Physical Geography - XI (NCERT)

Air is essential to the survival of all organisms. 

Atmosphere is a mixture of different gases and it envelopes the earth all round.

Life giving gases - Oxygen (O₂) for humans and animals,  Carbon dioxide (CO₂) plants.

Air is integral part of earth's mass. 

99% of total mass of atmosphere is confined to 32 km from earth's surface. 

Air - colourless and odourless

Atmosphere = gases + water vapour + dust particles

Note - Proportion of gases changes as we go up. 

No or negligible Oxygen (O₂) at height of 120 km and above.

Carbon dioxide (CO₂) and Water vapour are only found upto 90 km from the surface of the earth.

Q. Can you imagine what will happen to us in the absence of ozone in the atmosphere ?

 

Carbon dioxide (CO₂) - meteorologically a very important gas

                                      - transparent to incoming solar radiation 

                                      - opaque to the outgoing terrestrial radiation

                                      - It absorbs a part of terrestrial radiation and reflects back some part of it towards the earth’s surface.

                                      - It is largely responsible for the green house effect. (GHE) 

                                      - Volume of other gases is almost constant but volume of CO₂ is increasing (burning of fossil fuel).

                                      - increase in volume of CO₂ --> increase in temperature of air

                                      

 

Ozone (O₃)  - another important component of atmosphere

                     - found between 10 - 50 km above the earth's surface

                     - acts as a filter and absorbs UV-rays (ultra-violet rays) radiating from the sun

                     - prevent UV-rays from reaching the surface of the earth.

 

Water Vapour -  variable gas in the atmosphere

                         - decreases with altitude 

                        - warm and wet tropics -> water vapour  = 4% of air by volume.

                        - dry and cold areas of deserts and polar regions -> water vapour < 1% of the air

                        - Water vapour also decreases from the equator towards the poles.

                        - absorbs parts of the insolation from the sun and preserves the earth’s radiated heat.

                        - acts like a blanket allowing the earth neither to become too cold nor too hot.

                        - contributes to the stability and instability in the air.

 

Dust Particles -  Atmosphere has a sufficient capacity to keep small solid particles, which may originate 
                            from different  sources and include sea salts, fine soil, smoke-soot, ash, pollen, dust 
                            and disintegrated particles of meteors.

                        - concentrated in the lower layers of the atmosphere

                        - yet convectional air currents may transport them to great heights.

                        - subtropical and temperate regions  --> dry winds --> high concentration of dust particles. 

                        - equatorial and polar regions --> no dry winds --> low concentration of dust particles

                        - clouds formation --> Dust and salt particles act as hygroscopic nuclei around which water vapour                              condenses to produce clouds.

 

STRUCTURE OF THE ATMOSPHERE

 

atmosphere - combination of layers with varying density and temperature.

                        Density is highest near the surface of the earth and decreases with increasing altitude. 

                        Atmosphere column = five layers        [depending on temperature] 

                        Five layers are troposphere, stratosphere, mesosphere, thermosphere and exosphere.

   
Troposphere  - lowermost layer       
                       -  average height = 13 km [ extends upto 8 km near poles and about 18 km near equator ]             
                       - Thickness of the troposphere is greatest at the equator because heat is transported to great heights by strong convectional currents.
                       - composition = dust particles and water vapour
                       - temperature reduction with respect to height 1 degree / 165 m 
                       - all climate and weather changes take place in this layer
                       - all biological activity takes place in this layer

Tropopause :-  zone separating troposphere from stratosphere is known as tropopause
                       - air temperature = -80 degree C over the equator
                       - air temperature = -45 degree C over the poles
                       - temperature is nearly constant therefore name tropopause
                       

Stratosphere :- found above tropopause 
                       - extends upto height of 50 km 
                       - it contains the ozone layer 
                       - absorbs UV radiation 
                       - shields life on the earth from intense, harmful form of energy 
                       

Mesosphere :- lies above stratosphere
                       - extends upto height of 80 km
                      - in this layer also temperature starts decreasing with the increase in altitude 
                      - temperature = -100 degree C at 80 km 
                      - upper limit of the mesosphere is known as mesopause
                      

Ionosphere :- it is located between 80 and 400 km above mesopause
                     - it contains ions (electrically charged particles) so it is known as ionosphere
                     - radio waves transmitted from earth are reflected back to the earth by this layer
                     - tempreature starts increasing with height in this layer 
                     

Exosphere :-  the uppermost layer of the atmosphere above the thermosphere is known as the exosphere. 
                    -  this is the highest layer, very little is known about it
                    - its contents are extremely rarefied in this layer 
                    

Note:- geographers are concerned with the first two layers of the atmosphere.


Elements of weather and climate:- these elements influence human life on the earth

1. temperature
2. pressure
3. winds 
4. humidity 
5. clouuds 
6. precipitation