Introductory Agrometeorology/Earth Atmosphere


 * Objectives of this chapter:
 * To understand about earth's atmosphere, its composition, extent and structure

Atmosphere
Atmosphere
 * colourless, odourless and tasteless physical mixture of gasses.
 * surrounds earth on all sides.
 * mobile, compressible and expandable.
 * contains huge number of solid and liquid particles called aerosol.

The lower atmosphere where the chemical composition of gas is uniform is called homosphere. At higher levels the chemical composition of air changes considerably and known as heterosphere.

Atmosphere
 * 1) Provides oxygen which is useful for crop respiration
 * 2) Provides CO2 to build biomass in photosynthesis.
 * 3) Provides N, which is essential for plant growth.
 * 4) Acts as a medium for transportation of pollen.
 * 5) Protects crops and human beings from harmful UV rays.
 * 6) Provides rain to field crops.

Composition of atmosphere
The following all the different gases that are present in percentage by volume approximately.

a. gases
 * Nitrogen (N2) = 78.08
 * Oxygen (O2) = 20.95
 * Argon (Ar) =0.93 CO2 =0.03
 * Neon (Ne) = 0.0018 Helium(He) =0.0005
 * Ozone(O3) =0.00004 Hydrogen(H2) =0.00006
 * Methane (CH4) =0.00017

b. water vapour: 0.02% by volume in a cold dry climate to nearly 4% in the humid tropics.

c. dust particles:

Structure

 * Troposphere:Gr.‘tropos’=“mixing”/turbulence
 * avg. height is 14 km above sea level (poles is about 8 km, equator is about 16 km)
 * shallow layer separating troposphere from the next thermal layer of the atmosphere (stratosphere) is tropopause. Various clouds, thunderstorms, cyclones and anticyclones occur in this sphere because of the concentration of almost all the water vapour (4% of the atmosphere composition) aerosols in it.
 * decrease in temperature with increasing elevation up to 14 km.
 * Stratosphere: (16-30 km)
 * no visible weather phenomena occur above tropopause.
 * gradual temperature increase with height beyond 29 km.
 * Ozonosphere (or Mesosphere):
 * maximum concentration of ozone between 30 and 69 km above the surface of the earth
 * temperature increases with height @ 5oC/km.
 * Ionosphere:60 km above the earth’s surface
 * temperature falls again reaching a minimum of about 100oC at a height of 80 km above earth’s surface. Beyond this level, the temperature increases again
 * Exosphere: 400 and 1000 km
 * density of atoms in the atmosphere is extremely low.
 * Hydrogen and helium gases predominate in the outer most region

Lapse Rate
The decrease in air temperature with height is known as the normal/environmental lapse rate and it is 6.5°C/km.