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The atmosphere 
The air envelope that surrounds the earth is known as the atmosphere. It extends to a height of several kilometres from the surface of the earth. The atmosphere is held to the earth by gravitational force. 
Gases, moisture and dust particles are the major constituents of our atmosphere that blanket our earth. It is the gravitational attraction of the earth that holds the atmosphere close to the earth. The presence of an atmosphere extends to about 10000 kilometres from the earth's surface. But it is estimated that about 97 per cent of the atmospheric air remains within 29 kilometres from the earth's surface. The gases get rarefied with increasing altitude.

condensation nuclei
It is the water vapour and the dust particles in the atmosphere that help the formation of clouds and different forms of precipitation. Water vapour condenses to form clouds and fog. For the water vapour to condense, the dust in the atmosphere becomes the condensation nuclei. These are called aerosols.

Atmosphere and Temperature

The earth receives energy from the sun. The radiation coming from the Sun is called insolation. How does the atmosphere become hot?
The sun's rays reach the earth as short waves. This does not heat the atmosphere directly as it passes through. But the dust particles, the water vapour and other materials in the atmosphere directly absorb the heat of the sun's rays. The earth absorbs solar energy. Due to this, the earth's surface gets heated. The earth thus heated, emits out the heat as longwave radiation. This is called Terrestrial Radiation. The air particles that lie next to the surface of the earth get heated due to this terrestrial radiation. Through the processes like conduction, convection, advection and radiation the atmosphere gets heated up.

Processes of heat transfer in the atmosphere
Heat is transferred to that part of the atmosphere close to the hot surface of the earth.
Heated air expands and rises up.
Horizontal transfer of heat by the wind.
Terrestrial Radiation
Heat is transferred from the surface of the earth to the outer space in the form of long waves.

Heat budget
If the total radiation from the sun reaching the earth is considered to be 100 per cent, 35 per cent of it returns to the outer space after getting reflected from different regions of the atmosphere. Clouds and dust particles in the atmosphere absorb 14 per cent. Only the remaining 51 per cent reaches the earth's surface. This heats the surface following which it returns to the outer space as longwave terrestrial radiation.
How Do You Measure the Atmospheric Temperature?
Sometimes, we suffer from a high fever. We check our temperature then, with a thermometer. In the same way, we can measure the atmospheric temperature also. For this, a different type of thermometer is used. The thermometer displays the temperature in unit Degree Celsius (°C) and Degree Fahrenheit(°F).

The maximum intensity of the sun's rays is at 12 noon. But the maximum temperature of the atmosphere is felt at about 2 in the afternoon because it takes about two hours to heat the atmosphere by the terrestrial radiation. That is why the maximum temperature of the day is recorded at 2 in the afternoon. Similarly, atmospheric temperature decreases very slowly. It is only after 4 in the morning that the atmospheric temperature falls. The minimum
the temperature of a day is recorded at 5 in the morning. The difference between the maximum temperature and the minimum temperature in a day is called the diurnal range of temperature. The daily mean temperature is calculated by the formula
Maximum temperature in a day + minimum temperature/2

Maximum-minimum thermometer
This is the instrument for measuring the maximum and minimum temperature in a day. Here the two thermometers are connected using a U-shaped glass tube. The mercury-filled in the maxi-mum thermometer expands with the rise in temperature and pushes up the metal indicator. The indicator remains at the position showing the maximum temperature of the day. Thus the maximum temperature can be read at any time during a day. The minimum thermometer has alcohol-filled above the indicator. When the temperature falls, the indicator is pushed up as the alcohol contracts. The minimum temperature can be read from the position of the indicator at any time.

The temperature distribution map can be prepared based on the temperature recorded at different places. The imaginary lines connecting places having equal atmospheric temperature are called isotherms.

You know that temperature varies from place to place on the earth's surface. Let us find out the reason behind this.
Distribution of Temperature on the Earth's surface
The distribution of temperature on the surface of the earth will not be the same everywhere at all times. There are many factors that affect such distribution.  Let us see what they are.
• Latitude- At the Equator, where the sun's rays fall vertically on the earth, the temperature will be more. But on higher latitudes, the temperature will be less as they receive inclined rays of the sun. The temperature is less in the higher latitudes as the slanting rays of the sun spread to a larger area than the vertical rays. 

• Nearness to the sea-  Two places on the same latitude, one close to the sea and the other far from the sea will not have the same temperature. Places far away from the sea will have a lower temperature in winter and higher temperature in summer. But the places near the sea will have a moderate temperature due to the influence of the sea.

•  Ocean Currents:  Ocean currents influence the temperature of the coastal regions. The warm water that flows to the cold regions enhances the temperature in the areas close to the sea. Similarly, the cold currents that flow to the hot regions reduce the heat in those areas.

• Winds:  Like the ocean currents, the winds also influence the temperature of a place. While the hot winds raise the temperature of a place, the cold winds reduce the temperature. For example, the hot dry wind 'Loo' which blows over the plains of north India in summer, causes an increase in temperature there. But the extremely cold polar winds that blow from the northern polar region greatly reduce the temperature in Northern China.

• Altitude: Two places on the same latitude, one at sea level and the other at height, will not experience the same temperature. As the height above sea level increases, the temperature decreases. Why do people go to places like Ooty, Kodaikanal, Munnar etc during the summer? On mountains about 5000 meters above sea level, it is common to experience cold and snowfall. When you are going to a hill station from the low altitudinal regions you will be able to experience a decrease in temperature as you go up.

