ATMOSPHERE: STRUCTURE AND COMPOSITION

The mass of air surrounding the earth up to a height of several hundred kilometers, held close to earth because of
the earth’s gravitational force is called the Atmosphere.

STRUCTURE OF THE ATMOSPHERE:

A. ON THE BASIS OF COMPOSITION:

1. HOMOSPHERE:

Almost uniform chemical composition. Divided into three major layers:
I. The troposphere
II. The stratosphere
III. The mesosphere.

2. HETEROSPHERE:

Non-uniform chemical composition, Consists of:
I. Ionosphere
II. The exosphere

B. ON THE BASIS OF THERMAL STRUCTURE:

TROPOSPHERE

• Thickness: 8 Km (At Poles), 16 km (At Equator) More in summer than in winter.
• Contains about three fourth of the total molecular mass of the atmosphere
• Accounts for practically the entire water vapour, all dust particles and most of the carbon dioxide contained in the atmosphere. Due to this, all weather phenomena such as condensation, precipitation and storms etc. occur in the troposphere.
• Lowest part of the troposphere is the warmest and the temperature falls with increasing height at a rate of about 1°C per 165 metres or about 6.4° C per km. It is called the lapse rate.
• The lapse rate is not uniform and generally it is higher in the lower part of the troposphere and tends to decline with increasing height so that the upper limit of the troposphere, the tropopause, is marked by the fall of lapse rate to zero.
  Tropopause seperates troposphere from stratasphere.
• This rate is higher over the zone of higher ground temperature and lower over the areas of lower ground temperature. The lapse rate is thus higher over the tropical zone and lower over the polar areas.

 

STRATOSPHERE

• Extends up to about 50 km, where Stratopause separates it from the Mesosphere.
• Contains most of the atmospheric ozone which absorbs the sun’s ultraviolet radiation. As a result the temperature rises in this layer from about –60°C at the tropopause to 0°C.
• The part of the stratosphere in which there is a concentration of ozone is often called ozonosphere. Due to absorption of ultraviolet radiation by ozone the temperature in this part of the stratosphere rises more rapidly.
• Free from dust particles etc. and also from atmospheric turbulences. Hence, considered ideal for flying  of jet aircraft.
• Rare clouds- Nacreous clouds, found.

MESOSPHERE

• Extends above the stratopause, up to a height of about 80 km.
• The temperature decreases with height and it falls from about 0° C at its base to about –100°C at 80 km
  height
• Coldest layer of the atmosphere.
• Known for the presence of Noctilutent clouds
• The upper limit of the Mesosphere is marked by the Mesospause, a transitional layer seperating it from the ionosphere.

IONOSPHERE

• Extends from 80 km up to 600 kms.
• Called ionosphere because it contains electrically charged ions that reflect the radio waves back to the earth thus making radio communications possible.
• Absorption of solar radiation by ionized particles causes an increase in temperature with increasing height in the ionosphere.
• Large concentration of ionized particles hence acts as a protective layer against meteorites etc. that are burnt in this layer.

THERMOSPHERE

• Zone between the 85 and 400 km above the surface
• Thermopause generally taken at an altitude of about 600 km
• Day temperatures at this altitude exceed 1400° C while night temperatures remain about 225° C.
• Upper part of the thermosphere contains only the lighter gases like helium and hydrogen.

EXOSPHERE

• Outermost part of the atmosphere of the earth
• Extends up to a height of about 9600 km
• The upper limit of the exosphere is taken as the outer limit of the atmosphere.
• The upper limit of the exosphere is uncertain as this layer acts as a transitional layer between the earth’s atmosphere and the space.

MAGNETOSPHERE

• Outer part of the exosphere
• VAN ALLEN BELTS: High energy protons and electrons trapped in a belt about 3200 km and at 2600 km above the
  equator. These two belts carrying ionized particles are called Van Allen Belts.

2. COMPOSITION OF ATMOSPHERE

NITROGEN

• 78.09% by volume
• Moderating influence on temperature and controls combustion.
• Important nutrient for all vegetative growth.

OXYGEN

• 20.95% by volume
• Important for all breathing organisms.
• Necessary for the process of decomposition of organic matter and helps in circulation of nutrients in the biosphere.

CARBON DIOXIDE

• 0.03 % by volume
• Important for absorption of heat from the sun as well as from the earth
• Major nutrient and plants require it for producing organic matter.
• Largest component of all plant organic matter.
• A high concentration in the atmosphere leads to Greenhouse Effect.

WATER VAPOUR

• 0.20-4.0% by volume
• Immediate cause of condensation and precipitation
• Absorbs the isolation coining from the sun thereby reducing the amount reaching the earth’s surface.
• Helps in containing the temperature on the earth within tolerable limits.

ARGON

• 0.93% by volume
• Chemically inert gas with few industrial uses

DUST PARTICLES

• Scatter and diffuse isolation
• Acts as hygroscopic nuclei for condensation.

OZONE

• Present in stratosphere, Absorbs the ultraviolet radiation from the Sun and keeps the level of isolation reaching the earth within tolerable limits.
• In the absence of ozone not only the temperature of the earth will rise, a lot of harmful radiation will also reach the earth surface thereby adversely affecting all forms of life.
• An increased amount of ultraviolet radiation may lead to a high incidence of diseases like skin cancer