What is Atmosphere?

• The envelope of gases surrounding the earth is called the atmosphere.
• It forms a protective boundary between the outer space and the biosphere.
• The atmosphere is a dynamic collection of gases that constantly move and change.
• The gases of the present atmosphere are a product of progress through volcanic eruptions, hot springs, chemical breakdowns of solid matter and redistribution from the biosphere.
• The atmosphere is a significant component of the biospheric ecosystem because it protects the earth from harmful radiation from the sun and prevents the earth from becoming barren like the moon.
• It acts as a greenhouse by allowing short-wave radiation (from Sun) and trapping long-wave terrestrial radiation (from Earth’s surface).
• The atmosphere absorbs certain frequencies and lets through some other frequencies of solar radiation, which regulates the entry of solar radiation.
• The atmosphere also keeps the temperature over the earth’s surface within certain limits and prevents extremes of temperature from existing between day and night.
• The atmosphere takes care of extra-terrestrial objects like meteors that get burnt up while passing through the atmosphere due to friction in the mesosphere.

Composition of the atmosphere

• The atmosphere is composed of neutral, uncharged particles, except for the noble gases.
• Atoms in the gas phase share electrons with other atoms in chemical bonds for a more stable filled-shell configuration.
• The Earth’s atmosphere consists of noble gas atoms and many kinds of molecules.
• The atmosphere contains gases, vapor, and particulates.
• It also contains solid and liquid particles, known as aerosols.
• 

Gases

• Nitrogen and oxygen constitute almost 99% of the clean, dry air.
• Oxygen is the most important component among gases, although it constitutes only 21% of the atmosphere.
• Nitrogen accounts for 78% of the total atmospheric volume, and it dilutes oxygen to control combustion.
• Carbon dioxide constitutes about 0.038% of the dry air and is considered of great climatic significance.
• Carbon dioxide is increasing at an alarming rate due to the increased burning of fossil fuels.
• Argon constitutes about 0.93% of the atmosphere.
• Ozone is another important gas, constituting less than 0.00006% of the atmosphere, which blocks harmful ultraviolet radiation from the sun.
• Other gases found in almost negligible quantities in the atmosphere are neon, helium, hydrogen, xenon, krypton, methane, etc.
• The atmosphere is composed of gases, vapor, and particulates.
• Aerosols are huge numbers of solid and liquid particles found in the atmosphere.

Water Vapour

• The vapour content in the atmosphere ranges from 0 to 5% by volume.
• Atmospheric vapour comes from evaporation of moisture and water from various sources.
• Vapour content decreases from equator to poleward due to decreasing temperature.
• Vapour content is 2.6%, 0.9%, and 0.2% (by volume) in moist tropical areas, 50-degree and 70-degree latitudes, respectively.
• Vapour content decreases upward, with over 90% found up to 5 km in height.
• Atmospheric moisture leads to various forms of precipitation such as clouds, fogs, dew, rainfall, frost, hailstorm, ice, snowfall, etc.
• Vapour is almost transparent for incoming shortwave solar radiation, but less transparent for outgoing longwave terrestrial radiation, thus helping to heat the earth’s surface and lower atmosphere by absorbing terrestrial radiation.
• 

Particulate Matter

• Solid particles in the atmosphere consist of sand particles, pollen grains, small organisms, soot, ocean salts, and fragments of meteors.
• These particulates help in absorbing, reflecting, and scattering solar radiation, which causes the varied colors seen at sunrise and sunset.
• The sky appears blue due to the selective scattering of solar radiation by dust particles.
• Salt particles become hygroscopic nuclei and help in the formation of water drops, clouds, and various forms of condensation and precipitation.
• A hygroscopic nucleus is a microscopic particle (such as sulfur dioxide, salt, dust, or smoke) in the air onto which water vapor may condense to form droplets.

Structure of the Atmosphere

• The atmosphere can be divided into different layers according to composition, density, pressure, and temperature variations.

