Nayana Babu S.N.G.K B.Ed COLLEGE POTHENCODE (SOCIAL SCIENCE 2014-2015): October 2015

ONLINE ASSIGNMENT

STRUCTURE OF ATMOSPHERE

SUBMITTED BY:

NAYANA BABU

SOCIAL SCIENCE

B. Ed. TRAINEE

SNGK POTHENKODE

SUBMITTED TO:

TENNY VARGHESE

HoD. SOCIAL SCIENCE

SNGK B. Ed. COLLEGE

POTHENKODE

SUBMITTED ON- 20/08/2015

INDEX

CONTENT	PAGE NUMBER
INTRODUCTION	02
CONTENT	03
ATMOSPHERIC STRUCTURE	03
COMPOSITION OF THE ATMOSPHERE	04
TROPOSPHERE	06
STRATOSPHERE	07
MESOSPHERE	08
THERMOSPHERE	08
CONCLUSION	09
REFERENCES	10

INTRODUCTION

STRUCTURE AND COMPOSITION OF THE ATMOSPHERE

The atmosphere of Earth is mostly composed of nitrogen. It also contains oxygen used by most organisms for respiration and carbon dioxide used by plants, algae and cyanobacteria for photosynthesis. It protects living organisms from genetic damage by solar ultraviolet radiation, solar wind and cosmic rays. Its current composition is the product of billions of years of biochemical modification of the paleoatmosphere by living organisms.The term stellar atmosphere describes the outer region of a star, and typically includes the portion starting from the opaque photosphere outwards. Stars with sufficiently low temperatures may form compound molecules in their outer atmosphere.Initial atmospheric composition is generally related to the chemistry and temperature of the local solar nebula during planetary formation and the subsequent escape of interior gases. The original atmospheres started with the radially local rotating gases that collapsed to the spaced rings that formed the planets. They were then modified over time by various complex factors, resulting in quite different outcomes.

The Earth's atmosphere consists of a number of layers, that differ in properties such as composition, temperature and pressure. The lowest layer is the troposphere, which includes the planetary boundary layer or peplosphere at its base. Three quarters of the atmospheric mass resides within the troposphere, and the depth of this layer varies between 17 km at the equator and 7 km at the poles. The stratosphere, from 20 to 50 km, includes the ozone layer, located at altitudes between 15 and 35 km, which absorb ultraviolet energy from the Sun. The mesosphere, from 50 to 85 km is the layer in which most meteors burn up. The thermosphere extends from 85 km to the base of the exosphere at 690 km and contains the ionosphere, a region where the atmosphere is ionised by incoming solar radiation. The Kármán line, located within the thermosphere at an altitude of 100 km, is commonly used to define the boundary between the Earth's atmosphere and outer space. The exosphere extends from about 690 to 1,000 km above the surface, where it interacts with the planet's magnetosphere. Each of the layers has a different lapse rate, defining the rate of change in temperature with height.

CONTENT

ATMOSPHERIC STRUCTURE

The vertical distribution of temperature, pressure,density, and composition of the atmosphere constitutes atmospheric structure. Thesequantities also vary with season and location in latitude and longitude, as well as fromnight to day; however under the topic of atmospheric structure, the focus is on theaverage variations with height above sea level.Although it is impossible to define an absolute depth of the atmosphere, most ofthe atmosphere is confined to a narrow shell around the planet, with the pressure anddensity of air decreasing rapidly with altitude and gradually merging into the emptinessof space. Fifty percent of the mass of the atmosphere is within 5.5 kilometers (3.4 miles)of sea level; 90 percent is within about 16 kilometers (10 miles) of sea level, and 99.9percent is below 49 kilometers (about 30 miles). Since the mean radius of the Earth is6,370 kilometers (3,960 miles), the atmosphere is a very thin coating around our planet.

At altitudes of 500 to 600 kilometers (about 350 miles) it is still possible to detect air,although the density of gases there is less than 10-12(one trillionth) of that at sea level.

Figure displays the vertical temperature structure and the pressure distributionof the atmosphere. The names given to the various layers, defined based on thetemperature change with height, and the boundaries between these layers are also shown.The heights, pressures, and temperatures in the diagram are based on the U.S. StandardAtmosphere, which represents average conditions above the middle latitudes.

