2 ATMOSPHERE – COMPOSITION AND STRUCTURE
2.1. INTRODUCTION
- The Atmosphere is a thin layer of odourless, colourless, tasteless gases surrounding the earth up to a height of several hundred kms. It is close to the earth because of the earth’s gravitational force.
- Atmosphere is a mixture of different gases and it envelopes the earth all round. These gases support life over the earth’s surface.
- It contains life-giving gases like oxygen for humans and animals and carbon dioxide for plants.
- The air is an integral part of the earth’s mass and 99 per cent of the total mass of the atmosphere is confined to the height of 32 km from the earth’s surface. The air is colourless and odourless and can be felt only when it blows as wind.
METROLOGY is the scientific study of the Atmosphere focusing on weather processes and short term and it is the study of lower layer of the atmosphere.
2.2. CHARACTERISTICS OF ATMOSPHERE
1. The atmosphere of the earth is surrounded by gases which are retained by earth’s gravity.
2. The water vapour and dust particles present in the atmosphere are responsible for weather changes.
3. The presence of all these gases varies with quantity in the atmosphere according to heights.
4. The air is dense near the surface and becomes thinner and thinner with increasing height.
2.3. COMPOSITION OF THE ATMOSPHERE
- Nitrogen, Oxygen, Argon, Carbon dioxide, Neon, Helium, Krypton, Xenon and Hydrogen are the important gases present in the atmosphere.
- The proportion of gases changes in the higher layers of the atmosphere in such a way that oxygen will be almost in negligible quantity at the height of 120 km. Similarly, carbon dioxide and water vapour are found only up to 90 km from the surface of the earth.
Permanent Gases of the Atmosphere and their composition (particularly lower atmosphere)
| GASES OF ATMOSPHERE | PERCENTAGE IN ATMOSPHERE |
| Nitrogen (N2) | 78.08 |
| Oxygen (O2) | 20.95 |
| Argon (Ar) | 0.93 |
| Carbon dioxide (CO2) | 0.036 |
| Neon (Ne) | 0.002 |
| Helium (He) | 0.0005 |
| Krypton (Kr) | 0.001 |
| Xenon (Xe) | 0.00009 |
| Hydrogen (H2) | 0.00005 |
2.4. Important Gases
- Carbon dioxide is meteorologically a very important gas as it is transparent to the incoming solar radiation but opaque to the outgoing terrestrial radiation. It absorbs a part of terrestrial radiation and reflects back some part of it towards the earth’s surface.
- Hence it is largely responsible for the greenhouse effect.
- The volume of other gases is constant but the volume of carbon dioxide has been rising in the past few decades mainly because of the burning of fossil fuels. This has also increased the temperature of the air.
- Ozone is the another important component of the atmosphere found between 20 and 50 km above the earth’s surface and acts as a filter and absorbs the ultra-violet rays radiating from the sun and prevents them from reaching the surface of the earth.
Water Vapour
Dust Particles
- Atmosphere has a sufficient capacity to keep small solid particles, which may originate from different sources and include sea salts, fine soil, smoke-soot, ash,pollen, dust and disintegrated particles of meteors.
- Dust particles are generally concentrated in the lower layers of the atmosphere; yet, convectional air currents may transport them to great heights.
- The higher concentration of dust particles is found in subtropical and temperate regions due to dry winds compared to equatorial and Polar Regions.
- Dust and salt particles act as hygroscopic nuclei around which water vapour condenses to produce clouds.
Why is the Sky blue?
Sunlight reaches Earth’s atmosphere and is scattered in all directions by all the gases and particles in the air Blue light is scattered in all directions by the tiny molecules of air in Earth’s atmosphere. Blue is scattered more than other colors because it travels as shorter, smaller waves.
2.5. STRUCTURE OF THE ATMOSPHERE
- The atmosphere consists of different layers with varying density and temperature.
- Density is highest near the surface of the earth and decreases with increasing altitude.
- The column of atmosphere is divided into five different layers depending upon the temperature condition as troposphere, stratosphere, mesosphere, ionosphere and exosphere.
2.5.1. Troposphere
- The troposphere is the lowermost layer of the atmosphere and it contains 90% of the total mass of the atmosphere.
- Its average height is 13 km and extends roughly to a height of 8 km near the poles and about 18 km at the equator.
- Thickness of the troposphere is greatest at the equator because heat is transported to great heights by strong convectional currents.
- This layer contains dust particles and water vapour.
- All changes in climate and weather phenomenon (Condensation, precipitation, storms, etc.) take place in this layer. The temperature in this layer decreases at the rate of 1°C for every 165m of height (6.5° C per 1,000 m), because the atmosphere is more heated by long wave terrestrial radiation or infrared radiation
- This is the most important layer for all biological activity.
- Its thickness is more in summer than in winter
- The zone separating the troposphere from stratosphere is known as the tropopause.
- The air temperature at the tropopause is about minus 80°C at the equator and about minus 45°C over the poles
- The temperature here is nearly constant, and hence, it is called the tropopause.
2.5.2. Stratosphere
- The stratosphere begins at the tropopause, which forms its lower boundary and extends up to a height of 50 km.. The temperature in the lower part of this sphere does not change with altitude. In certain situations, there may be slight increase in temperature with elevation.
- There is a gradual temperature increase with height beyond 20 kilometers. This region is known as the upper stratosphere.
- Lower stratosphere is the ideal layer for flying of jet aircraft, because no air pockets and clouds. Therefore weather phenomena such as rain and lightning are also absent here.
- The total absence of water vapour in this layer prevents the formation of clouds, thus providing finest visibility,
- One important feature of the stratosphere is that it contains the Ozone layer.
