The atmosphere is a mixture of a layer of gases enveloping the earth, held to it by gravitational force. Almost all the atmosphere (97 per cent) lies within 29 km of the earth’s surface.
Beyond about 100 km, recent data from satellites suggest that the lightest gases separate out, forming several concentric layers around the earth. The innermost of these is the nitrogen layer (between 100-200 km); then comes oxygen (200-1100 km); helium (1100-3500) and then hydrogen only, to which there is really no clearly defined upper limit.
Up to about 50 km the atmosphere is composed of:
- Nitrogen – 78 09%
- Oxygen – 20 95%
- Argon – 0.93%
- Carbon dioxide – 0.03%
- (Others are Neon, Helium, Ozone, Hydrogen etc.,)
Water vapour, besides being the immediate cause of condensation and precipitation, absorbs the insolation coming from the sun, reducing the amount reaching the earth’s surface.
Carbon dioxide is important for absorption of heat from the sun as well as from the earth. A high concentration of carbon dioxide leads to Greenhouse Effect.
Dust particles scatter and diffuse insolation and also act as hygroscopic nuclei for condensation (for the formation of clouds).
Layers of Atmosphere
- Layer nearest to earth’s surface. Thickness varies from 8 km at the poles to 16 km at the equator.
- All weather phenomenons occur here.
- Densest of all and contains water vapour, moisture and dust.
- Dust particles present in this layer hold the water vapour and contribute to the occurrence of twilight and the red colours of sunlight and sunset.
- In this, at every 165 m there is a drop of 1°c (or 6.4°c per km). This is called Normal Lapse Rate of Temperature.
- Tropopause separates troposphere from stratosphere.
- Extends from 16 km to 50 km ht. The temperature ceases to fall with the increase of height in this layer.
- Little weather is generated here as there is very little water vapour and virtually no dust present.
- Stratosphere provides ideal conditions for flying large airplanes.
- Contains ozone (25-30 km from earth’s surface), region being called Ozonosphere. It absorbs the ultra-violet rays from the sun. This layer has a comparatively higher temperature due to the absorption of ultra-violet radiation from the sun (temperature increases as we go up).
- Up to a height of about 80 km.
- In this, the temp decreases with height and falls to about -100°c at 80 km ht.
- Extends to about 500-600 km.
- Called so as it contains electrically charged particles (ions) that reflect the radio waves back to the earth thus making radio communication possible.
- Also protects earth from harmful radiation. This causes increase in temperature with height in this layer.
- It also protects earth from falling meteorites, as most of them burn out in this region.
- Here the earth’s gravity is extremely weak.
- Upper limit quite uncertain.
- The outer part is called Magnetosphere.
- The ionized particles increase in frequency with increasing heights. There are 2 belts in the upper atmosphere having a high concentration of ionized particles. They are known as Van Allen’s Radiation Belts. The inner belt lies about 2600 km from the earth’s surface, while the outer lies at about 13,000 to 19,000 km from it. These belts represent concentrations of highly charged particles, protons and electrons from the sun, trapped within lines of force of the earth’s external magnetic field- the Magnetosphere.
- The final boundary between the earth and the outer space is called ‘Magnetopause’.
The auroras are produced by the charged particles from the sun captured by earth’s magnetic field at heights of about 100 km. it is a luminous phenomenon seen in the sky at night in high latitudes.
It may be visible as arcs of light or as coloured curtains, streamers or rays. Auroras occur most frequently during the intense periods of the 11-year sunspot cycle.
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