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CBSE Class 7 Social Science - The Composition and Structure of the Atmosphere. Learning the important concepts is very important for every student to get better marks in examinations. The concepts should be clear which will help in faster learning. The attached concepts made as per NCERT and CBSE pattern will help the student to understand the chapter and score better marks in the examinations.
Topic: The Composition and Structure of the Atmosphere
The word atmosphere is derived from Greek words atoms, meaning ‘vapour’ and sphaira, meaning ‘sphere’. The atmosphere is held in place by the gravitational pull of the earth. Its
density is the highest near the surface of the earth and goes on decreasing with height. The atmosphere extends upto a height of 16,00km above the mean sea level.
COMPOSITION OF ATMOSPHERE:
Air is a mixture of gases like nitrogen, oxygen and carbon dioxide.
Nitrogen makes up the bulk of air, constituting 78°/ of air volume. Oxygen constitutes 21% of air volume. other gases such carbon dioxide, argon, helium, ozone, hydrogen, etc., make up just 1 of the total volume of air. Nitrogen helps in the growth of living organisms. They get nitrogen from the bacteria that live in soil and their roots. These bacteria convert atmospheric nitrogen into a form that plants can use.
Oxygen: The second most abundant atmospheric gas, oxygen, is the breath of life. Humans and animals take in oxygen from the air they breathe. Green plants release oxygen into the air during photosynthesis. Oxygen is constantly being used up by humans and animals, and replenished by plants.
Carbon dioxide plays an important role. It traps heat from the sun and keeps earth warm enough to sustain life, it is this gas that green plant s use to make food and release oxygen. In the presence of sunlight plant takes in carbon dioxide and converts it into starch by the process of photosynthesis. While plants absorb the gas, humans and animals release carbon dioxide into the air when they breathe. Therefore, the amount of this gas in air also remains nearly constant.
Ozone is also an important gas as it protects us from the harmful ultraviolet rays of the sun.
Besides gases, air also contains water vapour and dust particles. The amount of water vapour and dust particles in the air varies with time and place. Water vapour plays an important role in determining the weather conditions. There would be no rainfall or snow fall in the absence of water vapours.
Dust particles help condense water vapour into water droplets to form clouds which eventually cause rainfall.
STRUCTURE OF ATMOSPHERE
Atmosphere is divided into five layers troposphere, stratosphere, mesosphere, thermosphere and exosphere.
1) The word ‘troop’ is taken from a Latin word which means ‘change’. This is the sphere in which changes related to weather conditions occur.
2) This is the lowest layer of the atmosphere and also the most important.
3) It protects us from the heat of the sun during the day and keeps the earth warm at night.
4) The troposphere is the densest layer of the atmosphere and its thickness varies from 8 km over the poles to 18 km over the equator.
5) In this layer, temperature decreases with height. The rate at which the temperature drops is 1ºC per 165m. This is also called the lapse rate.
6) All weather phenomena such as clouds, rain, storms, etc.occur in this layer.
7) The boundary between the troposphere and the stratosphere is called the tropopause
1) This layer extends up to 50 km above the surface of the earth. In this layer, there is no disturbance.
2) The layers of air are almost straight or horizontal and undisturbed, thus the name’strato’, which means straight.
3) There are no clouds, no weather changes and no disturbing elements. This is the layer in which the air travel is the safest. There are no bumpy pockets of air, therefore, jet aircraft usually fly in this sphere.
4) This layer has a band of ozone gas which protects us from the sun’s harmful ultraviolet rays.
5) The stratopause, the upper layer, separates it from the mesophere. The temperature remains more or less steady in its lower parts. But in the upper part, the temperature increases steadily to reach 0ºC at the stratopause.
1) This layer extends up to a height of 80 km and lies above the stratosphere.
2) Here the temperature decreases with height till it reaches 100ºC at the end of mesophere.
3) This layer has the lowest temperature in the atmosphere.
4) The mesopause, the upper part of this layer, separates it from the ionosphere.
1) The ionosphere contains electrically charged particles called ions, which help in transmitting communication signals. Radio waves are sent back to the earth from this layer
2) Communication satellites help in transmitting the signals back to earth.
3) The layer extends from 80 km to about 500 km.
1)This is the outermost layer of the atmosphere in which the air thins until it ultimately merges into outer space.
2) The temperature increases rapidly in this layer because of solar radiation.
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