Based on Composition:

• According to its composition, broadly it is divided into two layers- homosphere and heterosphere
• Homosphere:
  • The homosphere is the lower part of the atmosphere and has three regions: troposphere, stratosphere, and mesosphere.
  • The troposphere is the bottom layer and contains nearly all weather conditions. The temperature decreases as altitude increases.
  • The stratosphere is the middle region of the homosphere.
  • The mesosphere is the top layer of the homosphere.
  • All three regions have the same air composition, but air concentration decreases as altitude increases.
  • The homosphere extends from the Earth’s surface up to an altitude of 80 km.
• Heterosphere:
  • The heterosphere is a layer of the atmosphere where gases separate out by molecular diffusion with increasing altitude, causing lighter species to become more abundant relative to heavier species.
  • The heterosphere is divided into two regions: the thermosphere and the exosphere.
  • The thermosphere is the bottom region of the heterosphere.
  • The exosphere is the top region of the heterosphere.
  • Both the thermosphere and exosphere are considered outer space.
  • The heterosphere begins over 80 km and extends up to 10,000 km.
• 

Based on Change in temperature:

• The atmosphere can be divided into five layers according to the diversity of temperature and density. They are:
  • Troposphere
  • Stratosphere
  • Mesosphere
  • Thermosphere (Ionosphere)
  • Exosphere

• Troposphere:
  • The troposphere is the lowest layer of Earth’s atmosphere, extending up to 18km at the equator, 13 km at mid-latitude, and about 8km at poles.
  • Most of the atmosphere’s mass (about 75-80%) is in the troposphere, with the thickness greater at the equator due to heated air rising higher.
  • The troposphere ends with the Tropopause, which is the atmospheric boundary that separates the troposphere from the stratosphere.
  • Temperature in the troposphere falls at a rate of 5°C per kilometer upwards and reaches -45°C at the poles and -80°C over the equator at Tropopause.
  • The troposphere is marked by temperature inversion, turbulence, and eddies, and is also meteorologically significant since all weather phenomena occur here.
  • It is also called the convective region since all convection stops at Tropopause.
  • The troposphere is influenced by seasons and jet streams.
  • The tropopause is marked by constant temperatures.
• Stratosphere:
  • The stratosphere is above the troposphere and extends up to 50km.
  • Temperature in this layer remains constant for some distance but then rises to reach 0°C at 50km altitude.
  • The rise in temperature is due to the presence of ozone which absorbs harmful ultraviolet radiation.
  • This layer is almost free from clouds and associated weather phenomena.
  • The conditions in the stratosphere are ideal for flying airplanes, so they often fly in the lower stratosphere or upper troposphere where the weather is calm.
  • Sometimes, cirrus clouds are present at lower levels in this layer.
• Mesosphere:
  • The mesosphere is located between 50-80 km above Earth’s surface.
  • The temperature in this layer decreases and reaches an average minimum of -90°C.
  • A homogenous layer extends up to the mesosphere.
  • At the upper boundary of the mesosphere, there exists a layer of ions.
  • This ion layer helps in reflecting radio waves and aids in telecommunication.
• Thermosphere:
  • The thermosphere is a layer where the temperature rises rapidly with height.
  • The ionosphere is a part of the thermosphere that extends from 80-400 km.
  • The ionosphere helps in radio transmission and reflects radio waves back to earth.
  • There is extremely low pressure in this layer, so a person would not feel warm.
  • The International Space Station and satellites orbit in this layer.
  • The atmosphere is extremely rarified, so a person or an object in this layer doesn’t feel the heat.
  • Auroras are observed in lower parts of this layer.
• Ionosphere:
  • The ionosphere is an electrically charged layer located between 80 km and 400 km.
  • It lies from the upper mesosphere to the thermosphere.
  • The charged particles in this layer are ionized by absorption of cosmic rays, gamma rays, X-rays and shorter wavelengths of ultraviolet rays.
  • Incoming space vehicles and meteorites begin to heat due to friction in this layer.
  • Temperature starts increasing with height in this layer because of radiation from the sun.
• Exosphere:
  • The exosphere is the uppermost layer of the atmosphere.
  • It extends beyond the ionosphere at a height of about 400 km.
  • The air in this layer is extremely rarefied.
  • The temperature gradually increases through the layer due to exposure to direct sunlight.
  • Light gases such as helium and hydrogen float into space from this layer.
  • This layer coincides with space.