The atmosphere is a thick layer of gases which surrounds the Earth from outer space and is always attached to the Earth’s gravitational force.
The atmosphere shields the sun’s ultraviolet rays, as well as protecting us from the showers of meteors. It contains life-forming gases like oxygen for creatures and carbon dioxide for plants.
The atmosphere allows the short waves to pass through it and becomes opaque for the outgoing long-wave terrestrial radiation. Hence, the atmosphere works as a huge greenhouse and maintains an average temperature of 15 degrees Celsius on Earth’s surface.
Layers of Atmosphere:
The atmosphere of Earth has various layers that vary from one another based on density and temperature. The density of the atmosphere is highest near the surface of Earth, and it decreases with altitude.
The habitable temperature is around 15 degrees Celsius in the lower atmospheric region near the surface, and the temperature becomes very high near the outer surface.
On the basis of uniformity, atmospheric layers can be divided into two categories:
Homosphere:
In this zone, uniform mixing and composition of gases and other constituents are found. Major gases like nitrogen, oxygen, argon, carbon dioxide, and water vapour are found with uniform distribution.
The troposphere usually extends up to 88 km from sea level. This zone includes the troposphere, stratosphere, and mesosphere.
The concentration of the gases remains constant in this zone due to convective currents and effects of heat and moisture on airflow patterns near Earth’s surface.
Heterosphere:
This layer extends to 88-10000 km, and includes the ionosphere and exosphere. Due to a lack of convective current, the materials found in the heterosphere are layered according to their mass.
The lower levels of the heterosphere are dominated by nitrogen and oxygen, but lighter gases like helium and hydrogen dominate the upper levels.
The lower level of the heterosphere consists of a zone of ionic particles, known as the ionosphere.
On the basis of thermal conditions, the atmosphere can be divided into six different layers:
Troposphere:
The troposphere is the lowest layer of the atmosphere with an average height of 13 km. It generally extends up to 8 km in poles and more than 18 km near the equator.
More than 75% of the total gaseous masses, dust particles, and water vapour of the atmosphere are found in this region.
The troposphere experiences a sharp decline in temperature with an increase in height, 1 degree Celsius per 165 m in height. This is known as the normal lapse rate.
All weather and climate-related phenomena are found in this layer. The troposphere is separated from the Stratosphere by the tropopause, which has almost similar temperature.
Stratopause:
The stratosphere extends beyond the troposphere around 50 km in height, without any atmospheric turbulence, so jet aeroplanes are flown in this air. However, the lower stratosphere rarely has cirrus clouds.
In this layer, air temperature gradually increases with height due to the absorption of UV rays through the ozone layer, but it never goes beyond 0 degrees Celsius.
The stratosphere is separated from the mesosphere (the next layer) by the stratopause, where the thermal condition is relatively stable. The lower portion of the stratosphere contains maximum ozone, hence it is known as the ozone layer.
Mesosphere:
The mesosphere is the coldest atmospheric layer that extends up to 80 km from the stratopause. Temperature in this layer decreases with altitude and becomes as low as -100 degrees Celsius.
Meteors burn like a shooting star in the mesosphere, and this layer is separated from the thermosphere by the mesopause, which is the uppermost layer of the mesosphere and homosphere.
Thermosphere:
The thermosphere extends beyond mesopause up to 640 km. The lower portion of the thermosphere contains free radicals and ionic particles and is known as the ionosphere.
In this layer, temperature increases with height due to the interaction of the ionosphere and electromagnetic radiation from outer space of the sun.
In the thermosphere, the International Space Station (ISS) is present. Also, low-Earth orbit spacecraft fly and astronauts conduct spacewalks in this layer.
Ionosphere:
In this layer, radiowaves transmitted from the Earth are transmitted back to the Earth, and the temperature starts increasing with height.
The ionosphere contains many thermal layers, such as the D-layer, E-layer, etc. Each layer is separated from the other by gradients, also known as a pause.
Exosphere:
It is the uppermost layer of the atmosphere, extending beyond 640km- 10000 km. Gases are almost absent here, and charged particles dominate this space.
Molecules in the exosphere have so much kinetic energy that they can escape the gravity and fly off into space. Helium disappears in this layer.
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