Climatology/Atmospheric Composition and Structure

Meaning of Composition of Atmosphere
The literal meaning of composition is ‘ingredients’ or ‘constituents’ of something. In another words, it is a manner by which something is made up of. When we apply the same meaning with atmosphere, it signify the items or the elements with which our atmosphere is composed. Our atmosphere  is  composed  of  numerous  gasesand  other  substances,  hence,  it  is  a mechanical  mixture  of  the  gases,  water  vapour  and  dust  particles. Let us  discuss  about  the composition of atmosphere.

Composition of Atmosphere
The envelope  of  atmosphere  around  the  earth,a  mechanical  mixtures  of numerous gases and other substancesare very important to all living organisms of the planet. The four major gases – nitrogen, oxygen, argon and carbon dioxide together constitute 99.99% of the total volume of dry air. The maximum concentration is of nitrogen with more than 78 percent while the oxygen is a little less than 21 percent.

Nitrogen
Nitrogen is the most abundant found in atmosphere constituting 78.084 percent to the total volume of dry gases. This is almost chemically inactive and have nothing to do  with  any  sort  of  chemical  actions  in  the  atmosphere. It does  not  combine  freely  with other elements, hence, it is termed as neutral substance. This gas is found beyond a height of 100 km, but its concentration is below 50 km height from the sea level. This gas is significant for the growth and reproduction in plants and animals. Certain bacteria in the soil are capable of converting a very small amount of atmospheric nitrogen into nitrates and fix it to the soils and water  bodies  to  be  consumed  by  animals  and  plants. This process  is  called  as  Nitrogen fixation. The nitrogen  fixed  in  the  earth’s surface is again converted and sent back to the atmosphere by bacterial action through a chemical reaction called denitrification.

Oxygen
It is the  second  largest  gas  of  the  atmosphere  constituting  20.9476  percent  of  the total  dry  atmospheric  gases. It is  very  essential  for  the  survival  of many  of  the living organisms of  this  planet. It is chemically very  active  gas. It is combined  with  several other elements and forms varied compounds. Oxygen is vital for combustion of fuels. When anything burns, oxygen is consumed and helps in burning that substance. Though oxygen is found beyond  100 km  but  it  is  reasonably  in  good  proportion  within  16 km  of  height.With increasing  height,  the  amount  of  oxygen  decreases  very  rapidly. On mountain  slope,  the available oxygen for breath is very scanty and the mountaineers are supposed to carry oxygen for them.

Argon
In terms  of  percentage,  argon  is  the  third  largest  gas  in  the  atmosphere constituting 0.934  percent  of  total  dry  atmosphere. It is  an  inert  gas  and  chemically  it  is inactive. It is also found in the earth’s crust and sea water. It is used in electric bulb and fluorescent lights.

Carbon-dioxide
It is the fourth abundant gas of the atmosphere. It is densest gas and found in lower parts. It is found upto a height of about 30 km, it is concentrated in the lower strata. Its percentage is very low, i.e.,  0.04 percent but it is  most  vital  for the  growth  of vegetative  life  of  biosphere. It is  transparent  to  the  incoming  solar  radiation  but  does  not allow  to  escape  the  same. And hence,  it  is  called  as  greenhouse gas. It plays  a  very  crucial role in increasing the global temperature.

Methane
Methane is the also a greenhouse gas which absorbs the radiation and cause more temperature of the air. Paddy cultivation also generates methane in the air. It is also produced from the wetlands and waterlogged soils and released in the atmosphere. Fossil fuel is also a source to release of methane in the atmosphere. Its amount in the atmosphere is variable.

Ozone
Ozone’s concentration  lays  in  a  belt  between  the  heights  of  15  to  50 km  of atmosphere. Instead ofnormal two atoms of oxygen, ozone has three atoms of oxygen formed together denoted  by  O3. It is  formed  when  atmospheric  oxygen  molecules  are  broken  by ultraviolet  solar  radiation. It may  even  be  formed  at  the  time  of  electrical  discharge  during thunderstorms. This gas  is  also  termed  as  variable  as  its  formation  and  disintegration  is dependent upon numerous activities. Though ozone is very less in quantity (0.00006 percent), this thin layer is very significant for the survival of living worldas it absorbsthe dangerous ultraviolet rays  and  protects  the  earth. Neon, helium, krypton, hydrogen, xenon are  other minor gases. Some gases  are  still  extremely  less  in  quantity,  they  are  termed  as  trace  gases. Important among  them  are  ammonia,  carbon  monoxide,  sulphur  dioxide,  nitrogen  dioxide, nitrous oxide and sulphur hexafluoride etc.

Water Vapour
Water vapour is  small in amount  but  it is  one of the  most  important  part of atmosphere with respect to the distribution of vegetation and life. Water vapour exists all the time in  the  atmosphere  but  with  varying  degree  of  amount  depending  upon  the  season (temperature  condition)  and  the  supply  of  water  for  evaporation  and  evapotranspiration. Air is hardly  completely  dry. In summer,  the  water  holding  capacity  of  the  air  is large  as  the temperature is high while in winter it is low.

