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Home , Atmosphere of Earth, Homosphere

Lecture Note on

-Dr S P Singh

Department of , A N College, Patna

ORIGIN The origin of the ’s atmosphere is spanned over millions of years. It involves the following stages; Stage 1: Primitive Atmosphere

∑ The formation of earth comprising of primarily H2, (H2O), N2, CO2 and CO took place about 5 billion years ago from the solar nebula. These deliver to the because of having very high . ∑ There was no differentiated core resulting thereby weak earth’s gravity. ∑ About 3.5 billion years ago, the accretion of earth took place leading to an exothermic process. Heat was released and absorbed by noble gases; most of which escaped out. ∑ Many constituents chemically combined to form gases which held on the earth under its gravitational pull.

∑ The atmosphere consisted of H2, N2, CH4, NH3, H2O, and noble gases. Stage 2: Reducing Atmosphere ∑ This atmosphere existed from 3.5 to 2 billion years ago. ∑ *The formed about 4 billion years ago resulting in formation of the huge from condensed water vapour. ∑ The differentiation of layers took place into a solid core, liquid mantle and thin crust. The solid core resulted in a strong gravitational force. The released from accretion of and decay of radioactive isotopes caused massive volcanic eruptions.

∑ Gases dissolved in the molten magma were reduced. Large amounts of N2 and CO2 were released into the

atmosphere. Most of the CO2 dissolved in water leading to the formation of carbonate sediments. The other

gases present were H2, N2, CH4, NH3, H2S, SO2, Cl2 and CO in trace amounts, etc. but free (O2) was not present. Stage 3: Oxidizing Atmosphere

∑ Photosynthesizing were present around 2.7 billion years ago, but the O2 released during

was used in oxidation of metals like iron but free O2 started forming in the atmosphere around 2.4 billion years ago.

∑ The atmosphere became oxidizing and O2 consuming forms began appearing in the oceans on the earth.

∑ Photolysis of water from (UV) resulted in generation of O2.

∑ O2 molecules were absorbing the UV and getting converted to (O3). Soon, an was formed, which started protecting the surface of the earth from high energy UV radiations coming from the .

1OBJECTIVES

• To learn the importance of atmosphere.

• To to explain the composition of atmosphere.

• To differentiate Earth’s various atmospheric layers

• To know the characteristics of different layers of the atmosphere.

• To learn heat budget of the Earth Atmospheric system INTRODUCTION

• Atmosphere envelopes the mother earth. It comprises of various gases (air) along with water vapour (moisture) and particles.

• Gases create , and allow water vapour to exist on Earth’s surface. It is the gravitational pull of the earth which keeps on maintaining gases and moisture near to the Earth. Both of them are the essential components for existence of life.

• An imaginary line called the Karmin line demarcates the boarder of the atmosphere from the roughly at the height of 100 km from the level.

• The components of the atmosphere changes with the change in and place. It warms the Earth’s surface.

SIGNIFICANCE The atmosphere of earth possesses a lot of significance such as

∑ containing gases; O2 plays role in , CO2 in photosynthesis, larger % of to prevent excessive oxidation, moisture in creating comfortable environment, etc. ∑ having of heat retention to maintain warmth, energy absorption and radiation to maintain energy / heat balance to prevent excessive heating and cooling. ∑ protecting living beings from harmful UV rays from the Sun. ∑ allowing the bio-geo chemical cycles of C, N, O, P and S. ∑ helping in radio communications, air fly and dynamic processes of air flow, etc. COMPOSITION OF ATMOSPHERE

The atmosphere is divided into two layers (i) the heterosphere and (ii) the homosphere.

∑ The outermost sphere of the atmosphere is known as the heterosphere, where the gases are distributed on distinct layers in accordance with their atomic weight. Gravitational force also plays a vital role. In general, the lighter elements like and make up the outer layer and the heavier elements such as nitrogen and oxygen remain at the lower layer. ∑ The homosphere lies between the Earth’s surface and heterosphere. The gases are uniformly distributed in this layer. The envelop of gases that is what we call Earth’s Atmosphere is bound to remain with the planet more or less permanently due to gravity. Within 50miles above the surface, the air is so thoroughly mixed by that the variation of permanent constituent gases is minimal. Two gases nitrogen and oxygen comprise of 99% of dry gases by . Water vapour is a variable constituent which can increase upto 4%. There are many other gases which are anthropogenic leading to air .

