The Atmosphere class 7 notes

 Atmosphere

The earth is a unique planet because it is only planet having life on it. Its capacity to support life makes it different from all other planets. The earth is surrounded by an envelope of air which is called the atmosphere. The atmosphere extends to about 1,000 km. from the surface of the earth. But 99% of the total mass of the atmosphere is found within 32 km. The atmosphere is held to the earth by the force of gravity and it is made up of a mixture of gas molecules. A molecule is the smallest particle of a substance in a free state. Gas molecules move randomly in all directions.

COMPOSITION OF THE ATMOSPHERE :

Air, generally used as synonym for atmosphere, is not a specific gas but a mixture of gases, mainly nitrogen and oxygen.

 Nitrogen amounts to nearly 78 per cent of the volume of air while oxygen makes up about 21 per cent. The remaining one per cent of air includes all the remaining gases found in the atmosphere. All components of

 air are important to us. Oxygen is the 'life breath' as all living beings breathe in oxygen. Nitrogen helps in the growth of living organisms especially the plants. Carbon dioxide and water help plant growth. Rain and snowfall which provide most of the water to the plants come from the water vapours 

The four major layers of the atmosphere of the earth are : 

1. Troposphere 2. Stratosphere 3. Mesosphere 4. Thermosphere  

1. Troposphere : Troposphere is the lowest and the densest layer of the atmosphere. It extends upto a height of 16 kilometres over the Equator and about 8 kilometres over the Poles. The average height of this layer is taken as about 11 kilometres. About 80% of the total mass of the atmosphere is contained in this layer. Almost all dust particles and water vapours are contained in it. Thus all weather phenomena occur in this very layer. The temperature in this layer falls with increasing height.

. Stratosphere : The second layer of the atmosphere is called stratosphere, which extends from an average altitude of 11 to 50 kilometres. Most of the ozone of the atmosphere is contained in this layer. Meteors entering the earth's atmosphere generally get burnt in this layer. Long distance aircrafts fly in the lower part of the stratosphere so as to take advantage of the lack of air resistance. 

3. Mesosphere : Mesosphere is the third layer of the atmosphere, occurring between the stratosphere and the thermosphere. It lies between 50-80 km above the earth's surface. In the mesosphere, temperature falls with elevation. It is the coldest of the atmospheric layers. It is also the layer in which a lot of meteors bum up while entering the Earth's atmospehre.

 4. Thermosphere : It is the fourth layer of the atmosphere. It extends from 80-600 km above the earth's surface. It is located highest above earth's surface. Because there are relatively few molecules and atoms in the thermosphere, even absorbing small amount of solar energy can significantly increase the air temperature, making it the hottest layer in the atmosphere. It is divided into two parts

i) Ionosphere : This layer reflects the radio signals back to the earth thus making radio communications possible. 

(ii) Exosphere : The upper portion of the thermosphere is called exosphere. The atmosphere acts as a filter to the radiation coming from the sun. 

The ozone layer

The ozone layer forms a thick layer in stratosphere, encircling the earth, that has large amount of ozone in it. It protects our planet, i.e., Earth from the harmful radiations (Ultraviolet Radiations) that comes from the sun. The ozone layer has the capability to absorb almost 97-99% of the harmful ultraviolet radiations that sun emits and which can produce long term devastating effects on human beings as well as plants and animals.

 During the last several decades excessive release of Chloroflurococarbons (CFC's) have caused a severe damage to the ozone layer. CFC are used in spray cans, refrigerators, air conditioning systems and some fire extinguishers. Other ozone damaging chemicals are pesticides such as methyl bromide, halons, etc.

As already studied the Earth is wrapped in a blanket of air called the 'atmosphere', which is made up of several layers of gases. The sun is the main source of heat which radiate energy that travel through the atmosphere and reach the Earth. The rays of the sun warm the Earth, and heat from the Earth then travels back into the atmosphere. Several gases in the earth's atmosphere such as carbon dioxide, methane, water vapours work like a blanket to retain much of this heat.  

These gases are called greenhouse gases and the natural process between the sun, the atmosphere and the Earth is called the 'Greenhouse Effect'.

 To make sure that the Earth's temperature remains constant, the balance of these gases in the atmosphere must not be upset.

However many human activities like burning fossil fules, cutting of trees, etc., have contributed huge amount of additional greenhouse gases into the atmosphere, leading to global warming. 

Ways to Reduce global warming

To put a serious dent to global warming, we need to adopt certain preventive measure involving individual, community, country and even at international level.

- Plant more trees. 

- Ban of use of Chlorofluorocarbons. 

- Use of energy efficient vehicles. 

- Using alternative energy resources. 

- Following principle of 3R, i.e., Reuse, Recycle and Reduce.