Modern View regarding the structure of Atmosphere

a) Homosphere:

• This is the lower part of the atmosphere which extends to a height of about 88 km.
• It is characterized by uniformity in composition.
• It is further subdivided into 3 sub-layers a. troposphere b. stratosphere and c. mesosphere with some transition zones such as tropopause,stratopause and mesopause.

b) Heterosphere:

• The atmosphere above the homosphere is not uniform in its composition, so called heterosphere.
• The heterosphere is also referred as the thermosphere as the temperature goes on increasing on outermost boundary of this layer due to the photochemical action of the ultraviolet solar radiation on rarefied air mass.
• In this sphere, the gases are said to be arranged into following four roughly spherical shells having its own distinctive composition.

a) Nitrogen layer: 200 km above earth’s surface molecular N.

b) Oxygen layer: Average ht. 1120km (atomic oxygen)

c) Helium layer: Average ht. 3520km (atomic oxygen)

d) Hydrogen layer: This layer is arranged according to the weight of the gases.