TROPOSHERE (~10 °C to -60°C)

• The is the lowest layer of Earth's atmosphere.

• It ranges from Earth's surface to an average height of about 12 km (7.5 miles; 39,000 ft). Its varies from about 9 km (5.6 miles; 30,000 ft) at the geographic poles to 17 km (11 miles; 56,000 ft) at the .

• The troposphere contains roughly 80% of the of Earth's atmosphere. 50% of the total mass of the atmosphere is located in the lower 5.6 km (3.5 mi; 18,000 ft) of the troposphere.

• The troposphere is mostly heated through energy transfer from the surface. This results from the Sun's radiation striking the earth and the earth then warming the air above it. Thus the lower section is the warmest section of troposphere.

• The usually declines with increasing altitude in the troposphere. The rate of change of air temperature with height is called the "". In the troposphere, the lapse rate is generally about 6.5 deg C per kilometer increase in altitude.

• The troposphere is bounded above by the , a boundary marked in most places by a temperature (i.e. a layer of relatively warm air above a colder one), and in others by a zone which is isothermal with height. • Nearly all atmospheric water vapour or moisture is found in the troposphere, so it is the layer where most of Earth's takes place.

• Because warm air tends to rise and cool air tends to sink, the troposphere is a location of much movement of air, or "turbulence". Hence, the troposphere is described by meteorologists as being "well-mixed".

• If are injected into the troposphere, they are mixed throughout its depth in a few days and, usually within a week or so, will fall back to the ground with the (e.g., ). Thus, the troposphere has a self-cleaning mechanism.

• Most conventional activity takes place in the troposphere.

STRATOSPHERE (-60°C to 0°C)

• The lies above the troposphere and is separated from it by the tropopause.

• It ranges from roughly 12 km (7.5 miles; 39,000 ft) above Earth's surface () to an altitude of about 50 to 55 km (31 to 34 miles; 164,000 to 180,000 ft).

• It contains the ozone layer. Temperatures rises with increasing altitude due to absorption of ultraviolet radiation (UV) radiation from the Sun by the Ozone layer which, in turn, increases the motion of the ozone molecules. The ozone molecules then collide with other molecules in the air, increasing its temperature.

• The stratospheric temperature profile creates very stable atmospheric conditions, so the stratosphere lacks the weather-producing air turbulence.

• Particles that travel from the troposphere into the stratosphere can stay aloft for many years without returning to the ground. For example, large volcanic eruptions force ash to be projected into the stratosphere, where it may remain for years and causing slight global cooling in the process.

• The stratosphere is almost completely free from and other forms of weather. However, polar stratospheric or nacreous clouds are occasionally seen in the lower part of this layer of the atmosphere where the air is coldest.

• The importance of the ozone layer lies with the fact that (1) ozone helps the earth to maintain its heat balance, and (2) ozone reduces the amount of harmful UV radiation that reaches the earth's surface. Ozone is produced and destroyed as well in the stratosphere. Ozone destruction can be both natural (UV radiation or molecular collisions) or man- made (e.g., ). (0°C to -90°C)

• The mesosphere is the third highest layer of Earth's atmosphere that ranges from altitude of about 50 km (31 miles; 160,000 ft) to the at 80–85 km (50–53 miles; 260,000–280,000 ft) above sea level.

• Temperatures decreases with increasing altitude to the mesopause (the point of minimum temperature at the boundary between the mesosphere and the atmospheric regions).

• Due to lack of solar heating and very strong from CO2, the mesosphere is the coldest zone on Earth. It has an average temperature around −85 °C(−120 °F; 190 K).

• Below the mesopause, the air is so that even the very scarce water vapour at this altitude can be sublimated into polar-mesospheric noctilucent clouds. Noctilucent clouds, or shining clouds, are tenuous like phenomena in the upper atmosphere of Earth.

• The mesosphere is also the layer where most metereors burn up upon atmospheric entrance. A is a small rocky or metallic body in outer space.

