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In my opinion the blue color is totally due to the ozone gas. Where it is situated, in the stratosphere, this pale blue gas becomes a deep blue liquid and, in the intensely cold areas such as over the poles, they might even become dark blue black. The usual color of the sky is due to the ‘dilution’ effect from layers of dust particles in the intervening troposphere. Please read the recently published article of mine on this subject, which I’ll copy and paste below. I hope the article comes through intact. After reading it, please let me know what you think!Why the sky is bluePuthalath Koroth Raghuprasada)2400 E. 8th Sreet, Odessa, Texas 79761, USA(Received 13 April 2013; accepted 14 February 2017; published online 3 March 2017)Abstract: The “Tyndall effect” and “Rayleigh scattering” are the accepted explanations for theblue color of the sky. However, since heavy rainfall is known to remove particulates as well as thegases and yet the sky actually becomes a deeper blue in color, both explanations are probablyinvalid. The current author proposes the following explanation for the blue color of the sky: This isthe pale blue of ozone gas, which will appear deeper blue when there are sufficient quantities of it.Further, ozone attains an even deeper blue color when it becomes a liquid at around a temperatureof 161K (-112 C) and a blue to violet-black solid at temperatures below 82K (-193.2 C). The latteris the case in the lower Stratosphere, especially near the poles. Also, since ozone absorbs ultravioletradiations, it is likely that some of the spectra close to UV (such as violet, indigo, and blue)radiations are also absorbed or scattered by ozone and this may add to the blue color of the ozonelayer. To an observer on the surface of the earth, the many layers of dust and other particulates inthe intervening Troposphere, which dampen the deep blue of the ozone layer, will make the “sky”appear less blue. How much each of the above factors contributes to the color of the sky is notknown but jointly, they can explain all the observed phenomena. VC 2017 Physics EssaysPublication. [Why the sky is blue]Re´sume´ : L’effet Tyndall et la diffusion de Rayleigh sont les explications admises de la couleurbleue du ciel. Les fortes pluies sont cependant connues pour e´liminer les matie`res particulariseainsi que les gaz tout en donnant au ciel une couleur bleue plus fonce´e, ce qui signifie que ces deuxexplications sont probablement incorrectes. L’auteur de cette e´tude propose l’explication suivantepour la couleur bleue du ciel: il s’agit du bleu paˆle de l’ozone gazeux, qui apparaıˆt plus fonce´lorsqu’il est pre´sent en quantite´s suffisantes. En outre, l’ozone acquiert une couleur bleue encoreplus fonce´e lorsqu’il devient liquide au voisinage de la tempe´rature de 161K (112 C), et unecouleur entre le bleu et le violet-noir lorsqu’il devient solide a` moins de 82K (193,2 C). Cettedernie`re situation existe dans la stratosphe`re infe´rieure, en particulier pre`s des poˆles. Sachant quel’ozone absorbe le rayonnement ultraviolet, il est probable qu’une partie du spectre au voisinage durayonnement UV (notamment le violet, l’indigo et le bleu) est aussi absorbe´ ou diffuse´ par l’ozone,ce qui accentuerait la couleur bleue de la couche d’ozone. Pour un observateur a` la surface de laterre, les nombreuses couches de poussie`res et autres matie`res particulaires dans la troposphe`re(qui occupe une position interme´diaire) atte´nuent le bleu fonce´ de la couche d’ozone et donnentune apparence moins bleue au « ciel ». La contribution de chacun des facteurs ci-dessus a` la couleurdu ciel n’est pas connue, mais ensemble ils peuvent expliquer tous les phe´nome`nes observe´s.Key words: Tyndall Effect; Rayleigh Scattering; Ozone; Stratosphere.I. INTRODUCTIONThe blue color of the sky was originally attributed byscientists to the Tyndall effect. Described by John Tyndall in1859, this effect is a preferential scattering of shorter wavelengthblue light by particulate matter (particles 40–900 nmin size, the wavelength of visible light being 400–750 nm) inthe atmosphere.1–3 The longer wavelength spectra of sun’slight such as red and yellow have the ability to coursethrough these particles. Thus, while we see sky as beingblue, the sun and the sunlight take on a yellow hue. A classicexample of this phenomenon is the bluish tinge assumed byan opalescent stone when a beam of white light traverses thestone, while the light that exits on the opposite side takes ona yellowish tinge. Experiments to prove the above explanationinvolved passing light through containers of water witha suspension of colloidal substances; milk has been a popularingredient in these experiments. The demonstration that thecontainer turns bluish in color, while the beam of light thatexits the container is more yellow/red has been quoted asconfirming the phenomenon and as the explanation of thecolor of sky. The scientific explanation of related observationssuch as why near the horizon the sky appears less blueand at dawn and dusk the sun and the sky appear orange-redis also attributed to the same scattering effect. In the former,more scattering of blue light by the increased amount of particlesin the air closer to the earth is given as the reason. Thelatter effect is attributed to “rescattering” of blue light so thata) the orange and red are accentuated. [email protected] 0836-1398 (Print); 2371-2236 (Online)/2017/30(1)/116/4/$25.00 116 VC 2017 Physics Essays PublicationPHYSICS ESSAYS 30, 1 (2017)The Tyndall effect was discarded later as scientists couldnot explain why the sky does not appear bluer by theincreased amounts of water vapor and dust in rainy conditions.And, after the sky is cleared of dust and particles, thesky does appear bluer, not less. Further study by Lord Rayleighdiscovered that the amount of light scattered wasinversely proportional to the fourth power of the wavelengthof light, for finer particles (particle size below 40nm).4,5Based on these studies, the current explanation is that the scatteringof blue light is due to the oxygen and nitrogen in theair, rather than the particulate matter. Thus, the shorter wavelengthblue light is scattered by these molecules, whereas thelonger wavelength yellow and red pass through relativelyunaffected. Larger particles in the atmosphere such as watervapor are credited with scattering of light of all wavelengthsand this is called Mie scattering, named after the GermanPhysicist Gustav Mie.6 Due to this scattering of light of allwavelengths, the clouds at sunset appear gray/white while therest of the sky appears yellow-red due to Rayleigh scattering.This paper proposes that the colors associated with thesky at different locations, at different times of the day and indifferent weather conditions are due to very different phenomena.In the subsequent paragraphs, we will explain howthe color of ozone in the ozone layer imparts different huesof blue, depending on the location, the ambient temperature,prevailing weather conditions, etc. When appropriate, wewill also assign appropriate role for actual “removal” ofshorter wavelength light by dust/particles in the atmosphere.II. DISCUSSIONIn preparation for this paper, a thorough review of thescientific literature available both in print and online wasmade. What was discovered was that most authors narrate aconfused array of explanations and they could not satisfactorilyexplain all the observed phenomena and in all circumstances.Their attempts to explain the varied phenomenasuch as the usual blue of the sky, the lighter blue of the horizon,the multicolored sunsets and the blue haze of distantobjects, all being attributable to Rayleigh scattering or Tyndalleffect can be questioned. We will discuss below whytheir explanations fail in some situations and why the inclusionof the ideas expressed in this paper more completelyexplains the observations.This author had long been skeptical about using the scatteringof blue radiations by the particulates or molecules asthe explanation of why the sky is blue. If blue light is preferentially“scattered” by the molecules (or particulates), thenmore molecules closer to the earth at horizon should makethe sky appear deeper blue, not less. And, if the blue color isscattered all through the atmosphere, then the clouds shouldappear blue and not white, as we are looking at the cloudsthrough layers of the atmosphere. The dusty atmosphere ofMars makes its sky appear red in the usual conditions; thishas been attributed to the high concentration of iron oxide inits atmosphere.7 Images of Martian sky sporting a pale bluecolor when the dust settles down has been noted, and reportsof not only the presence of ozone in its atmosphere but alsoat least two layers of it have been presented.8,9 In the case ofthe earth’s moon, the atmosphere is black but its atmosphereis not devoid of gases and particulate matter; it has significantquantities of dust, Helium, Neon, and Hydrogen.10However, it is unknown if the amount of dust and the gasesare inadequate to produce Rayleigh’s scattering and that iswhy the sky on the moon is black. The absence of ozone inmoon’s atmosphere is noteworthy, however and this, webelieve, is the real reason for the absence of blue color inmoon’s sky. Our review of NASA’s website did not revealthe presence of a blue sky in any of the other terrestrial planets.Ozone is conspicuously absent in all their atmospheres,unlike in the earth’s and Mars’.Ozone is situated in a region of the atmosphere in the middlepart of the Stratosphere, between 10 and 50 km, like anenvelope around the earth. This layer of ozone can impart ablue color in at least two ways. First, the natural color of ozonegas is a light blue; one could argue that a column of such gasor innumerable layers of the gas when viewed as a wholemight appear even bluer. Also, the ambient temperature in theStratosphere is cold enough so that some of this ozone mayturn into a liquid and hence impart a deeper blue hue.11–13 It isalso known that the pale blue color of ozone gas turns to adeep blue black when it becomes solid at even lower temperatures.Thus, the ozone layer of the Stratosphere will appearblue of different degrees, depending on the prevailing temperaturein each region, including a deep blue-black in the poles.Ozone may impart a blue color to the Stratosphere inanother way. Ozone absorbs the harmful UV radiations fromsunlight. Since the UV spectrum is very close to the violet,indigo, and blue spectra, the aggregate might be a hue ofblue. This may be because quantitatively the blue light dominatesor because the aggregate of blue, indigo, and violet isstill registered as blue by the cones (the color-sensitive photoreceptorcells in the retina that decipher color, and whichare most sensitive to red, green, and blue colors). How muchof this particular effect contributes to the overall blue colorof the ozone layer, or whether other physical phenomena inthe Stratospheric region, such as the very low gravitationaleffect from the earth is also important, is not known.It is proposed that this ozone layer, which exists literallyas a globe around the earth, with its trapped blue color is thereason for the illusion that the sky is blue in color. This willthen explain why in cloudy conditions the color gets lessintense and after a heavy rain has cleared the atmosphere ofparticulate matter, including dust, and even substantialamounts of the gases, the sky appears deeper blue. Numerousscientific studies have provided evidence of clearance of particulatesby the process of “impaction” by raindrops, as wellas by gravity and, of the gases by a process called“scavenging.”14–22 This clearance of the particulates andgases by rain deepening the blue color of the sky will makethe Tyndall Effect and Rayleigh’s scattering untenable asexplanations for the blue color of the sky. The above also suggeststhat in normal circumstances, the pale blue color of thesky is due to some dampening effect from the light traversingthe particulate matter in the earth’s atmosphere. This hypothesisfurther explains why the clouds, which are much closer toan observer on the surface of the earth, in the Troposphere,than the ozone layer, (which is in the Stratosphere) thereforePhysics Essays 30, 1 (2017) 117do not take on the color of the sky. The above two observationsalso cannot be explained by either the Tyndall or Rayleighscatterings. When viewed from the space (beyond theStratosphere), for example, from the space shuttle, the earthhas a deep blue color; this is because the observer is viewingthe proper color of the ozone layer, without the diluting effectof the earth’s atmosphere. The haziness near the horizonwhen the sun is overhead, like during the midday, is also easyto explain by the current hypothesis; the increased dust closeto the surface of the earth obscures the color of the sky to agreater extent than the sky overhead, when the line of theobserver’s view is perpendicular to the sun’s rays.Similar explanations will account for the deep blue colorof sky in pristine locations in nature (such as the forests) andincreasingly lighter blue to gray shades in dusty locations.The orange-red appearance of sunsets and sunrises is due tothe Sun’s rays traversing through the thick layers of dustclose to the earth, and in the process the dust particles progressivelyallowing the longer-wavelength rays such as yellowand red preferentially to pass; the present author is inagreement with the conventional explanation, only in thisphenomenon. It is also worth noting that the orange/red colorimparted by this phenomenon paints the clouds and the interveningsky, as well as the sun, unlike the blue color residingonly in the sky during the rest of the daylight hours.A related phenomenon is how the moon takes on a red tingejust before the total eclipse of the moon; here the sunlightskimming the layers of dust close to the earth before reachingthe moon is filtering out the shorter-wavelength blues.The finding of large areas of deeper blue in the PolarRegions has been ascribed to loss of ozone (“ozone hole”).The current author believes this to be an erroneous assumption.While it is true there is loss of ozone, presumably fromthe use of Chlorofluorocarbons (CFCs), that will not increasethe blue color of the sky in any region. Besides, even reductionsin the ozone from aerosols will be near the landmass ofthe earth in areas other than the poles, and particularly inregions with heavy human habitation. The poles have few ifany human activity and it is more likely the atmosphere thereis pristine. Instead, since the temperatures around the Polesare much lower than elsewhere, it is much more likely thateven if there were diminished amounts of ozone in thoseregions, it will still appear to be a deeper blue in color, simplybecause the ozone is partly in a liquid or even a solidstate. There are reports of increases in ozone in the PolarRegions in the spring and this has been attributed to “ozonerichair rushing” in from lower latitudes.23 One could insteadargue that the lower ozone gas measured at the poles in thewinter may be due to the inability of the currently availablemethods to measure liquefied or solidified ozone in theFIG. 1. (Color online) View of the Earth from the space. This is an image of the earth and moon from the outer space. Note the deep blue color of the globe.Also, there is a clear strip of blue envelope around the globe, clearly visible on the left edge of the image of the earth, with a sharply delineated edge. The blackcolor outside the earth is the deep space, and the moon is the small gray ball near the right top corner. This image is from http://www.nasa.gov/multimedia/imagegallergy/index.html118 Physics Essays 30, 1 (2017)intense cold of the winter. The increases in ozone in thespring in these regions may simply be the warmer temperatureconverting some of the liquid or solid ozone to thegaseous form and thus becoming measurable.Proof for the current explanation of why the sky is bluecan be found in the fact that astronauts in space shuttlesnotice a completely black sky, whereas travelers in the commercialaircrafts, which fly in the Stratosphere see the sky asdeep blue in color. The ozone layer is in the Stratospherelevel (approximately 10–50 km above the earth), whereas thespace shuttles and most of the satellites travel or are situatedaround 300–400 km range above the surface of the earth, inthe Mesosphere. Even more suggestive is the appearance ofthe earth itself from outer space; there is a strip of deep bluecolor extending from the surface of the earth to a short distance,all around the globe. This blue color abruptly endswith a sharp edge as if enclosed within an envelope. This canbe used as the most compelling argument in favor of the bluecolor of sky residing primarily in the ozone layer. If, on theother hand, the blue of sky is solely due to scattering by moleculesclose to the earth, the diminishing quantities of thesesame molecules farther and farther out from the surface ofthe earth should produce the appearance of gradually diminishingblue, rather than an abrupt cut-off (Fig. 1).If human activities manage to destroy all ozone aroundthe earth, we will then probably see the sky as being black aswell, here on earth. Experiments to prove (or disprove) ourcurrent explanation might be difficult, if the extremely lowtemperatures, low gravity, and the rarefied air, as well as acertain concentration of ozone are critical for the blue colordisplay. Obviously, such experiments can only be arrangedin the outer space, to reproduce the low gravitational state, ifthat is also critical. If the color is reproduced by such experiments,removing the ozone from the system and demonstratingthat the blue color is lost, will clinch the proof.III. CONCLUSIONThis paper presents arguments about the contributionsmade by ozone in the ozone layer surrounding the earth, tothe blue color of the sky. The current author suggests that theblue of the overhead sky is due to the color of ozone, whilethe haze in the horizon is due to increased dust close to thesurface of the earth, physically obstructing the blue of theozone. The orange-red of the sunsets and sunrises are due topreferential passage of the longer wavelength yellow and redby the same dust but while the sun’s rays are traveling parallelto the earth’s surface and in the line of an observer’sview. This article thus emphasizes that different phenomenasurrounding the sky and the atmosphere have different physicalbases, rather than only one is offered to explain all thevaried observations.ACKNOWLEDGMENTSI wish to acknowledge the excellent secretarial assistanceof Ms. Rosie Gonzales in the preparation of this manuscriptand in her assistance in the submission to PhysicsEssays.1R. Petrucci, W. Harwood, F. Herring, and J. Madura, General Chemistry:Principles and Modern Applications, 9th ed. (Prentice Hall, Upper SaddleRiver, NJ, 2007).2J. H. Seinfeld and S. N. Pandis, Atmosphere Chemistry & Physics, 2nd ed.(John Wiley & Sons, Hoboken, NJ, 2006), Chap. 15.1.1.3See http://www.desy.de/user/projects/Physics/General/Bluesky/blue_sky.htmlfor an explanation of why the sky is blue.4K. Rajagopal, Textbook on Engineering Physics (PHI, New Delhi, India,2008), Part I, Chap. 3.5S. Chakraborti, Am. J. Phys. 75, 824 (2007).6G. Mie, Ann. Phys. 330, 377 (1908).7See Mars Fact Sheet for Mars’atmospheric composition.8S. Lebonnois, E. Que´merais, F. Montmessin, F. Lefe`vre, S. Perrier, J.-L.Bertaux, and F. Forget, J. Geophys. Res: Planets 111, E09S05 (2006).9S. Perrier, J. L. Bertaux, F. Lefe`vre, S. Lebonnois, O. Korablev, A. Fedorova,and F. Montemessin, J. Geophys. Res.: Planets 111, E09S06 (2006).10See Moon Fact Sheet for theMoon’s atmospheric composition.11A. G. Streng, J. Chem. Eng. Data 6, 431 (1961).12See http://www.epa.gov/ozone/science/sc_fact.html for a discussion ofozone and the dangers of its depletion in the ozone layer.13V. Ramaswamy, M. L. Chanin, J. Angell, J. Barnett, D. Gaffen, M.Gelman, P. Keckhut, Y. Koshelkov, K. Labitzke, J. J. R. Lin, A. O’Neill,J. Nash, W. Randel, R. Rood, K. Shine, M. Shiotani, and R. Swinbank,Rev. Geophys. 39, 71 (2001).14J. M. Hales, Atmos. Environ. 6, 635 (1972).15T. D. Davies, Atmos. Environ. 10, 879 (1976).16S. Kumar, Atmos. Environ. 19, 769 (1985).17T. Y. Chang, Atmos. Environ. 18, 191 (1984).18W. H. Chen, Atmos. Environ. 38, 1107 (2004).19B. E. A. Fisher, Atmos. Environ. 16, 775 (1982).20R. Naresh, Int. J. Nonlinear Sci. Numer. Simul. 4, 379 (2003).21R. Naresh and S. Sundar, Nonlinear Anal.: Modell. Control 12, 227(2007).22R. Naresh, S. Sundar, and J. B. Shukla, Nonlinear Anal. RWA 8, 337(2007).23See http://.albany.edu/faculty/rgk/atm101/ozmeas.htm for ozone measurementin the poles in the winter versus in late spring.Physics Essays 30, 1 (2017) 119

Sorry, Snehil, I am going to give a contrarian answer. I disagree with the teaching about what makes earth’s and Mars’ skies appear blue. I’ll copy and paste my paper on this subject that was published a year ago. It is a little long; please bear with me. Here goes….WHY THE SKY IS BLUEPuthalath Koroth Raghuprasad(Received 13 April 2013; accepted 14 February 2017; published online 3 March 2017)ABSTRACT:The “Tyndall effect” and “Rayleigh scattering” are the accepted explanations for the blue color of the sky. However, since heavy rainfall is known to remove particulates as well as the gases and yet the sky actually becomes a deeper blue in color, both explanations are probably invalid. The current author proposes the following explanation for the blue color of the sky: This is the pale blue of ozone gas, which will appear deeper blue when there are sufficient quantities of it. Further, ozone attains an even deeper blue color when it becomes a liquid at around a temperature of 161K (-112 C) and a blue to violet-black solid at temperatures below 82K (-193.2 C). The latter is the case in the lower Stratosphere, especially near the poles. Also, since ozone absorbs ultraviolet radiations, it is likely that some of the spectra close to UV (such as violet, indigo, and blue) radiations are also absorbed or scattered by ozone and this may add to the blue color of the ozone layer. To an observer on the surface of the earth, the many layers of dust and other particulates in the intervening Troposphere, which dampen the deep blue of the ozone layer, will make the “sky” appear less blue. How much each of the above factors contributes to the color of the sky is not known but jointly, they can explain all the observed phenomena.VC 2017 Physics EssaysPublication. [Why the sky is blue]Re´sume´ : L’effet Tyndall et la diffusion de Rayleigh sont les explications admises de la couleur bleue du ciel. Les fortes pluies sont cependant connues pour e´liminer les matie`res particularise ainsi que les gaz tout en donnant au ciel une couleur bleue plus fonce´e, ce qui signifie que ces deux explications sont probablement incorrectes. L’auteur de cette e´tude propose l’explication suivante pour la couleur bleue du ciel: il s’agit du bleu paˆle de l’ozone gazeux, qui apparaıˆt plus fonce´ lorsqu’il est pre´sent en quantite´s suffisantes. En outre, l’ozone acquiert une couleur bleue encore plus fonce´e lorsqu’il devient liquide au voisinage de la tempe´rature de 161K (112 C), et une couleur entre le bleu et le violet-noir lorsqu’il devient solide a` moins de 82K (193,2 C). Cette dernie`re situation existe dans la stratosphe`re infe´rieure, en particulier pre`s des poˆles. Sachant que l’ozone absorbe le rayonnement ultraviolet, il est probable qu’une partie du spectre au voisinage du rayonnement UV (notamment le violet, l’indigo et le bleu) est aussi absorbe´ ou diffuse´ par l’ozone, ce qui accentuerait la couleur bleue de la couche d’ozone. Pour un observateur a` la surface de la terre, les nombreuses couches de poussie`res et autres matie`res particulaires dans la troposphère`re (qui occupe une position interme´diaire) atte´nuent le bleu fonce´ de la couche d’ozone et donnent une apparence moins bleue au « ciel ». La contribution de chacun des facteurs ci-dessus a` la couleur du ciel n’est pas connue, mais ensemble ils peuvent expliquer tous les ph´nome`nes observe´s.Key words: Tyndall Effect; Rayleigh Scattering; Ozone; Stratosphere.I. INTRODUCTIONThe blue color of the sky was originally attributed by scientists to the Tyndall effect. Described by John Tyndall in 1859, this effect is a preferential scattering of shorter wavelength blue light by particulate matter (particles 40–900 nm in size, the wavelength of visible light being 400–750 nm) in the atmosphere.1–3 The longer wavelength spectra of sun’s light such as red and yellow have the ability to course through these particles. Thus, while we see sky as being blue, the sun and the sunlight take on a yellow hue. A classic example of this phenomenon is the bluish tinge assumed by an opalescent stone when a beam of white light traverses the stone, while the light that exits on the opposite side takes on a yellowish tinge. Experiments to prove the above explanation involved passing light through containers of water with a suspension of colloidal substances; milk has been a popular ingredient in these experiments. The demonstration that the container turns bluish in color, while the beam of light that exits the container is more yellow/red has been quoted as confirming the phenomenon and as the explanation of the color of sky. The scientific explanation of related observations such as why near the horizon the sky appears less blue and at dawn and dusk the sun and the sky appear orange-red is also attributed to the same scattering effect. In the former, more scattering of blue light by the increased amount of particles in the air closer to the earth is given as the reason. The latter effect is attributed to “rescattering” of blue light so that the orange and red are accentuated.The Tyndall effect was discarded later as scientists could not explain why the sky does not appear bluer by the increased amounts of water vapor and dust in rainy conditions. And, after the sky is cleared of dust and particles, the sky does appear bluer, not less. Further study by Lord Rayleigh discovered that the amount of light scattered was inversely proportional to the fourth power of the wavelength of light, for finer particles (particle size below 40nm).4,5 Based on these studies, the current explanation is that the scattering of blue light is due to the oxygen and nitrogen in the air, rather than the particulate matter. Thus, the shorter wavelength blue light is scattered by these molecules, whereas the longer wavelength yellow and red pass through relatively unaffected. Larger particles in the atmosphere such as water vapor are credited with scattering of light of all wavelengths and this is called Mie scattering, named after the German Physicist Gustav Mie.6 Due to this scattering of light of all wavelengths, the clouds at sunset appear gray/white while the rest of the sky appears yellow-red due to Rayleigh scattering. This paper proposes that the colors associated with the sky at different locations, at different times of the day and in different weather conditions are due to very different phenomena. In the subsequent paragraphs, we will explain how the color of ozone in the ozone layer imparts different hues of blue, depending on the location, the ambient temperature, prevailing weather conditions, etc. When appropriate, we will also assign appropriate role for actual “removal” of shorter wavelength light by dust/particles in the atmosphere.II. DISCUSSIONIn preparation for this paper, a thorough review of the scientific literature available both in print and online was made. What was discovered was that most authors narrate a confused array of explanations and they could not satisfactorily explain all the observed phenomena and in all circumstances. Their attempts to explain the varied phenomena such as the usual blue of the sky, the lighter blue of the horizon, the multicolored sunsets and the blue haze of distant objects, all being attributable to Rayleigh scattering or Tyndall effect can be questioned. We will discuss below why their explanations fail in some situations and why the inclusion of the ideas expressed in this paper more completely explains the observations.This author had long been skeptical about using the scattering of blue radiations by the particulates or molecules as the explanation of why the sky is blue. If blue light is preferentially “scattered” by the molecules (or particulates), then more molecules closer to the earth at horizon should make the sky appear deeper blue, not less. And, if the blue color is scattered all through the atmosphere, then the clouds should appear blue and not white, as we are looking at the clouds through layers of the atmosphere. The dusty atmosphere of Mars makes its sky appear red in the usual conditions; this has been attributed to the high concentration of iron oxide in its atmosphere.7 Images of Martian sky sporting a pale blue color when the dust settles down has been noted, and reports of not only the presence of ozone in its atmosphere but also at least two layers of it have been presented.8,9 In the case of the earth’s moon, the atmosphere is black but its atmosphere is not devoid of gases and particulate matter; it has significant quantities of dust, Helium, Neon, and Hydrogen.10 However, it is unknown if the amount of dust and the gases are inadequate to produce Rayleigh’s scattering and that is why the sky on the moon is black. The absence of ozone in moon’s atmosphere is noteworthy, however and this, we believe, is the real reason for the absence of blue color in moon’s sky. Our review of NASA’s website did not reveal the presence of a blue sky in any of the other terrestrial planets. Ozone is conspicuously absent in all their atmospheres, unlike in the earth’s and Mars’.Ozone is situated in a region of the atmosphere in the middle part of the Stratosphere, between 10 and 50 km, like an envelope around the earth. This layer of ozone can impart a blue color in at least two ways. First, the natural color of ozone gas is a light blue; one could argue that a column of such gas or innumerable layers of the gas when viewed as a whole might appear even bluer. Also, the ambient temperature in the Stratosphere is cold enough so that some of this ozone may turn into a liquid and hence impart a deeper blue hue.11–13 It is also known that the pale blue color of ozone gas turns to a deep blue black when it becomes solid at even lower temperatures. Thus, the ozone layer of the Stratosphere will appear blue of different degrees, depending on the prevailing temperature in each region, including a deep blue-black in the poles.Ozone may impart a blue color to the Stratosphere in another way. Ozone absorbs the harmful UV radiations from sunlight. Since the UV spectrum is very close to the violet, indigo, and blue spectra, the aggregate might be a hue of blue. This may be because quantitatively the blue light dominates or because the aggregate of blue, indigo, and violet is still registered as blue by the cones (the color-sensitive photoreceptor cells in the retina that decipher color, and which are most sensitive to red, green, and blue colors). How much of this particular effect contributes to the overall blue color of the ozone layer, or whether other physical phenomena in the Stratospheric region, such as the very low gravitational effect from the earth is also important, is not known.It is proposed that this ozone layer, which exists literally as a globe around the earth, with its trapped blue color is the reason for the illusion that the sky is blue in color. This will then explain why in cloudy conditions the color gets less intense and after a heavy rain has cleared the atmosphere of particulate matter, including dust, and even substantial amounts of the gases, the sky appears deeper blue. Numerous scientific studies have provided evidence of clearance of particulates by the process of “impaction” by raindrops, as well as by gravity and, of the gases by a process called “scavenging.”14–22 This clearance of the particulates and gases by rain deepening the blue color of the sky will make the Tyndall Effect and Rayleigh’s scattering untenable as explanations for the blue color of the sky. The above also suggests that in normal circumstances, the pale blue color of the sky is due to some dampening effect from the light traversing the particulate matter in the earth’s atmosphere. This hypothesis further explains why the clouds, which are much closer to an observer on the surface of the earth, in the Troposphere, than the ozone layer, (which is in the Stratosphere) thereforedo not take on the color of the sky. The above two observations also cannot be explained by either the Tyndall or Rayleigh scatterings. When viewed from the space (beyond the Stratosphere), for example, from the space shuttle, the earth has a deep blue color; this is because the observer is viewing the proper color of the ozone layer, without the diluting effect of the earth’s atmosphere. The haziness near the horizon when the sun is overhead, like during the midday, is also easy to explain by the current hypothesis; the increased dust close to the surface of the earth obscures the color of the sky to a greater extent than the sky overhead, when the line of the observer’s view is perpendicular to the sun’s rays. Similar explanations will account for the deep blue color of sky in pristine locations in nature (such as the forests) and increasingly lighter blue to gray shades in dusty locations. The orange-red appearance of sunsets and sunrises is due to the Sun’s rays traversing through the thick layers of dust close to the earth, and in the process the dust particles progressively allowing the longer-wavelength rays such as yellow and red preferentially to pass; the present author is in agreement with the conventional explanation, only in this phenomenon. It is also worth noting that the orange/red color imparted by this phenomenon paints the clouds and the intervening sky, as well as the sun, unlike the blue color residing only in the sky during the rest of the daylight hours. A related phenomenon is how the moon takes on a red tinge just before the total eclipse of the moon; here the sunlight skimming the layers of dust close to the earth before reaching the moon is filtering out the shorter-wavelength blues.The finding of large areas of deeper blue in the Polar Regions has been ascribed to loss of ozone (“ozone hole”). The current author believes this to be an erroneous assumption. While it is true there is loss of ozone, presumably from the use of Chlorofluorocarbons (CFCs), that will not increase the blue color of the sky in any region. Besides, even reductions in the ozone from aerosols will be near the landmass of the earth in areas other than the poles, and particularly in regions with heavy human habitation. The poles have few if any human activity and it is more likely the atmosphere there is pristine. Instead, since the temperatures around the Poles are much lower than elsewhere, it is much more likely that even if there were diminished amounts of ozone in those regions, it will still appear to be a deeper blue in color, simply because the ozone is partly in a liquid or even a solid state. There are reports of increases in ozone in the Polar Regions in the spring and this has been attributed to “ozone rich air rushing” in from lower latitudes.23 One could instead argue that the lower ozone gas measured at the poles in the winter may be due to the inability of the currently available methods to measure liquefied or solidified ozone in theFIG. 1. (Color online) View of the Earth from the space. This is an image of the earth and moon from the outer space. Note the deep blue color of the globe. Also, there is a clear strip of blue envelope around the globe, clearly visible on the left edge of the image of the earth, with a sharply delineated edge. The black color outside the earth is the deep space, and the moon is the small gray ball near the right top corner. This image is from http://www.nasa.gov/multimedia/imagegallergy/index.html118 Physics Essays 30, 1 (2017)intense cold of the winter. The increases in ozone in the spring in these regions may simply be the warmer temperature converting some of the liquid or solid ozone to the gaseous form and thus becoming measurable.Proof for the current explanation of why the sky is blue can be found in the fact that astronauts in space shuttles notice a completely black sky, whereas travelers in the commercial aircrafts, which fly in the Stratosphere see the sky as deep blue in color. The ozone layer is in the Stratosphere level (approximately 10–50 km above the earth), whereas the space shuttles and most of the satellites travel or are situated around 300–400 km range above the surface of the earth, in the Mesosphere. Even more suggestive is the appearance of the earth itself from outer space; there is a strip of deep blue color extending from the surface of the earth to a short distance, all around the globe. This blue color abruptly ends with a sharp edge as if enclosed within an envelope. This can be used as the most compelling argument in favor of the blue color of sky residing primarily in the ozone layer. If, on the other hand, the blue of sky is solely due to scattering by molecules close to the earth, the diminishing quantities of these same molecules farther and farther out from the surface of the earth should produce the appearance of gradually diminishing blue, rather than an abrupt cut-off (Fig. 1).If human activities manage to destroy all ozone around the earth, we will then probably see the sky as being black as well, here on earth. Experiments to prove (or disprove) our current explanation might be difficult, if the extremely low temperatures, low gravity, and the rarefied air, as well as a certain concentration of ozone are critical for the blue color display. Obviously, such experiments can only be arranged in the outer space, to reproduce the low gravitational state, if that is also critical. If the color is reproduced by such experiments, removing the ozone from the system and demonstrating that the blue color is lost, will clinch the proof.III. CONCLUSIONThis paper presents arguments about the contributions made by ozone in the ozone layer surrounding the earth, to the blue color of the sky. The current author suggests that the blue of the overhead sky is due to the color of ozone, while the haze in the horizon is due to increased dust close to the surface of the earth, physically obstructing the blue of the ozone. The orange-red of the sunsets and sunrises are due to preferential passage of the longer wavelength yellow and red by the same dust but while the sun’s rays are traveling parallel to the earth’s surface and in the line of an observer’s view. This article thus emphasizes that different phenomena surrounding the sky and the atmosphere have different physical bases, rather than only one is offered to explain all the varied observations.ACKNOWLEDGMENTSI wish to acknowledge the excellent secretarial assistance of Ms. Rosie Gonzales in the preparation of this manuscript and in her assistance in the submission to Physics Essays.BIBLIOGRAPHY1 R. Petrucci, W. Harwood, F. Herring, and J. Madura, General Chemistry: Principles and Modern Applications, 9th ed. (Prentice Hall, Upper Saddle River, NJ, 2007).2J. H. Seinfeld and S. N. Pandis, Atmosphere Chemistry & Physics, 2nd ed.(John Wiley & Sons, Hoboken, NJ, 2006), Chap. 15.1.1.3Seehttp://www.desy.de/user/projects/Physics/General/Bluesky/blue_sky.htmlfor an explanation of why the sky is blue.4 K. Rajagopal, Textbook on Engineering Physics (PHI, New Delhi, India, 2008), Part I, Chap. 3.5 S. Chakraborti, Am. J. Phys. 75, 824 (2007).6 G. Mie, Ann. 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