COMPOSITION AND STRUCTURE OF ATMOSPHERE
Introduction
The word Atmosphere is derived from two Greek words ‘Atmos’ and ‘Sphaira’.
Atmos means ‘vapor’ and Sphaira means ‘sphere’.
Atmosphere is the name given to the thin layer of gaseous matter encircling the earth as an envelope or a blanket.
The Gravitational force of the earth is responsible for the existence of the atmosphere.
The stabilization of the atmosphere in its present form took place in the Cambrian period (About 600 million years ago).
All scientific evidence till the present day has indicated that it is only the earth which has atmosphere containing the gases required for all known forms of life like plants and animals.
At the same time, the atmosphere has protected the life on the earth’s surface by absorbing dangerous ultraviolet (UV) rays of the Sun and it has kept the earth warm enough by trapping the temperature.
COMPOSITION OF ATMOSPHERE
Atmosphere is composed of three elements such as gases, water vapour and the dust particles.
Water vapor is added to the atmosphere through the process of evaporation.
The amount of water vapor varies from one place to another place, as well as from one season to another season.
The dust and other microscopic solid particles of the atmosphere are known as ‘Aerosols’. They are confined only to the troposphere.
The main gases and their proportions have been shown in the below figure.
A unit of mass of dry air is made up of 78.08 percent Nitrogen (N2), 20.94 percent of oxygen (O2), 0.93 percent of Argon (A), 0.03 percent of Carbon dioxide (CO2) and smaller proportions of rare gases like Helium, Neon, Methane and Hydrogen.
Composition of Gases
Gases | Percentage |
Nitrogen (N2) | 78.08% |
Oxygen (O2) | 20.95% |
Argon (Ar) | 0.93% |
Neon, Helium, Krypton | 0.0001% |
Carbon dioxide (CO2) | 0.038% |
STRUCTURE OF ATMOSPHERE:
The distribution of temperature is not uniform at different heights of the atmosphere. Along with the variation of temperature, there are unique features at different heights based on these characteristics atmosphere is divided into five concentric zones namely
Troposphere
Stratosphere
Mesosphere
Ionosphere or Thermosphere
Exosphere
The First two layers are called Homospheres as the gases are highly mixed-up.
In the upper two layers the gases are in the form of separate layers. So they are called
1. TROPOSPHERE
It is the lowest layer of the atmosphere and lies closer to the earth.
Its average height is 13 km and extends roughly to a height of 8 km near the poles and about 18 km at the equator.
It is thickest at the equator because strong convection currents transport heat to such great heights. It contains 75 percent of the total gaseous mass of the atmosphere.
This layer contains dust particles and water vapor also. The temperature in this layer decreases at the rate of 1°C for every 165m of height (or at a mean rate of 6.5 degree C /km).The decrease occurs because air is compressible and its density decreases with height allowing rising air to expand and thereby cool.
It is interesting to note that the lowest temperature in the entire troposphere is found over the equator and not at the poles. The air temperature at the top of troposphere is about minus 80°C over the equator and about minus 45°C over the poles.
Word ‘troposphere’ is derived from the Greek word ‘tropos’ means ‘mixing’.
All changes in climate and weather take place in this layer. Clouds formation, thunderstorms etc. occur in this layer. Wind velocity increase with height and attain the maximum at the top.
At the top of the troposphere there is a shallow layer separating it from the next thermal layer of the atmosphere. This is known as tropopause.
2. STRATOSPHERE
The stratosphere is found above the tropopause.
The stratosphere ranges from 19 to 50 km above the Earth’s surface on the equator.
The temperature in the stratosphere varies from -57o C at the tropopause to 0o
This layer is free of any clouds of weather changes. It is an ideal place for flying of big planes. At about 50 km, the temperature begins to fall. This is the end of stratosphere, and is called the stratopause.
The sphere is characterized by the presence of the ozonosphere or ozone layer.
Ozone is highly reactive oxygen molecule made up of three oxygen atoms (O3).
3. MESOSPHERE
The mesosphere lies above the stratopause, and extends up to a height of 50 km to 80km.
In this layer, once again, the temperature starts decreasing with the increase in altitude and reaches up to minus 100° C at the height of 80 km.
It is the coldest layer in the atmosphere.
The exact upper and lower boundaries of the mesosphere vary with latitude and with season, but the lower boundary of the mesosphere is usually located at heights of about 50 km above the Earth’s surface and the mesopause is usually at heights near 100 km.
In summers, the height of the mesosphere descends down to 85km at middle and high latitudes. The upper limit of mesosphere is known as the mesopause.
4. IONOSPHERE / THERMOSPHERE
The thermosphere is located between 80 and 400 km above the mesopause.
In this layer the temperature increases rapidly with increase in height. It is estimated that the temperature reaches 1500 degree C.
The air is so thin that a small increase in energy can cause a large increase in temperature. Because of the thin air in the thermosphere, scientists can’t measure the temperature directly.
The Earth’s thermosphere also includes the region called the ionosphere. It contains electrically charged particles known as ions, and hence, it is known as ionosphere.
Ionization of molecules and atoms occurs mainly as a result of ultra-violet, x-rays and gamma radiations. The high temperatures in the thermosphere also cause molecules to ionize. This is why an ionosphere and thermosphere can overlap.
This layer also protects the earth from meteorites and remains of abandoned satellites. They are burned and reduced to ashes due to high temperature as they enter this layer.
It also forms Aurora or Natural bonds of light. They are Aurora Borealis and Aurora Australis.
Aurora Borealis
Aurora Borealis is a phenomenon of coloured lights seen in the sky in the Northern Hemisphere – 66½º North to 90º North.
Aurora Australis
A similar phenomenon in the Southern hemisphere is called Aurora Australis – 66½º South to 90º South.
5. EXOSPHERE
The uppermost layer of the atmosphere above the thermosphere is known as the exosphere.
This is the highest layer but very little is known about it. It lies beyond 400km to 1000s of kms where it merges with outer space.
At such great height the density of atoms is extremely low.
It is largely home to Helium and Hydrogen. Temperature increases with height and may cross 50000 o
The gravity of the Earth is too weak in this layer.
Magnetosphere is found above this layer.