The atmospheric whether is a shallow gaseous envelope surrounding the Earth of which thickness is less than 1% of the Earth’s radius (Earth’s radius is 6371 km). The present form of atmosphere and composition evolved at least 400 million years ago. The estimated mass of the Atmospheric Whether is 56 x 1014 metric tons, and it extends over about 400 km in height (variable), and meteorological events and effects occur in it. Earth’s atmosphere contains air which is a mechanical mixture of gases. The mean sea level temperature and pressure (450N latitude) of the atmosphere are 150 and 1013.250 mb, respectively.
Table of Contents
The earth is elliptical. It has three spheres.
They are as follows:
- Atmosphere – the gaseous portion
- Hydrosphere – The water portion
- Lithosphere – The solid portion
The Usefulness of the Atmospheric Whether
- It fulfills the biological oxygen demand (BOD) of the animal life.
- It supplies the necessary precipitation or moisture required for plants, animals etc.
- It protects the planet’s biological life from harmful extraterrestrial radiations like UV by absorbing them through ozone.
- It maintains warmth of the planet through its greenhouse effect, avoiding the temperature to falls to too extreme limits. [The earth’s temperature in absence of atmosphere would have been +95oC (day) and–145oC (night)]. Therefore, acts as both greenhouse and protective thermal blanket for earth.
- It provides the necessary CO2 which is basic input required to run photosynthesis process in plants to build biomass.
- It provides the necessary medium for the transport of pollens, seeds, spores,
insects, and conducts sound. - Many physical, chemical and hydrological processes responsible for weather
and climate occur in atmosphere only. - The Atmospheric Whether is a big reservoir of nitrogen. Some plants and microbes can fix this nitrogen for plant growth e.g. Azolla, Pinata, Azotobacter
Composition of the Atmospheric Whether
Atmospheric Whether is a huge envelope of mixture of gases extending several hundreds of kilometers from earth surface. The various constituents/ components of the atmosphere can be divided into following three categories.
Gases: The atmosphere is a mixture of many gases. Nitrogen (78%) and oxygen (21%) make up approximately 99% of gases constitution and 1% by other gases. Four gases viz., nitrogen, oxygen, argon, and carbon dioxide account for 99.98% of the dry air by volume. Some gases of the atmosphere remain constant at the surface of the globe up to the height of 80 to 88 km. These gases are called non-variable components/permanent gases. They are nitrogen, oxygen, argon, neon, helium, krypton, xenon, and radon. Some gases or components of the atmosphere change with changes in time, space, season, etc. These components are called, Variable components.
Moisture: Moisture in the form of water vapor exist in the atmosphere. It is released through the process of evaporation from open water bodies like sea, ocean, lake, river, etc. and also released by plants through transpiration. Water vapor comprises up to 4% of the Atmospheric Whether by volume (about 3% by weight) near the surface, but it is almost absent above 10 to 12 km. Water vapor plays a large role in absorption of long wave radiation. It is also important component of hydrological cycle (precipitation). The proportion of water vapor in the atmosphere always changes according to space and time.
Solid impurities or Aerosols: These are suspended solid particles of dust, salt, and carbon also some liquid particles like water droplets. These particles are dispersed in the atmosphere and are known as aerosols. Aerosols enter the Atmospheric Whether by man made pollution and by agricultural practices as well as through forest fires, wind raised dust, sea spray and volcanic activity. Bacteria, spores, pollens, seeds, smokes are also present in the atmosphere. They are also called as aerosols
Extent and Structure of the Atmospheric Whether
The Atmospheric Whether can be divided into two spheres on the basis of its chemical
composition occurring with height.
1) Homosphere: In the lower region up to the height of 88 km the various gases are
thoroughly mixed and are homogeneous by the processes of turbulant mixing and
diffusion. This sphere is called as Homosphere. The gases composition in this
sphere remains normally constant.
2) Heterosphere: In Heterosphere, gaseous composition changes and various gases
form separate compositional layering individually. Satellite data has shown the
presence of different Chemospheres as follow:
a) Nitrogen and oxygen layer – From 88 to 115 km
b) atomic oxygen layer – 115 to 965 km
c) Helium layer – 965 to 2400 km
d) Hydrogen layer – 2400 to 10,000 km.