- This layer absorbs ultra-violet radiation and shields life on the earth from intense, harmful form of energy.
- Temperature increases with height.
- The top edge of the stratosphere (upper stratosphere) is rich in ozone (O3). They capture the harmful ultraviolet rays of the sun and prevent the harmful effects, in the earth surface. Unfiltered radiation from the sun can destroy all animal tissue and cause skin diseases and cancer.
- It is free from dust particles
2.5.3. Upper Stratosphere – Ozonosphere
- Ozone layer or ozonosphere is a region of ozone concentration in the upper stratosphere extending from 25 to 50 kms from the surface of the earth.
- It protects the surface of the earth by absorbing high energy ultraviolet rays from the sun.
- It is made up of three atoms of oxygen.
Formation of Ozone: When an oxygen molecule is broken into two atoms by ultra-violet radiation and the free unstable atoms combine with two other oxygen molecules to form ozone.
If minute ultraviolet rays reach the earth,
- Helps in production of certain vitamins and promotes the growth of some virus and bacteria.
- It also has its role in the process of photosynthesis
Ozone hole in Antarctica
- Reduction of up to 70% in the ozone is observed in the southern hemisphere over Antarctica and was first reported in 1985. It is still continuing phenomenon.
- Decline upto 30% are in the winter and spring, when the stratosphere is colder.
- Reactions that take place on Polar Stratospheric Clouds (PSCs) play an important role in enhancing ozone depletion. PSCs form more readily in the extreme cold of Antarctic stratosphere. This is the reason for formation of ozone holes over Antarctica.
- In middle Latitudes, ozone depletion occurs rather than holes.
In Tropics, no significant trends in formation of ozone holes.
2.5.4. Mesosphere
- The mesosphere lies above the stratosphere, which extends up to a height of 80 km. In this layer, once again, temperature starts decreasing with the increase in altitude and reaches up to minus 100°C at the height of 80 km.
- The upper limit of mesosphere is known as the mesopause.
- Temperature decreases with increasing height and it is the coolest layer of the atmosphere.
- Meteors burn in this layer due to friction of the atmosphere.
2.5.5. Thermosphere
- The part of the atmosphere beyond mesopause is known as thermosphere wherein temperature increases rapidly with increasing height.
- It is estimated that the temperature at its upper limit (height undecided) becomes 1700°C.
2.5.6.IONOSPHERE (Lower Thermosphere)
- Free ions and electrons occur and been created by ionization of gas molecules through incoming solar ultraviolet and x- radiation.
- 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. These layers reflect radiomagnetic waves back to earth thus making radio communications.
- D-layer (between heights of 60 km – 99 km) reflects the signals of low frequency radio waves but absorbs the signals of medium and high frequency waves. This layer disappears with the sunset because it is associated with the solar radiation.
- E-layer, known as Kennelly – Heavyside layer, is confined in the height between 99 km – 130 km. This layer reflects the medium and high frequency radio waves back to the earth. This layer is produced due to the interaction of solar ultra-violet photons with nitrogen and nitrogen molecules and thus it also disappears with the sunset.
- Sporadic E layer is associated with high velocity winds and is created under special circumstances. This layer reflects very high frequency radio waves.
- F layer consists of two sub-layers e.g. F1 and F2 layers (150km – 38-km) and are collectively called ‘appleton layer’. These layers reflect medium and high frequency radio waves back to the earth.
- G layer (400km – above) most probably persists day and night but is not detectable.
- Absorption of solar radiation by ionized particles causes an increase in temperature with increasing height.
- Ionized particles protect the earth surface against meteorites that are burnt in this layer.
Aurora Borealis
- The aurora is a glow observed in the night sky, usually in the polar zone.
- It is also known as “northern lights” or “aurora borealis” Latin “northern dawn” since in Europe especially, it often appears as a reddish glow on the northern horizon as if the sun were rising from an unusual direction.
- In the Southern Hemisphere, it is known as “Aurora Australis”
2.5.7. Upper Thermosphere
- Concentration of ions that comprise the Van Allen radiation belt is the torus of energetic charged particles (i.e. plasma) around Earth and it is trapped by Earth’s magnetic field.
- When the belts “overload”, particles strike the upper atmosphere and fluoresce, causing the polar aurora.
IONOSPHERE
- It stretches from 80km to 500 km.
- It is called Ionosphere because; in this part of the atmosphere the sun’s radiation is ionized.
- It reflects the radiowaves back to the earth’s surface which are useful for modern communication systems.
- The colourful displays of auroras are called the Northern lights of Aurora borealis in the Northern Hemisphere, the Southern lights of Aurora Australis in the Southern Hemisphere.
- The ionosphere is located between 80 and 400 km above the mesopause.
- It contains electrically charged particles known as ions, and hence, it is known as ionosphere.
- Radio waves transmitted from the earth are reflected back to the earth by this layer.
- Temperature here starts increasing with height.
2.5.8. EXOSPHERE
- The uppermost layer of the atmosphere above the ionosphere is known as the exosphere.
- This is the highest layer but very little is known about it.
- Contents in this layer are extremely rarefied, and it gradually merges with the outer space. Lighter gases like hydrogen and helium are present over here.
Homosphere
Troposphere, Stratosphere and Mesosphere –extends upto an altitude of about 90 km from sea level. It is a region of uniform mixing and composition which have higher densities of its gaseous constituents.
Heterosphere
Above 90 km, the composition begins to change with progressive increase in lighter gases. The molecules and atoms tend to separate and arrange themselves in layers each with different composition.