COMPOSITION OF THE ATMOSPHERE

Air is a mixture of a number of gases, but the most abundant are molecular nitrogen (N2) and molecular oxygen (O2), with a tiny amount of the inert gas argon (Ar). These gases make up more than 99.9 percent of the mass of dry air; the ratio of the number of molecules of each is nearly constant up to a height of about 80 or 90 kilometers (about 60 miles). Other gases, whose relative concentrations vary, exist only in small quantities.

The most important of these are water vapor (H2O) and carbon dioxide (CO2), which absorb and emit longwave radiation, and ozone (O3). The distribution of these “trace” gases therefore affects the vertical temperature distribution. In addition to the layers of the atmosphere shown in Figure , which are defined based on temperature, there are also atmospheric layers defined based on the composition of air. The region below 80 to 90 kilometers is called the homosphere because the main constituents of air are homogeneously distributed regardless of weight. Above this level, molecules and atoms tend to separate, with the heavier gases beneath the lighter ones, in a layer called the heterosphere. Above about 64 kilometers (40 miles), in the upper homosphere and through the heterosphere, gases can be readily ionized by very shortwave radiation; that is, they can lose an electron from their atoms. Thus free electrons and ions are plentiful, giving this region the name ionosphere. The ionosphere is very important in long-range radio transmission. Ultraviolet radiation from the Sun interacts with oxygen molecules in the stratosphere to form ozone (O3). The peak ozone concentration is found at about 25 kilometers (15.5 miles). Although the total amount is very small—only about 10 molecules of O3 per million molecules of air at highest concentrations—ozone blocks much ultraviolet radiation that would otherwise damage living things. The amount of water vapor in the air is quite variable and drops off rapidly with height. At peak concentrations near the surface, water vapor can make up around 4 percent of the mass of air, but in total water vapor makes up only around 0.33 percent of the mass of the atmosphere. At altitudes of about 10 kilometers the concentration of water vapor is only about 1 percent of that at the ground, and higher levels have much less. This occurs primarily because very little water vapor is needed to saturate cold air, and excess water will precipitate as rain or snow. Unlike water vapor and ozone, carbon dioxide is fairly well mixed in the air. Despite the low concentration of CO2 (of 1 million molecules of air near the surface, only about 400 are CO2) it is vital for photosynthesis as well as to maintaining the radiation balance. Water vapor and carbon dioxide both absorb little of the Sun’s radiation, but they do absorb some of the Earth’s radiation and reradiate it both upward and downward. This gives rise to the greenhouse effect, which keeps surface temperatures much warmer than they would otherwise be.

TROPOSPHERE

The lowest atmospheric layer, the troposphere, is the thinnest of the layers,but it contains about 80 percent of the mass of the atmosphere. This is the region wheremost of what we know as “weather” takes place. Almost all clouds and precipitation formin the troposphere; weather fronts, hurricanes, and thunderstorms are tropospheric phenomena. Weather activity produces much upward and downward motion, so thetroposphere is a region of mixing: the prefix “tropo” comes from the Greek word for“turning over.”The lowest part of the troposphere—usually the lowest kilometer or so—is calledthe planetary boundary layer, where winds and temperatures are affected bycharacteristics of the underlying surface such as the height of vegetation and thetemperature of the ground or water surface. Close to the surface, changes from daytime,when the Earth gains heat from the Sun, to nighttime, when it loses heat, are muchgreater than at higher altitudes in the troposphere.The rate at which temperature decreases with height is called the lapse rate.Above the boundary layer the troposphere’s lapse rate averages about 6° to 7°C perkilometer, but the actual lapse rate in a given situation can be quite different. Sometimesthe temperature is constant with height, or it may even increase with height in a shallowlayer called an inversion.When a dry air parcel rises without mixing heat with the environment, the lapserate is approximately 10°C per kilometer. The average tropospheric lapse rate is less thanthis value primarily due to latent heating from the condensation of water vapor (althoughother processes such as large scale dynamical motions and radiation are important inparts of the troposphere as well). As warm, humid parcels rise, the air becomes coldenough for condensation to occur. The condensation results in a heating of the air parcels,with temperature increases of up to 50°C from condensation if all the water vapor of ahumid parcel condenses out. Thus the air cools less rapidly as it rises and the lapse rate is

