Please refer to the examination notes which you can use for preparing and revising for exams. These notes will help you to revise the concepts quickly and get good marks.

Class 7 Social Science – Air (Exam Notes, Set A)

Introduction

1. The vast blanket of air that surrounds the earth is known as the atmosphere.
2. It is roughly estimated that the upper limit of the atmosphere reaches up to a height of about 10,000 km from the surface of the earth.
3. The total weight of the atmosphere is approximately 5000 million million tonnes.
4. The atmosphere is held close to the earth by the force of gravity.
5. About 50 per cent of the total mass of air is found within 5 km of the earth's surface.
6. It exists in several layers around the earth.
7. The density of air decreases as one goes to higher altitudes. So it is hard to breathe as one goes up.
8. The atmosphere protects us from the harmful effects of the sun's rays.
9. Life on earth depends on the atmosphere for its survival.
10. The atmosphere has the gases oxygen and nitrogen.
11. The atmosphere helps in retaining the warmth on the earth and also helps in the circulation of water vapour.
12. Without this vast blanket of air, we would be baked alive by the extreme heat of the sun during the daytime and would be frozen during the night.
13. The existence of the air around the earth has made it a unique planet, as the temperature on the earth is suitable for the growth of life.

Composition of the atmosphere

1. The atmosphere is a mixture of many gases, tiny dust particles, smoke and water vapour.
2. The composition is not fixed. It varies from place to place and from season to season. In the lower layers, the composition of dry air is uniform.
3. Nitrogen and oxygen are the two main gases, which account for about 99 per cent of the atmosphere.
4. Other gases present in the atmosphere include carbon dioxide, helium, ozone, argon and hydrogen.
5. Apart from the gases, the atmosphere also has varying quantities of dust particles and water vapour.
6. Nitrogen is the most abundant gas and accounts for about 78 per cent of the clean, dry air. Nitrogen, a colourless inert gas, dilutes the oxygen and slows down the process of oxidation. Plants need nitrogen for their survival. They cannot use it directly and obtain it from the soil.
7. Oxygen is the second most important gas in the atmosphere. Its share is about 21 per cent of the clean, dry air. It is found only in the lower layers of the atmosphere. All living beings need it for breathing. The share of oxygen in the atmosphere remains constant because plants produce oxygen during photosynthesis.
8. Carbon dioxide is needed by plants for their survival. Human beings and animals release carbon dioxide, and the amount released is about equal to the amount used by plants. This balance of carbon dioxide in the atmosphere is being disturbed by the increased burning of fossil fuels like coal and petroleum.
9. The amount of water vapour present in the atmosphere depends upon the temperature. Water vapour is added to the atmosphere by evaporation from oceans, seas, lakes, rivers and so on, by transpiration from vegetation, and by the respiration of animals. Water vapour is responsible for all forms of condensation and precipitation.
10. The atmosphere also has a large amount of fine dust particles, salt particles, pollen from plants, ash from volcanic eruptions, smoke from fires and so on. They are found in the lower layers only. These particles act as nuclei around which water vapour forms raindrops. They also scatter the sun's radiation during sunrise and sunset.

Constituents of air

Structure of the atmosphere

1. The atmosphere has a layered structure.
2. Each layer has different characteristics.
3. The upper layers press down on the lower layers.
4. Thus, the upper layers are less dense than the lower layers.
5. It is estimated that about 90 per cent of the total mass of the atmosphere lies within a height of about 20 km from the earth's surface.
6. On the basis of temperature and atmospheric pressure, the atmosphere can be divided into five layers starting from the earth's surface. These are the troposphere, stratosphere, mesosphere, thermosphere and exosphere.

Troposphere

1. This is the most important layer of the atmosphere. It is the lowest and the densest layer.
2. The height of the troposphere varies from about 8 km at the poles to about 18 km near the equator.
3. About three-fourths of the total mass of the atmosphere is found in this layer.
4. The air we breathe is available in this layer only.
5. In this layer, the temperature of the air decreases with an increase in height.
6. Almost all types of weather phenomena, like rainfall, fog, clouds and hailstorms, occur in this layer.
7. Water vapour and dust particles are found only in this layer.
8. This layer is heated more from below than from above.
9. The upper limit of the troposphere is known as the tropopause.

Stratosphere

1. This layer lies above the tropopause and extends up to a height of about 50 km from sea level.
2. The temperature in the lower part of this layer is mostly constant.
3. The air is generally thin, cold and dry.
4. Strong winds, known as jet streams, blow horizontally from west to east in this layer.
5. The stratosphere is almost free from weather phenomena, and its lower part is completely free from water vapour and dust particles.
6. Thus, the lower part of the stratosphere is ideal for flying jet aircraft.
7. The upper limit of the stratosphere is known as the stratopause.
8. An important feature of the stratosphere is the presence of ozone in its lower part. The ozone absorbs the incoming ultraviolet rays of the solar radiation. Thus, this ozone layer is essential for sustaining life on the earth. Scientists have observed a decline in the total global ozone. This can cause global warming.

Mesosphere

1. This layer lies above the stratosphere and extends up to a height of about 90 km from sea level.
2. The temperature in this layer decreases with height and reaches about −110°C near the upper limit.
3. It is the coldest layer in the atmosphere.
4. Meteors burn up in this layer on entering from space.
5. The upper limit of the mesosphere is known as the mesopause. From the mesopause onwards, the temperature increases.

Thermosphere

1. This layer lies above the mesosphere and extends up to a height of about 400 km from sea level.
2. In this layer, the temperature increases rapidly with an increase in height.
3. The ionosphere is a part of this layer. It contains electrically charged particles called ions. The ions reflect radio waves back to the earth's surface. This enables us to have wireless communication.

Exosphere

1. This is the uppermost layer of the atmosphere and lies between 400 km and 1500 km above the earth.
2. Here the air is very thin, as the main gases present are hydrogen and helium.
3. The temperature increases with height and may reach up to 5000°C.
4. This layer merges gradually with interplanetary space.

Importance of the atmosphere

The atmosphere is extremely significant for the growth and development of life on the earth.

1. The atmosphere protects us from the harmful ultraviolet rays coming from the sun.
2. It controls the extremes of temperature during the daytime and the night-time through the greenhouse effect.
3. It protects us from the meteors coming towards the earth from outer space.
4. Air has weight and exerts pressure. Being pressed down, it always remains in contact with land and water.
5. The change in weather conditions is exclusively due to the presence of the atmosphere.
6. Gases like oxygen and carbon dioxide have made life possible on the earth.
7. The ionosphere helps us in wireless and long-distance communication.
8. Sound waves can travel only through the air.
9. The energy of the sun has made the atmosphere dynamic.
10. Nitrogen is required to maintain the fertility of the soil and is vital for plants.
11. Oxygen is the gas that animals breathe in.
12. Carbon dioxide helps plants to produce food in the presence of sunlight. It also keeps the earth warm by absorbing the heat radiated from the surface. This is called the greenhouse effect.
13. The water vapour in the atmosphere causes precipitation.
14. The dust particles form the nuclei for condensation, which results in the formation of clouds that cause rainfall and snowfall.