In this article of Climatology, you are going to read the composition and structure of the atmosphere.
So, the first question arises in mind is that What is the atmosphere?
Let us start our discussion with the Atmosphere!
What is the Atmosphere?
The envelope of gases surrounding the earth is called the atmosphere. It forms a protective boundary between the outer space and the biosphere. It is a mixture of gases that is odorless, colorless, tasteless and formless mixed and blended so thoroughly that it acts as a single gas.
The gases of the present atmosphere are not the direct residue of the early stage of earth’s formation. They 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 life on the earth’s surface is because of this atmosphere otherwise the earth would have become barren like the moon.
- The atmosphere contains living gases like oxygen for man and animal and carbon dioxide for plants(important for survival).
- It protects the earth from the harmful radiation from the sun. It acts as a greenhouse by allowing short-wave radiation (from Sun) and trapping long-wave terrestrial radiation (from Earth’s surface).
- All life forms need a particular range of temperature and a specific range of frequencies of solar radiation to carry out their biophysical processes. The atmosphere absorbs certain frequencies and lets through some other frequencies of solar radiation. In other words, the atmosphere regulates the entry of solar radiation.
- The atmosphere also keeps the temperature over the earth’s surface within certain limits. In the absence of the atmosphere extremes of temperature would exist between day and night over the earth’s surface.
- The atmosphere also takes care of extra-terrestrial objects like meteors that get burnt up while passing through the atmosphere (mesosphere to be precise) due to friction.
Composition of the atmosphere
The atmosphere is composed of –
The atmosphere is a mixture of many gases. In addition, it contains huge numbers of solid and liquid particles, collectively called aerosols.
- Nitrogen and oxygen make up nearly 99% of the clean, dry air. The remaining gases are mostly inert and constitute about 1% of the atmosphere.
- Oxygen, although constituting only 21% of the total volume of the atmosphere, is the most important component among gases. All living organisms inhale oxygen. Besides, oxygen can combine with other elements to form important compounds, such as oxides. Also, combustion is not possible without oxygen.
- Nitrogen accounts for 78% of total atmospheric volume. It is a relatively inert gas and is an important constituent of all organic compounds. The main function of nitrogen is to control combustion by diluting oxygen. It also indirectly helps in the oxidation of different kinds.
- Carbon Dioxide which constitutes only about 0.038% of the dry air and is a product of combustion. Green plants, through photosynthesis, absorb carbon dioxide from the atmosphere and use it to manufacture food and keep other biophysical processes going.
- Being an efficient absorber of heat, carbon dioxide is considered to be of great climatic significance. Carbon dioxide is considered to be a very important factor in the heat energy budget.
- With increased burning of fossil fuels – oil, coal and natural gas – the carbon dioxide percentage in the atmosphere has been increasing at an alarming rate.
- More carbon dioxide in the atmosphere means more heat absorption. This could significantly raise the temperature at lower levels of the atmosphere thus inducing drastic climatic changes.
- The third important gas is Argon which constitutes only about 0.93%.
- Ozone (03) is another important gas in the atmosphere, which is actually a type of oxygen molecule consisting of three, instead of two, atoms. It forms less than 0.00006% by volume of the atmosphere and is unevenly distributed. It is between 20 km and 25 km altitude that the greatest concentrations of ozone are found. It is formed at higher altitudes and transported downwards.
- Ozone plays a crucial role in blocking the 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 vapour content in the atmosphere ranges from 0 to 5 % by volume.
- The atmospheric vapour is received through the evaporation of moisture and water from the water bodies (like seas and oceans, lakes, tanks and ponds, rivers, etc.), vegetation and soil cover.
- Vapour depends on temperature and therefore it decreases from the equator poleward in response to decreasing temperature towards the poles.
- The content of the vapour in the surface air in the moist tropical areas, at 50-degree and 70-degree latitudes, is 2.6%, 0.9% and 0.2% (by volume) respectively.
- The content of vapour decreases upward.
- More than 90% of the total atmospheric vapour is found up to the hight of 5 km.
- The moisture content in the atmosphere creates several forms of condensation and precipitation e.g. clouds, fogs, dew, rainfall, frost, hailstorm, ice, snowfall, etc.
- Vapour is almost transparent for incoming shortwave solar radiation so that the electromagnetic radiation waves reach the earth’s surface without many obstacles but vapour is less transparent for outgoing shortwave terrestrial radiation and therefore it helps in heating the earth’s surface and lower portion of the atmosphere because it absorbs terrestrial radiation.
- The Solid Particles present in the atmosphere consist of sand particles (from weathered rocks and also derived from volcanic ash), pollen grains, small organisms, soot, ocean salts; the upper layers of the atmosphere may even have fragments of meteors which got burnt up in the atmosphere.
- These particulates help in the absorbing, reflecting and scattering of the solar radiation which adds the varied charming colour of red and orange at sunrise and sunset.
- The sky appears blue in colour due to the selective scattering of solar radiation by dust particles.
