Hi guys, today we are revising “atmosphere” – the heartbeat for our survival :-). So let’s start with its meaning.
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The word atmosphere has been derived from a Greek word “atmos” and “sphaira” which means “vapour” and “sphere” respectively. It is a gaseous envelope that completely surrounds the earth by the gravitational force of the earth. The presence of oxygen, greenhouse gases and the ozone layer sustain and protect life on Earth by providing the air we breathe, regulating temperature, and shielding us from harmful UV rays.
- Before heading further, keep in mind that among the frequencies of electromagnetic radiations, Visible light is abundantly emitted by the sun. On the other hand, the earth also emits light, but since it is much cooler than the sun it emits infrared light (which our eyes can’t see).
Extent of Atmosphere – It extends to about 1000 Km from the surface of the earth, but 99% of the total mass of the atmosphere is found within 32 Km.
Thickness of the Atmosphere – As compared to the thickness of the earth, the thickness of the atmosphere is very thin. It’s difficult to get an exact measurement of the atmosphere because it becomes very thin with height.
Composition of Atmosphere
Water Vapour (H2O)
- Water vapours are very important constituents of the atmosphere.
- Volume of water vapour contents in the atmosphere is 0-5%.
- About 90% of water vapours found in within 5 Km of height from sea level. At the height of 2 Km above sea level their quantity falls to half.
- It includes dust particles, salt particles, smoke particles, pollen grains, volcanic ashes etc.
- It helps in scattering of solar radiation and result produced colour of red and orange at the time of sunrise and sunset respectively
- Sky appears blue in colour because of scattering of solar radiation by dust particles.
Layers of the Atmosphere
There are different layers of the atmosphere that lie above the earth’s surface. These are explained as below:
Troposphere – It is the lowest layer of the earth’s atmosphere and extends to 14.5 Km height from the earth’s surface.
- Its height changes according to season. In summers it goes high and in winters it goes down.
- Almost all the weather phenomenon such as fog, cloud, frost, rainfall, hailstones, storms, winds, cloud thunder and lightning etc. occurs in this layer.
- Water vapours exist in this layer.
- In this, temperature decreases with height because the density of heat decreases with height, thus the heat is absorbed less. (At 165 m there is a drop of 1°c which is known as normal lapse rate). At 1000 m there is a drop of 5°c.
- The height where temperature stops decreasing is known as tropopause.
- The thickness of layer varies from 8 Km at poles and 16 Km at the equator.
- Being the lowest layer, it contains over 3/4th of earth’s atmospheric gas.
- The uneven heating of the regions of the troposphere by the sun (the sun warms the air at the equator more than the air at the poles) causes convection currents.
Stratosphere – It begins above the troposphere and extends to height 50 Km.
- It provides ideal conditions for flying large airplanes as this layer is free from clouds.
- Temperature increases with height due to the absorption of ultraviolet radiation by the ozone present in this layer. It is also known as inversion of temperature.
- The maximum concentration of ozone lies within 20 – 35 Km, which is known as ozonosphere.
- The end of the stratosphere (at about 50 Km) where the temperature begins to fall again is called the stratopause.
Mesosphere – It begins above the stratosphere and extends to the height of 85 Km.
- In this, temperature decreases with height and falls to about – 100°c at 80 Km.
- Meteors burn up in this layer.
- The end of the mesosphere is called mesopause.
- Same as the troposphere, it promotes vertical circulation (convection) that results from differences in density brought by differences in temperature.
Thermosphere – It begins above the thermosphere and extends to the height of 600 Km.
- In this layer temperature increases to 1700°c due to the absorption of X-rays and ultraviolet rays by the gas molecules present in this layer.
- Aurora (e.g. Northern lights) and satellites happen in this layer.
With this we have completed Today’s topic. Hope you have learned today.
We will post Quiz on Atmosphere in Today’s Cracking Session at 5 p.m.
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