Aerosols
Aerosols are extremely fine-sized solid particles or liquid droplets which continue to be  in  suspended  form  in  gas  for  very-very  long  time. They could  be  seen  when  their concentration  is  more  otherwise  they  are  invisible. Aerosols themselves are  non-gaseous microscopic substance released in the atmosphere from various sources –natural and human created. They could be pollen, minute earthly dust, sea salt, carbon soot from burning fuels, volcanic dust  etc. Human activities  also  help  the  aerosols  to  enter  the  atmosphere. Their concentration is  more  over  the  industrial  and  urban  areas. Burning of  fossil  fuels  and generation of smoke also pump the  aerosols in  the air.



Meaning of Structure of Atmosphere
Structure means the arrangement of different part into one. In another words, it is the skeleton or organization or  anatomy  of  a  whole by looking  at the relationships  with  its  parts. According to this background, the study of different parts of the atmosphere and the relationship with its  parts  is  said  to  be  the  structure  of  the  atmosphere. Vertically, the atmosphere  is divided  into different layers/  parts. Therefore, the  study  of  different  layers  is  known  as structure of atmosphere.

Structure of Atmosphere
Based on chemical composition, the atmosphere is classified into two. They are homosphere and heterosphere.

Homosphere
Homosphere is that part of atmosphere where the chemical composition of the air is uniform or similar. It is the lowest layer in terms of chemical composition. It extends from the earth’s/ ocean surface  to  about  85 km. the basis of the changes in temperature, the atmosphere is divided into five layers out of them, three lower layers falls under homosphere (i.e. within 85 km of altitude). They are troposphere, stratosphere and mesosphere.

Troposphere
It is the lowest and densest layer of the atmosphere. It extends till a height of about 8 km over pole but over equator, it is 18 km. About 80 percent of the total mass of the atmosphere lays in this layer. With increase in height, the temperature keeps on declining till the limit of this layer. On an average, the decrease in temperature with height is 60 Celsius par km. The upper boundary is known as troposphere laying between 8 and 18 km. At this level, the average temperature reaches to minus 500 to minus 600 Celsius.

Stratosphere
Stratosphere is  the  upward  second  layer as  well  as  middle  layer  of  the homosphere. It starts from tropopause to approximate height of 50 km. The temperature at the tropopause remains almost constant till the height of 20 km. After that, it starts increasing and continue the trend till the height of 50 km. At this level, the estimated temperature is about  minus 100 to minus 150Celsius. Though the  temperature  is  on  rise,  but  there  is  no atmospheric  turbulence. This layer  is  completely  free  from clouds  and  other  weather conditions. That is  why,  it  has  an  advantage  for  flying  long-distance supersonic jets.

Mesosphere


Mesosphere is the third but the upper-most layer of the homosphere. After this layer, heterosphere  starts. The literal  meaning  of  mesosphere  is  the  middle  sphere. It is separated   by   tropopause   below   from   troposphere   and   mesopause   on   the   top   from thermosphere. It is extended from 50 km to 85 km from the earth’s surface. The air pressure is very low. It is 1 millibar at the lower limit whereas it is 0.01 millibar at the highest limit. This layer  is  characterised  by  decreasing  temperature  and  the coldest/ lowest atmospheric temperature is recorded in this layer. The lowest temperature estimated near the mesosphere is  around  minus  1300Celsius. It is  colder  that  the  lowest  temperature  recorded over Antarctic.

Heterosphere
The atmosphere laying beyond the homosphere is termed as heterosphere. The term itself is self-explanatory and it is used for that part of atmosphere where the air is not uniform. In this part of  atmosphere,  the  air  is  rare  and  the  molecules  are  wide  apart. Relatively heavier  gas molecules are concentrated in the lower part whereas the lighter are forced to be above. Beyond 85 km  height,  the  composition  of  the  atmosphere  with  increasing  altitude  vary significantly. Different layers of prominently different gases are nitrogen layer, oxygen layer, helium layer and  hydrogen  layer  are  differentiated. However, the  heterosphere,  is  divided  into  two main spheres –thermosphere and exosphere.

Thermosphere


This sphere  extends  from  mesopouse. it is 85 km to  about  650 km  from earth. The temperature is  on  rise  in  this  layer  due  to  absorption  of  solar  radiation  by  small amount  of  oxygen  molecules  present. It is  highly  dependent  upon  the  solar  activities. The temperature reaches  beyond  12000C  at  an  altitude  of  about 350 km  but  by  650 km  it  may even rise to 20000C. This much high temperature is primarily defined by average speed with which molecules are moving. Because of this, the temperature may be high.

Exosphere
Exo means external. Therefore, exosphere the external or the outer most layer of the atmosphere. Its lower  boundary  starts  from  the  thermopause 650 km to 10000 km. This much distance is little less than the diameter of the earth. It is really a very big size of the limit of the atmosphere.

Lesson Summary

 * Different temperature gradients create different layers within the atmosphere. The lowest layer is the troposphere, where most of the atmospheric gases and all of the planet's weather are located.
 * The troposphere gets its heat from the ground, and so temperature decreases with altitude. Warm air rises and cool air sinks and so the troposphere is unstable.
 * In the stratosphere, temperature increases with altitude. The stratosphere contains the ozone layer, which protects the planet from the Sun's harmful UV. The higher layers contain few gas molecules and are very cold.