- Salt particles become hygroscopic nuclei and thus help in the formation of water drops, clouds and various forms of condensation and precipitation.
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-
In the Homosphere, there are three regions: The Troposphere, the Stratosphere, and the Mesosphere.
Although the composition of air is the same throughout these three regions, the concentration of air decreases significantly with increasing altitude.
- The Troposphere is the earth’s weather layer. It contains nearly all weather conditions. As you go up in altitude the temperature goes down. It is the bottom-most layer of the
- The Stratosphere is the middle region of the Homosphere.
- The Mesosphere is the top layer of the Homosphere.
In the Heterosphere, there are two regions: The Thermosphere and the Exosphere. These two regions are considered outer space. The gases in this layer are not evenly mixed. The Ionosphere overlaps the Mesosphere and the Thermosphere.
- The thermosphere is the bottom region of the Heterosphere.
- The exosphere is the top region of the Heterosphere.
Homosphere: It extends from the earth’s surface up to the altitude of 80km. even though the atmosphere rapidly decreases in density with increasing altitude, the composition of the gases remains uniform in the homosphere. The exceptions in the homosphere are the concentration of Ozone (O3) in the stratosphere from almost 19-50 km and variation of water vapour and dust particles in the lower atmosphere. This uniform composition was attained approximately 600 million years ago.
Heterosphere: the gases in this layer are not evenly mixed. It begins over 80km and extends up to 10,000 km. however, for all scientific purposes, the upper limit of the atmosphere is taken as 480km as earth’s gravitational pull becomes negligible after it. The atmosphere above it is called the exosphere and it contains individual atoms of light gases like hydrogen, helium, etc.
Based on Change in temperature
On the basis of change in temperature the atmosphere is broadly divided into four layers:
- It is the lowermost layer of the atmosphere. It extends up to 18km at the equator, 13 km at mid-latitude and about 8km at poles.
- It contains approximately 90% of the total mass of the atmosphere.
- The entire weather phenomenon takes place in this layer. It contains all the water vapour, dust particles, clouds, etc.
- In the troposphere, the temperature decreases with an increase in height.
- The average rate of decrease of temperature with height is called a normal lapse rate and it is equal to 6.4 degrees C/km. the rate of decrease of temperature is not constant everywhere.
- The local rate of decrease is called the local lapse rate. The minimum temperature attained in this layer is -57 degree C.
- Tropopause: It is the topmost layer of the troposphere. It acts as a boundary between the troposphere and stratosphere. This layer is marked by constant temperatures.
- It lies above the troposphere and extends uniformly across the globe up to 50km.
- In this layer, the temperature increases with an increase in height. The temperature varies from -57 to 0 degree C.
- This layer is characterized by the presence of the Ozonosphere. Ozone is a highly reactive oxygen molecule made up of three atoms.
- Ozone absorbs the high-frequency ultraviolet radiation. Because of this absorption the temperature of the layer increases.
- The energy absorbed is used in chemical reactions causing the formation of ozone gas.
- Ultraviolet rays are highly harmful to living organism including plants, animals as well as humans. Absorbing these radiations ozone layers makes a protective layer around us.
- The mesosphere extends from 50 – 80 km.
- The temperature again decreases in this layer and reaches its minimum mark averaging -90o C. Although this temperature can vary.
- The homogenous layer extends up to the mesosphere. At the upper boundary of the mesosphere, there exists a layer of ions extending in the other layer. This layer of ions or charged particles is helpful in reflecting the radio waves and helps in telecommunication.
- This is a region extending from 80km to 480km.
- It contains a functional ionosphere. The temperature rises very sharply in this layer as the gas molecules absorb the short wave radiations coming from the sun.
- The temperature can reach as high as 1200o C, but despite such high temperatures, the thermosphere is not as ‘hot’ as we expect it to be.
- As the density of air is so low in this layer, the energy is not easily transferred; hence the hotness is not felt.
- This is the zone containing charged particles called ions. It lies from the upper mesosphere to the thermosphere.
- The charged particles are ionized by absorption of cosmic rays, gamma rays, X-rays and shorter wavelengths of ultraviolet rays.
- It is in this layer that incoming space vehicles and meteorites begin to heat due to friction.
- Above this layer i.e. above 480km, atomic oxygen is prevalent and beyond that first helium is more common and then hydrogen atoms predominate.
- The ionosphere is a deep layer of electrically charged molecules and atoms (which are called ions) in the middle and upper mesosphere and the lower thermosphere, between about 60 and 400 kilometers (40 and 250 miles). The ionosphere is significant because it aids long-distance communication by reflecting radio waves back to Earth.
- It is also known for its auroral displays, such as the “northern lights” that develop when charged atomic particles from the Sun are trapped by the magnetic field of Earth near the poles. In the ionosphere, these particles “excite” the nitrogen molecules and oxygen atoms, causing them to emit light, not unlike a neon light bulb.