THERMOSPHERE (-90°C to 500-1500°C)

• The thermosphere of Earth's atmosphere ranges from an altitude of about 80 km (50 miles; 260,000 ft) up to the at an altitude range of 500–1000 km (310– 620 mi; 1,600,000–3,300,000 ft).

• The temperature of the thermosphere gradually increases with height. The temperature of this layer can rise as high as 1500 °C (2700 °F). This layer is completely cloudless and free of .

• The lower part of the thermosphere, from 80 to 550 kilometres (50 to 342 mi) above Earth's surface, contains the .

EXOSPHERE

The lies over the thermosphere. It is the uppermost region of Earth’s atmosphere. It gradually fades into the vacuum of space. It is almost the same as the airless void of outer space. Particles, in principle, are still bound gravitationally to the Earth. There is no clear upper boundary of this layer. However, the outermost limit of the exosphere places the uppermost edge of Earth's atmosphere around 190,000 km (120,000 miles), about halfway to the . CHANGES IN AIR PRESSURE & AIR TEMPERATURE WITH HEIGHT Air Pressure with Height • Pressure decreases with height more rapidly near the ground because the atmosphere composed of can compress in response to the earth's gravitational pull. • If two air columns of same height are next to one another, but one with more molecules packed into it than the other, there is horizontal variations of pressure resulting high and low pressure systems. • The rate at which air pressure changes with height is determined primarily by the average temperature in the column under consideration. Air Temperature with Height • Temperature has a more complicated structure, mostly because the temperature of the air relies on the energy its molecules receive from radiation. • The Sun & the Earth are the two main sources of radiation in the atmosphere. The sun's radiation includes mostly near (37%), visible (44%), and ultraviolet (7%) while the earth's radiation is mostly far infrared. • Infrared is generally what we feel as "heat", visible is what we see, and ultraviolet is what our skin absorbs to make us tan or burn. The temperature structure of the atmosphere is controlled significantly by whichever of these three types of radiation are affecting the region.

HEAT BUDGET OF THE EARTH’S ATMOSPHERIC SYSTEM • Sun is the ultimate source of energy. The Earth receives energy from the Sun via insolation (all forms of electromagnetic radiation) and distributes it throughout its components. • The Earth radiates energy via atmospheric and terrestrial radiation (shifted to longer electromagnetic wavelengths) back to outer space. • The balance between incoming heat / energy from the Sun and outgoing heat /energy from the Earth is termed as the earth-atmosphere energy balance or Earth’s energy budget. On an average net Radiation Budget= Incoming Radiation – Outgoing Radiation =Zero • Most of the received by earth is scattered light.

• The differential heat received from sun by different regions on earth is the ultimate reason behind all climatic features.

• The amount of insolation received varies from latitude to latitude.

• Regions within the equator and 40° N and S latitudes receive abundant and hence more heat will be gained than lost. Hence they are energy surplus regions.

• Regions beyond 40° N and S latitudes lose more heat than that gained from sunlight. Hence they are energy deficit regions (This is because of slant sunlight and high albedo of polar regions). Going by this logic, the tropics should have been getting progressively hotter and the poles getting progressively cooler. And the planet would have been inhospitable except for few regions near mid-latitudes. But, in reality, this does not happen.

• The atmosphere (planetary ) and the oceans ( currents) transfer excess heat from the tropics (energy surplus region) towards the poles (energy deficit regions) making up for heat loss at higher latitudes.

• And most of the takes place across the mid-latitudes (30° to 50°) and hence much of the stormy weather is associated with this region.

• Thus, the transfer of surplus energy from the lower latitudes to the deficit energy zone of the higher latitudes, maintains an overall balance over the earth’s surface.

EARTH’S THERMODYNAMIC ENERGY BUDGET • The Earth system is maintained in a state so far away from Thermodynamic equilibrium despite the natural direction towards mixing and depleting sources of free energy (matter and energy are conserved obeying 1st law of thermodynamics). • When the energy transfer takes place, matter becomes less organized with time. It is a critical component that allows systems to evolve away from thermodynamic equilibrium without violating the second law of thermodynamics.

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