Atmosphere Of The Earth

Atmosphere Of The Earth: The atmosphere of Earth is a layer of gases surrounding the planet Earth that is retained by Earth's gravity. The atmosphere protects life on Earth by absorbing ultraviolet solar radiation, warming the surface through heat retention (greenhouse effect), and reducing temperature extremes between day and night.

Atmospheric stratification describes the structure of the atmosphere, dividing it into distinct layers, each with  specific characteristics such as temperature or composition. The atmosphere becomes thinner and thinner with increasing altitude, with no definite boundary between the atmosphere and outer space. An altitude of 120 km (75 mi) is where atmospheric effects become noticeable during atmospheric re-entry of spacecraft. The Karman line, at-100 km (62 mi), is often regarded as the boundary between atmosphere and outer space.

Air is the name given to atmosphere used in breathing and photosynthesis. Dry air contains roughly (by volume) 78.09% nitrogen, 20.95% oxygen, 0.93% argon, o.039% carbon dioxide, and small amounts of other gases. Air also contains a variable amount of water vapour, on average around 1%. While air content and atmospheric pressure varies at different layers, air suitable for the survival of terrestrial plants and terrestrial animals is currently known to be found only in Earth's troposphere and artificial atmospheres.

Air is mainly composed of nitrogen, oxygen, and argon, which together constitute the major gases of the atmosphere. The remaining gases are often referred to as trace gases, among which are the greenhouse gases such as water vapour, carbon dioxide, methane, nitrous oxide, and ozone. Filtered air includes trace amounts of many other chemical compounds. Many natural substances may be present in tiny amounts in an unfiltered air sample, including dust, pollen and spores, sea spray, and volcanic ash. Various industrial pollutants also may be present, such as chlorine (elementary or in compounds), fluorine compounds, elemental mercury, and sulphur compounds such as sulphur dioxide [SO.].

Earth's atmosphere can be divided into five main layers. These layers are determined mainly by the fact that whether temperature increases or decreases with altitude. From highest to lowest, these layers are:

Exosphere: The outermost layer of Earth's atmosphere extends from the exobase upwards. It is mainly composed of hydrogen and helium. The particles are so far apart that they can travel hundreds of kilometres without colliding with one another. Since the particles rarely collide, the atmosphere no longer behaves like a fluid. These free-moving particles follow ballistic trajectories and may migrate into and out of the magnetosphere or the solar wind.

Thermosphere: Temperature increases with height in the thermosphere from the mesopause up to the thermopause; then remains constant with height. The temperature of this layer can rise up to 1,500 °C (2,730 °F), though the gas molecules are so far apart that temperature in the usual sense is not well defined. The International Space Station orbits in this layer, between 320 and 380 km (200 and 240 mi). The top of the thermosphere is the bottom of the exosphere, called the exobase. Its height varies with solar activity and ranges from about 350-800 km (220-500 mi; 1,100,000-2,600,000 ft).

Mesosphere: The mesosphere extends from the stratopause to 8o-85 km (50-53 mi; 260,000-280,00o ft) upwards. It is the layer where most meteors burn up upon entering the atmosphere. Temperature decreases with height in the mesosphere. The top of the mesosphere is the coldest place on Earth and has an average temperature around -85 °C(-121 "F; 188.1 K). Due to the cold temperature of the mesosphere, water vapours get frozen, forming ice clouds (or Noctilucent clouds). A type of lightning referred to as either sprites or ELVES, form many miles above thunderclouds in the troposphere.

Stratosphere: The stratosphere extends from the tropopause to about 51 km (32 mi; 170,000 ft) upwards. Temperature increases with height, which restricts turbulence and mixing The stratopause, which is the boundary between the stratosphere and mesosphere, typically is at 50 to 55 km (31 to 34 mi; 160,000 to 180,000. The pressure here is 1/1,000 sea level,

Troposphere: The troposphere is the lowermost layer of the atmosphere. It is the most important layer because almost  all of the weather phenomena (e.g. fog, cloud, dew, frost, rainfall, hailstorm etc.) occur in this layer. Temperature decreases with increase in height at the rate of 6.5" per 1,00o m. This rate of decrease in temperature is called normal lapse rate. There is seasonal variation in the height of troposphere In height changes from equator towards the poles (decreases) and from one season of a year to another season (increases during summer and decreases during winter), The average height of troposphere is about 16 km over the equator and 6 km over the poles. The upper limit of the troposphere is called tropopause, whích is about 1.5 km thick.

Other layers: Within these major layers of the atmosphere, there are some minor layers:

The ozonosphere is the lower portion of the stratosphere having maximum concentration of ozone. It is confined between the height of 15 km to 35 km from sea level. About 90% of the ozone in our atmosphere is contained in the stratosphere.The ionosphere extends from 80 km to 640 km. There are a number of ionic layers (with increasing heights) in this sphere, e.g. D layer, E layer, F layer, and G layer. The ionosphere radiates signals of various frequencies of radio waves to the earth's surface. It is also responsible for auroras. On the basis of chemical composition, the atmosphere is divided into two broad zones viz.

(i) homosphere,

(ii) heterosphere.

Homosphere is the lower portion of atmosphere lying up to the height of 90 km from the sea level. Troposphere, Stratosphere and Mesosphere form the parts of this layer.

Heterosphere extends from 90 km to 10,000 km. There are four distinct layers of gases in this sphere with molecular nitrogen in the lowest portion, followed by oxygen layer and helium layer. Hydrogen layer is found at its top.The average temperature of the atmosphere at the surface of the Earth is 14°C (57°F).