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Development Team Paper No: 8 Atmospheric Processes Module: 2 Structure of Atmosphere Development Team Principal Investigator Prof. R.K. Kohli & Prof. V.K. Garg &Prof. Ashok Dhawan Co- Principal Investigator Central University of Punjab, Bathinda Dr. Sunayan Saha Paper Coordinator Scientist, ICAR-National Institute of Abiotic Stress Management, Baramati, Pune Dr. Nilimesh Mridha Content Writer ICAR-Indian Council of Agricultural Research, New Delhi Dr. Puneeta Pandey Content Reviewer Central University of Punjab, Bhatinda Anchor Institute Central University of Punjab 1 Atmospheric Processes Environmental Sciences Structure of Atmosphere Description of Module Subject Name Environmental Sciences Paper Name Atmospheric Processes Module Name/Title Structure of Atmosphere Module Id EVS/AP-VIII/2 Pre-requisites Know about the vertical structure of the atmosphere Objectives Know temperature distribution, electrical properties and gaseous composition of layers Interesting phenomena happening in various layers Temperature profile, Troposphere, Stratosphere, Mesosphere, Thermosphere, Keywords Ionosphere 2 Atmospheric Processes Environmental Sciences Structure of Atmosphere Module: 2 Structure of Atmosphere TABLE OF CONTENTS 1. Learning Outcomes 2. Introduction 3. Structure of atmosphere based on vertical temperature profile 3.1. Troposphere: structure and characteristics 3.1.1. Thickness and factors affecting it 3.1.2. Chemical composition 3.1.3. Temperature characteristics 3.1.4. Tropopause-Isothermal layer 3.1.5. Interesting facts 3.2. Stratosphere: structure and characteristics 3.2.1. Temperature stratification and ozone layer 3.2.2. Interesting facts 3.3. Mesosphere: structure and characteristics 3.4. Thermosphere: structure and characteristics 3.5. Exosphere: structure and characteristics 4. Structure of atmosphere based on gaseous composition 5. Layers of atmosphere based on electrical properties 5.1. Ionosphere and its sub layers 5.2. Some important phenomena of Ionosphere 6. Summary 3 Atmospheric Processes Environmental Sciences Structure of Atmosphere 1. Learning outcomes After studying this module, you shall be able to: Know about the vertical structure of the atmosphere Know temperature distribution, electrical properties and gaseous composition of layers Interesting phenomena happening in various layers 2. Introduction The atmosphere can be generally defined as an invisible gaseous blanket around the earth held by earth’s gravitational force. In one way or another, it affects all the entity exists on the earth - it is closely connected to our lives. Without this blanket, it would be unimaginably cold at night and unbearably hot during the day. Unlike other planets or extraterrestrial bodies, the planet earth is very unique in a sense that it is covered with atmosphere that provides optimum condition to support life to flourish. The earth’s atmosphere are composed of two primary gases, viz., nitrogen and oxygen and water vapor that back up the life whereas carbon dioxide, hydrogen or helium gases are abundant with no water vapor in the atmospheres of other planets. Detailed study of the structure of earth’s atmosphere is extremely important to gather knowledge regarding its origin, atmospheric composition and processes, different phenomena related to weather and climate, aviation, communication system etc. If we look into the earth’s atmosphere, it is seen that the atmosphere is not physically uniform throughout but has got considerable variations, especially in terms of pressure and temperature with height. An examination of vertical profile of the atmosphere reveals that it is comprised of a series of layers. These layers could be defined in several ways such as based on The vertical temperature distribution Electrical properties of the layer, and The gaseous composition of it These categorization of atmosphere is elaborated below: 4 Atmospheric Processes Environmental Sciences Structure of Atmosphere 3. Layers of atmosphere based on vertical temperature profile Air temperature throughout atmosphere shows complicated vertical profile and may be divided into five main distinguished layers, namely troposphere, stratosphere, mesosphere, thermosphere and exosphere. They are layered one atop the other and each one is very much needed to support and protect the life on the earth.The atmospheric also contains transition layers between two successive main layers.The nomenclature of these transitional layers zones contains the term – pause at its end. These are tropopause (from the end of troposphere and till the beginning of stratosphere), stratopause (boundary separating stratosphere from mesosphere), mesopause (transition region between mesosphere and thermosphere) and thermopause (between thermosphere and exosphere). Fig. 1. A schematic of the vertical profile of the atmosphere with relative position of its various layers. It also depicts the pattern of temperature changes with height from the ground and inside these layers. Approximate pressure levels at different altitude can also be noted. (Source: www.teachertech.rice.edu) 3.1. Troposphere: structure and characteristics The word ‘troposphere’ originates from the Greek word ‘tropein’ which means to change, circulate or mix. Troposphere is the lowest layer of the atmosphere which is close to the earth’s surface where almost all the weather phenomenon that we experience like convection, turbulence and most of the cloud formation occur. 