Structure of the atmosphere

• According to Peterson, the atmosphere is divided into the following most significant spheres.

i) Troposphere

ii) Stratosphere

iii) Ozonosphere

iv) Ionosphere

v) Exosphere

I. Troposphere (0-16km):

• Lowermost part of atmosphere
• It contains about 75% of the total gases mass of atmosphere and practically all the moisture and dust particles.
• The average height of this lowermost layer of the atmosphere is placed at about 14 km above sea level. Under normal conditions, the height of the troposphere at the poles is about 8 kilometers, while at the equator it is about 16 kilometers
• Troposphere is marked by turbulence and eddies. It is also called connective region. Various types of clouds, thunderstorms as well as cyclones and anticyclones occur in this sphere because of the concentration of almost all the water vapour & aerosols in it.
• Wind velocities increase with height and attain maximum at the top.
• The most important feature is decrease in temperature with increasing elevation up to 14km, at the mean lapse rate of about 6.5oc per km.
• Tropopause is a shallow layer separating troposphere from the next thermal layer of the atmosphere i.e., stratosphere. The height of the tropopause is about 1 to 2 km
• Tropopause is a transition zone, and above which the temperature increases slightly with increasing elevation. This is the isothermal layer forming a barrier between the troposphere and stratosphere.

II. Stratosphere (16-30 km):

• Lower stratosphere is isothermal in character, however, there is a gradual temperature increase with height beyond 20 km ( i.e. upper stratosphere) (temperature inversion)
• Above tropopause, no visible weather phenomenon occurs, however, cirrus clouds, called the mother of pearl clouds (high speed circulating cloud) occasionally form in lower stratosphere.
• Thickness of stratosphere is highest at the poles
• The upper boundary of the stratosphere is called the stratopause. It is the transitional zone of maximum temperature between the stratosphere and the mesosphere
• Above this level, there is a steep rise in temperature.

III. Ozonosphere (Mesosphere) (30-60km):

• There is maximum concentration of ozone.
• Ozone layer acts as a filter for the UV rays of sun which is harmful to the flora and fauna on earth.
• However, the emission of nitrogen oxide causes deterioration of ozone layer with the resultant serious damage to flora and fauna.
• Here, temperature increases with height at the rate of 5oC/km.
• Because of the preponderance of chemical process, this layer is also called chemosphere.

IV. Ionosphere (60 km and above):

• Regions of Earth’s atmosphere in which the number of electrically charged particles—ions and electrons—are large enough to affect the propagation of radio waves.
• In this layer, ionization of molecules and atoms occurs mainly as a result of ultraviolet, x-ray and gamma radiation.
• Above ozonosphere, the temperature falls again reaching a minimum of about -1000C at a height 80 km above earth’s surface.
• Beyond this level, the temperature increases again due to the absorption of short-wave solar radiator by the atoms of O & N in this ionosphere and temperature could reach more than 1000 ⁰

Ionosphere consists of many layers

a) D-layer (60-89km) reflects low frequency radio wave but absorb medium & high frequency radio waves.

b) E-layer (90-130km) is also called Kennelly- Heaviside layer which reflects medium and high frequency waves. It does not exist at night.

c) E1 Sporadic –layer (110 km) occurs under special circumstances. This affects very high frequency radio waves.

d) E2-layer (150km) appears at day time and vanish at the sunset, produce by UV-photon acting on oxygen molecule.

e) F1-layer play important role in long distance radio communication. It reflects medium and high frequency radio waves. And it disappears at night.

f) F2- layer (150-380km) also play important role in long distance radio transmission and characterized by diurnal as well as seasonal variability. It appears as directly related to sunspot activity. Its maximum development occurs shortly after local noon and during the middle of the winter.

g) G-Layer (400 km and above) is explored recently, functioning or showing same characteristics as F-layer but interaction of UV photon with nitrogen atoms results free electron in this layer.

V. Exosphere (400-1000 km):

• The atmosphere in this region is so rarefied that it resembles a nebula.
• Hydrogen and helium gas are predominant in this outermost region.
• At outermost boundary of our atmosphere, the kinetic temperature may reach around 5,680 ⁰C

VI. Thermosphere (90 to 600 km):

• The thermosphere includes the exosphere and part of the ionosphere.
• In the thermosphere, temperature increases with altitude.