The distribution of the gases is governed by the earth’s gravitational field. Thus,
heavier gases sink downward while the lighter gases like hydrogen remain at
higher altitude. Since the gases in the Atmospheric Whether are free to expand,
there is no sharp boundary between the air and extra-terrestrial space. It is difficult
to ascertain the height of the atmosphere. Half of the total gas exists below a height
of 5.5km. It is thought that up to a height of 400 km air exists in perceptible
quantity.
Layering of Atmospheric Whether
Thermosphere
Lower atmosphere:
Troposphere: The word “tropo” means mixing or turbulence and “sphere” means region. The lowest layer of the atmosphere is called the troposphere. The altitude of the troposphere changes according to latitude. It has an elevation of about 16 km at the equator and only 8 km at the poles. Its average altitude is about 11km. In this layer about 75% of the gaseous mass of the total atmosphere, water vapor and aerosols are present. It is the realm of clouds, storms, cyclones, anticyclones and convective motion. Because of these atmospheric activities occurs in this layer, the
layer is called as “Seat of weather phenomena”.
The outstanding characteristic of the troposphere is that there is a decrease of temperature with increase in altitude (height) at mean lapse rate of about 6.5o C/km or 3.6o F/1000ft until minimum temperature of -500C to -600 C is reached. (The temperature decrease with increase in height is called as lapse rate of temperature (LRT).) Thermal convection is better developed in tropics and hence the troposphere has higher altitude at equator. Because of thermal convection, the height of troposphere is more in summer than in winter. At the top of the troposphere there is a shallow layer separating it from the stratosphere which is “tropopause”.
The thermal layer marking the end of temperature decrease is called ‘tropopause’ and it separates troposphere and stratosphere. Tropopause acts as a lid at the top of the troposphere. Height of the tropopause is not constant, either in space or time. The temperature in the tropopause is almost constant (about -57 Oc). As the temperature is constant, the lapse rate in tropopause is zero. This layer contains relatively warm air at the top of troposphere. The Atmospheric Whether pressure in the troposphere at earth surface (mean sea level) is 1013.25 mb and it decreases with height.
Stratosphere: This is the second Atmospheric Whether layer above tropopause which extends upwards from tropopause to about 50 km (30 miles). The stratosphere contains much of the total atmospheric ozone. The density of the ozone is maximum at 22 to 24.5 km approximately. In stratosphere temperature increases with increase in height. The atmospheric pressure decreases from 110 mb to 1mb. The ozone at the upper layer of the atmosphere absorbs the ultraviolet rays from the sun and temperature may exceed 0oC (from –60oC to 0oC). Higher temperature occurs in the stratosphere because of absorption of ultraviolet radiations by ozone. Stratopause is a isothermal boundary layer at top of the stratosphere which separates stratosphere from the mesosphere.
Upper atmosphere:
Mesosphere: This is the third layer of atmosphere. Above the warm stratopause,
temperature decreases with increase in height to a minimum of about –90oC at
about 80 km height. This layer is also known as chemosphere. This layer extends
upwards from the stratopause to about 80km. Pressure in this layer is very low and
decreases from 1mb at about 50 km to about 0.01mb at 80 km nearly. The thin
isothermal layer which separates mesosphere from thermosphere is called
‘mesopause’. Gaseous composition is homogenous up to mesopause. Above 80 km
temperatures again rise with height and this inversion is referred to as mesopause.
Thermosphere: Outermost shell is known as thermosphere. It lies about 80 km height. In this layer, the atmospheric densities are extremely low. Atomic oxygen is important constituents of this sphere. In the thermosphere the temperature increases with height due to absorption of ultraviolet radiation from the sun by atomic Oxygen. Probably temperature reaches to 9500 C at 350 km and 17000 C at an undefined upper limit, but these temperatures are essentially theoretical. Such temperatures are not felt by the hands exposed by astronaut or the artificial satellite because of rare, filled air
IMPORTANCE OF RADIATION IN CROP PRODUCTION
- It provides the necessary energy for all the phenomena concerning biomass
production. - Photosynthetically Active Radiations ( PAR) are the real source of energy for
photosynthesis process. Plants are efficient biological convertors of solar energy
into biomass. Radiation defines the yield of a crop in particular region. - It also provides the energy for the physical processes taking place in plant soil
and atmosphere. - It conditions (governs) the distribution of temperature and hence crop
distribution on the earth surface. - Almost all growth, development and yield governing processes such as
germination, elongation formative activity, reproduction, flowering, photoperiodism, leaf enlargement, pigmentation etc. are affected by radiation.