5 Atmospheric Processes Environmental Sciences Structure of Atmosphere 3.1.1. Thickness and factors affecting it The thickness of this layer varies from 7-8 km at poles to 16-18 km over the equator. The depth of the troposphere is influenced by the latitude, season and time of the day. The reasons behind the non-uniformity in thickness of the troposphere are: I. Equator gets high insolation and strong convective fluxes As equator gets highest amount of insolation, it becomes warmer than other parts of the earth. According to Charles’ law, density reduces with increase in temperature at constant pressure which implies more the temperature over a place, more the air gets heated and rises upward due to decrease in density. Hence, due to differential heating of the earth’s surface, equatorial regions experience more amounts of convective fluxes. Simply, we can say that although both equator and poles have the same amount of atmospheric mass over it, the thermal expansion of air over equator is more than the poles and that makes the atmosphere thicker over equator than over the poles. II. Higher gravitational pull on atmospheric gases at poles Due to ellipsoid shape of the earth, the gravitational pull is not uniform throughout and it is more over poles. As a result atmospheric molecules are drawn with more force near the poles causing contraction of the atmosphere. III. Equator has highest centrifugal force due to earth’s rotation The atmosphere over equator tends to swell due to maximum speed of the rotating earth and the minimum Coriolis effect. 3.1.2. Chemical composition The chemical composition of this layer is basically uniform, with the important exception of water vapor. About 99% of the total water vapor in the atmosphere is present in the troposphere itself. The concentration of water vapor varies with latitudes. Equatorial region has maximum concentration of water vapor (up to 3%) and reduces towards polar region. At the same time water vapor is more at 6 Atmospheric Processes Environmental Sciences Structure of Atmosphere the surface and decreases with altitude.The troposphere primarily comprises of nitrogen (78%) and oxygen (21%) with very little amount of trace gases. Nearly 75% of the total mass of atmosphere is contained in troposphere and thus denser than the overlaying layers. 3.1.3. Temperature characteristics Lapse rate: We usually feel cooler on the top of a mountain than in the valley below and chances of encountering snow or ice is greater in a higher hill than a hill with low altitude. Such thing happen as, troposphere, in general, shows a decreasing trend of air temperature with elevation. Air temperature usually declines from the earth’s surface with increasing height up to about 11 km due to the fact that air near the surface gets heated up much more by earth’s emitted long wave radiation energy than the air which is away from it.The rate at which the air temperature reduces with altitude is called the temperature lapse rate and average valueof it is about 6.5 °C per km. This lapse rate is just an average value of day to day variations. Inversion: Sometimes, an increase in the air temperature with height may actually happen. Such a condition is known as temperature inversion and this prevents the vertical mixing of air. Temperature inversion plays an important role in air pollution where pollutants released at ground level get confined in the lower layers of atmosphere thus affecting the human and animal lives. 3.1.4. Tropopause-Isothermal layer It can be observed that just above a certain height of the troposphere, the lapse rate becomes zero which means the air temperature (about - 60°C) remains constant with altitude in that region i.e. an isothermal (equal temperature) layer. The bottom of this layer represents top of the troposphere and its upper limit as the starting point of the stratosphere. This transitional boundary differentiating the stratosphere from the troposphere is called the tropopause. It is typically higher over equatorial regions and lower in polar region. The height of tropopause generally increases in
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