Structure of the Atmosphere
From the earth's surface upwards, there is a change in temperature. Based on this change in temperature the atmosphere has been divided into different layers.
The composition of the atmosphere is somewhat uniform up to about 90 Kilometres from the surface of the earth. This part of the atmosphere is called homosphere. Beyond this there is no uniformity in the gaseous composition. So the part of the atmosphere beyond 90 Km from the earth is called as heterosphere. Based on the temperature at different levels, the atmosphere can be divided into different layers. Each layer in the atmosphere has its own indigenous characteristics.

Troposphere:-  The atmospheric layer close to the earth's surface is the troposphere.
The word 'troposphere' is derived from the Greek word 'tropos' which means 'to turn'. Air is very turbulent in this region.
According to the latitude and seasons, the height of the troposphere varies. At poles, it is about 8 kilometres and at the equator, it is about 17 kilometres., The height of the troposphere is more in the regions where the amount of heat received is more and it is less in regions which are colder. In the tropics, convection currents of air extend to greater heights, whereas in cold regions, convection currents are limited to lower heights. The differences in the altitudinal extent of convection currents cause the change in the height of the troposphere. It is now clear that the extent of the troposphere is influenced by the changes in temperature. The temperature in the troposphere decreases steadily with increasing
height. That is, for every 165 metres of height, temperature decreases by 1-degree celsius. This is called the normal lapse rate. 
Troposphere plays a vital role in shaping human life on earth. The phenomena like clouds, rain, snow, thunderstorms etc. are found only in the troposphere. Water vapour and dust particles in the air are also found only in this layer of the atmosphere. 
Above the troposphere is the tropopause where the temperature is steady. This is the transitional zone between the troposphere and stratosphere.
Important facts
* This layer extends up to about 90 Km from the surface of the earth and lies close to the earth.
* The height of the troposphere is more in the equatorial regions because of strong convection here. (about 18 Km)
* Almost all the weather and climatic phenomena such as cloud formation, rain, snow, wind, thunder and lightning etc. take place in this layer.
* In the troposphere the temperature decreases at a uniform rate of 1°Celcius for every 165 metres of altitude. This is called Normal Lapse Rate.
* The zone of transition above the troposphere is called the tropopause. 
Transition zones
Transition zones are the parts of the atmosphere separating the major atmospheric layers.
Tropopause, stratopause and mesopause are the respective transition zones between troposphere, stratosphere, mesosphere and thermosphere.

Stratosphere:- In the lower part of the stratosphere, the temperature is constant. This means that as the height increases, the temperature does not vary. For about 30 kilometres from the surface of the earth, the temperature remains steady. Then up to 50 kilometres, it increases. The ultraviolet rays of the sun absorb the oxygen molecules and split them into oxygen atoms which aid the formation of ozone in this part of the atmosphere. As a result, latent heat is released. This causes a rise in temperature in this part. The ozone that is produced in this layer is known as the ozone layer which protects the earth. The transitional zone above the stratosphere is stratopause.
Jet planes that fly very high create a white line in the sky. This is because when the
planes travel in the stratosphere, the steam they release condenses and forms a white line. Since there is less movement of air in this sphere, the line is visible to us for a longer time.

Ozone formation
At an altitude of about 20 to 50 km in the atmosphere, the ultraviolet rays from the sun split up ordinary oxygen molecules to single atom oxygen molecules. This single atom oxygen molecules react with ordinary oxygen molecules to form tri atomic ozone gas. This process is called as Ozonisation.

Important facts
* Extends up to a height of about 50 Km from the earth and lies above the tropopause.
* In the lower parts of the stratosphere there is no change in temperature with altitude. This zone is called an isothermal zone. Beyond this, there is an increase in temperature with altitude.
* Ozone layer which forms part of this layer absorbs the harmful ultraviolet rays from the sun and prevents it from reaching the earth.
* Allows the free movement of jet aircraft through clear atmospheric conditions and absence of air gutters.
* The zone of transition above the stratosphere is called stratopause.

Mesosphere:- The atmospheric layer that extends from 50 km to 80 km above the sea level is the mesosphere. As the height increases, the temperature in the mesosphere decreases. In the upper part of the mesosphere, the temperature decreases from -83 degree and  -100 degree Celsius. The transitional zone above this where the temperature
remains steady is called the mesopause. 
Important facts
* Extends from about 50 to 80 Km altitude from the earth.
* Temperature decreases with altitude and the lowest temperature of the atmosphere is felt at the mesopause. ( -80°C to -100°C)
* The meteors on reaching the mesosphere get burnt out due to friction. The zone of transition above the mesosphere is called mesopause.
The stratosphere and the mesosphere are sometimes collectively referred to as the "middle atmosphere", which spans altitudes between approximately 10 and 100 kilometres above the Earth's surface. 

Thermosphere:-This is an area where the temperature increases abruptly. The part of the thermosphere, from about 80 kilometres to 400 kilometres, above the earth surface is called the ionosphere. Due to the ionization process, the temperature in this part of the atmosphere increases considerably. Ionization occurs more during the day and the zone is inactive during night. The short waves in the electromagnetic spectrum, like X - rays cause ionization. Ionosphere reflects radio waves. During day times the ionosphere is well developed and hence it reflects radio waves effectively. As the height increases, the temperature in the thermosphere also increases.
At an altitude of about 80 to 400 Km in the atmosphere the intense solar rays such as
Ultraviolet, X-rays etc. react with gaseous molecules to form ions.  This process is called ionization and this part of the atmosphere is called the ionosphere. Ions conduct electricity. As the radio waves are electromagnetic waves this layer is made use for long-distance radio transmission.
Important facts
 * Extends from about 80 to 600 Km altitude.
 * There is a considerable increase in temperature with altitude.
 * The lower part of the thermosphere is known as the ionosphere. Ionosphere helps in the transmission of radio waves.
The International Space Station orbits the Earth within the middle of the thermosphere, between 408 and 410 kilometres (254 and 255 mi).
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