What Is Atmosphere?
The atmosphere is a thick gaseous envelope which surrounds the earth from all sides and is attached to the earth’s surface by gravitational force.
• The height of the atmosphere is estimated between 16 to 29 thousand kilometers from the sea level.
• It is estimated that 97% of the effective atmosphere is up to the height of 29km.
• The atmosphere is composed of gases, water vapour and particulates.
• It is the lower part of the atmosphere which has interested man from times immemorial, but from the beginning of the 21st century, when Aeroplanes and radio were invented, the knowledge of the upper part of the atmosphere became rather essential.
•Much help was received in this difficult task by weather balloons, Aeroplanes, rockets, sound and radio waves, satellites of various kind and sputnik etc.
Structure of Atmosphere
• The Earth’s atmosphere consists of a few zones or layers like spherical shells. The layered structure of the atmosphere has been classified on two major considerations &ndash
Thermal characteristics, Chemical characteristics
• On the basis of the characteristics of temperature and air pressure S. Petterson divided the atmosphere into five vertical zones e.g.
Troposphere,
Stratosphere,
Ozonosphere
Ionosphere
Exosphere
• It may be mentioned that majority of scientists have included Ozonosphere into Stratosphere and Ionosphere into Exosphere.
TROPOSPHERE
• The lower most layer of the atmosphere is known as troposphere which was named by Teisserence de Bort.
• Troposphere comes from the Greek word ‘tropos’ Meaning ‘mixing’ or ‘turbulence’. That is why sometime this zone is also called as Turbulent zone
• This layer is also known as Convective layer.
• Almost all of the weather phenomena occur in this layer.
• The average height of the troposphere is about 16km over the equator and 6 km over the poles.
• The height of troposphere changes from the equator towards the poles(decreases) and from one season to other season.
• Temperature decreases with increasing height at the rate of 6.50c per 1000metre.this is called normal lapse rate.
• The boundary of the upper limit of the troposphere is called Tropopause ( first used by Napier Shaw),which is about 1.5 km thick.
• The decrease of temperature stops at this point and hence it represents a cold point.
• The word tropopause means ‘where the mixing stops’.The turbulent mixing stops at tropopause.
• The air pressure at tropopause is only 100 milibars and about 250 milibars over the equator and poles respectively.
STRATOSPHERE
• The layer just above the tropopause is called stratosphere which was first discovered and studied by Teisserencede Bort in the year 1902.
• The average height of the stratosphere is taken to be 50 km.
• In this layer, temperature rises with increasing altitude. Because of absorption of ultraviolet solar radiation by ozone and lesser density of air.
• This layer is characterized by near absence of weather phenomena because of stable condition, dry air, concentration of ozone gas. Sometimes cirrus clouds known as ‘mother-of-pearl cloud’ appear in the lower stratosphere.
• The lower part of stratosphere is very important for life-forms in the biospheric ecosystem because there is concentration of ozone(O3) between the height of 15 &ndash 35 km.this portion of stratosphere is known as ozonosphere.
• The main culprit of ozone destruction are halogenated gases called Chlorofluorocarbons, Halons and Nitrogen oxide.
• Depletion of ozone would result in the rise of temperature of the ground surface and lower atmosphere.
• The upper limit of the stratosphere is known as Stratopause.
MESOSPHERE
• Meso means middle. It extends between 50 and 80 km.
• Temperature decreases with increasing height at a rate of 50c/ km.
• At the uppermost limit of mesosphere temperature becomes - 800c and may go down as low as -1000c to &ndash 1330c. This limit is called Mesopause.
• In this layer, Noctilucent clouds formed through the process of condensation in association with meteoric dusts and some moisture transported upward by convective mechanism, are noticed during summer season over polar areas.
• The layer is characterized by very low air pressure ranging between 1.0 milibar at 50 km altitude representing stratopause and 0.01 milibar at mesopause (at 90-100 km).
THERMOSPHERE
• It is above the Mesopause layer.
• Temperature increases rapidly with increasing height.
• It is divided into two layers :
I. Ionosphere
• It extends from 80 km to 640 km.
• There are many inonic layers in this sphere
D Layer reflects the signals of low frequency radio waves but absorbs the signals of medium and high frequency waves. This layer disappears with the sunset.
E layer also known as Kennelly &ndash Heaviside layer, reflects the medium and high frequency radio waves back to the earth.
SPORADIC E layer is associated with high velocity winds. This layer reflects very high frequency radio waves.
E2 layer is produced due to reaction of ultra-violet solar photons with oxygen molecules and this layer also disappear during night.
F layers consists of two sub layers F1 and F2 layers, collectively they are called ‘ Appleton layer’. This layer reflect medium and high frequency radio waves back to the earth.
G layer most probably persists day and night but is not detectable.
II. Exosphere
• It represents the uppermost layer of the atmosphere. The density becomes extremely low and the atmosphere resembles a nebula because it is highly rarefied.
• The temperature becomes 55680c at its outer limit.
• It may be mentioned that the atmosphere above ionosphere is called outer atmosphere having exosphere and magnetosphere.
• This zone is characterized by van allen radiation belts having charged particles trapped by earth’s magnetic field, aurora australis and aurora borealis.
• Aurora means dawn while borealis and australis mean northern and southern respectively.
• Aurora are cosmic glowing lights produced by a stream of electron discharged from the sun’s surface due to magnetic storm.
• Auroras are seen as unique multicoloured fireworks hanging in the polar sky during mid night.
This article is shared by Priyanka Duta. Priyanka is a Guest Lecturer of Geography at New Alipore College, Kolkata.
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