less. Condensation of water vapor is also a fundamentalprocess in driving large scale circulations on Earth and in the dynamics of hurricanes.The top of the troposphere, the tropopause, averages about 11 to 12 kilometersabove sea level, but it can be as low as 7 to 8 kilometers in polar regions and as high as16 to 18 kilometers in the tropics. The coldest tropopause temperatures are over thetropics, where it can be -70°C or colder, whereas over the poles tropopause temperaturesof -40°C are found. The tropopause is generally higher in summer than in winter, and isexpected to rise in warmer climates as well. The tropopause is not always one continuous surface. Often there is a distincttropopause above the tropics with a break at around 30° latitude where a new tropopauseforms at a lower level. That tropopause slopes downward toward the pole and sometimesshows another break. A jet stream is often found in these tropopause breaks, whichbecome regions of mixing between troposphere and stratosphere.

Stratosphere

Above the tropopause the stratosphere begins. The temperature usuallystops decreasing; it becomes roughly constant at first and then begins to increase withheight. The air in the stratosphere is very dry, generally having less than 0.05 percent ofthe maximum amount of water vapor found near the ground. Clouds are very rare here,except for occasional thunderstorm penetration in the lower part. The stratosphere ends atabout 50 kilometers (31 miles) above the surface at the stratopause. Here the density ofthe air is only about one thousandth of that at sea level, but the temperature may be about0°C and is sometimes near 20°C. Temperature increases in the stratosphere because ofthe presence of ozone. Ozone’s absorption of ultraviolet radiation leads to warmer air thatcan sometimes reach temperatures as high as those found at the ground. This temperaturedistribution—warmer air above colder air—dampens vertical motion and mixing. Thisproduces a stratified distribution of material, hence the name of this layer. There is littlemixing across the tropopause; thus material injected into the stratosphere from anexplosive volcano, for instance, can remain there for several years. Above the Stratosphere about 99.9 percent of the mass of the atmosphere is in thetroposphere and stratosphere combined. Of the remaining mass, about 99 percent is in thenext layer, called the mesosphere.

Mesosphere

The mesosphere, or middle atmosphere, begins abovethe stratopause. The amount of ozone here is very small, so the temperature ceases to increase and begins to drop as the air loses heat to space by radiation, mainly from carbondioxide. The temperature drops to near -90°C at the top of this layer, the mesopause, at analtitude of about 85 to 90 kilometers. The mesosphere is where meteors heat up andbecome visible.Above the mesopause, in the thermosphere, the temperature begins to increaseagain because the gases there absorb the very short ultraviolet waves of the Sun’sradiation; that radiation does not penetrate any lower.

Thermosphere

The thermosphere, together withthe upper reaches of the mesosphere, is the region of the ionosphere. Auroras form in thethermosphere above both polar regions when gases in this layer are excited by particlesfrom the Sun. The top of this layer, the thermopause, is not well defined. It is estimated tobe at between 500 and 1,000 kilometers and changes radically with the amount of

sunlight falling on it. The temperature in this region is not well defined either, but valuesover 1,000°C are sometimes reported. Beyond the thermosphere is a region called the exosphere. In this layer, the last ofthe atmospheric “spheres,” the air is so rarefied that gas molecules may not collide witheach other, and a few escape Earth’s gravitational field altogether. In the exosphere theatmosphere gradually gives way to the radiation belts and magnetic fields of outer space.

CONCLUSION

The fact that the moon's surface is covered with meteorite impact craters is obvious to us today. Though the moon is not far from us, impact craters are few and far between on Earth. As it turns out, Earth has received just as many incoming meteorites as the moon, but the presence of the atmosphere has determined the fate of many of them. Small meteorites burn up in the atmosphere before ever reaching Earth. Those that do hit the surface and create an impact crater are lost to us in a different way – the craters are quickly eroded by weather generated in the atmosphere, and the evidence is washed away. The moon, on the other hand, has no atmosphere, and thus every meteor aimed at the moon hits it, and the craters have remained essentially unchanged for 4 billion years (Figure 1).