Climate – Composition and Structure of Atmosphere
The atmosphere of Earth is a layer of gases surrounding the planet Earth that is retained by Earth’s gravity. The atmosphere protects life on Earth by absorbing ultraviolet solar radiation, warming the surface through heat retention (greenhouse effect), and reducing temperature extremes between day and night (the diurnal temperature variation).
Composition of the Atmosphere
The atmosphere is composed of gases, water vapour and dust particles. Nitrogen, Oxygen, and Argon, which together constitute the major gases of the atmosphere. The proportion of gases changes in the higher layers of the atmosphere.
Water Vapour :
- Water Vapour is also a variable gas in the atmosphere, which decreases with altitude.
- Water vapour also decreases from the equator towards the poles.
- It also absorbs parts of the insolation from the sun and preserves the earth’s radiated heat.
- Water vapour also contributes to the stability and instability in the air
- Dust particles are generally concentrated in the lower layers of the atmosphere; yet, convectional air currents may transport them to great heights.
- Dust and salt particles act as hygroscopic nuclei around which water vapour condenses to produce clouds.
Structure of the Atmosphere
The atmosphere consists of different layers with varying density and temperature. Density is highest near the surface of the earth and decreases with increasing altitude.
The column of atmosphere is divided into 5 different layers depending upon the temperature condition. They are :
- The troposphere is the lowest layer of Earth’s atmosphere. It extends from Earth’s surface to an average height of about 13 km, although this altitude actually varies from about 8km at the poles to 18km at the equator.
- Thickness of the troposphere is greatest at the equator because heat is transported to great heights by strong convectional currents.
- This layer contains dust particles and water vapour.
- All changes in weather and climate take place in this layer.
- The temperature in this layer decreases at the rate of 1 degree Celsius for every 165m of height.
- This is the most important layer for all biological activity.
The Zone separating the troposphere from stratosphere is known as the tropopause. The air temperature at the tropopause is about -80 degree Celsius over the equator and about -45 degree Celsius over the poles. The temperature here is nearly constant and hence the name tropopause.
- The Stratosphere is found above the tropopause and extends up to a height of 50 km.
- One important feature of this layer is that it contains the Ozone layer. This layer absorbs ultra-violet radiation and shields life on the earth from intense, harmful form of energy.
- The Mesosphere lies above the stratosphere, which extends up to a height of 80 km.
- In this layer, Temperature starts decreasing with the increase in altitude and reaches up to -100 degree Celsius at the height of 80 km.
The upper limit of Mesosphere is known as the Mesopause.
- The Ionosphere is located between 80 to 400 km above the Mesopause.
- It contains electrically charged particles known as ions, and hence, it is known as ionosphere.
- Radio waves transmitted from the earth are reflected back to the earth by this layer
- Temperature here starts increasing with height.
- The uppermost layer of the atmosphere above the ionosphere is known as exosphere.
- Whatever contents are here, these are extremely rarefied in this layer and it gradually merges with the outer space.
- Here Temperature decreases with increasing height.