I. Atmosphere and atmospheric layers

Definition

The Earth's atmosphere is a protective layer of gases that surrounds the planet, providing a habitat for life and playing a crucial role in sustaining ecosystems. It consists of distinct layers, each with its unique characteristics and significance. Understanding these layers is fundamental to comprehending how our atmosphere functions.

Salient Facts about Earth’s Atmosphere

I.1. The Atmosphere's Composition:

The atmosphere is primarily composed of nitrogen (about 78%) and oxygen (about 21%). Other gases such as argon (0.93 %), carbon dioxide (0.04 %), water vapor, and trace amounts of various gases are also present.

I.2. Layers of the Atmosphere:

The atmosphere is divided into five different layers depending upon the temperature conditions – troposphere, stratosphere, mesosphere, thermosphere and exosphere.

1.  Troposphere:

It is the lowermost layer of the atmosphere, where weather phenomena occur. All the climatic and weather changes take place in this layer of the atmosphere. It contains the air we breathe. The average height of the troposphere is 13 km; its height is about 8 km near the poles and about 18 km at the equator. At the equator, its thickness is greatest because heat is transported to great heights by strong convection currents. Temperature decreases with altitude; for every 165 m of height, the temperature decreases by 1℃ (normal lapse rate).

Tropopause is a zone that separates the troposphere from the stratosphere. The temperature in this zone is nearly constant (-80℃ over the equator and about -45℃ over the poles) and hence, it is called the tropopause. 

2.  Stratosphere:

It is the second layer of the atmosphere, just above the troposphere and extends up to a height of 50 km. This layer of the atmosphere contains the ozone layer which absorbs ultraviolet radiation from the sun and protects life from harmful forms of energy. The UV radiations absorbed by the ozone layer gets converted into heat, that is why the stratosphere gets heater with increasing altitude (unlike the troposphere). Weather-related phenomena are absent in this layer of the atmosphere, that is why aero planes fly in the stratosphere for a smooth ride.

Stratopause separates the stratosphere and mesosphere.

3.  Mesosphere:

Mesosphere is the third layer of the atmosphere which extends up to a height of 80 km. In this layer, temperature decreases with increasing altitude and drops down to minus 100℃ at the height of 80 km. Meteorites burn in this layer on entering the atmosphere from outer space. Its upper limit is mesopause which separates the mesosphere and thermosphere.

4.  Thermosphere:

The ionosphere lies within the thermosphere. It is located between 80 and 400 km above the mesosphere and contains electrically charged particles called ions, hence the name ionosphere. In this layer of the atmosphere, temperature increases with increasing height. Radio Waves transmitted from the earth are reflected back to the earth by this layer. Satellites orbit in the upper part of the thermosphere.

5.    Exosphere:

Exosphere is the outermost layer, transitioning into space. It is composed of very sparse gases and gradually merges with interplanetary space.

I.3. Importance Atmosphere’s layers for Life: