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Exploration of a Natural Colour Phenomenon Why does the sky colour change? To answer this question, one should be aware of the Earth's atmosphere, and light. What is the atmosphere? The sky is filled with air which is a mixture of tiny gas molecules and small bits of solid stuff, like dust (Why Is the Sky Blue? ). The air constitutes the atmosphere, a gaseous shell surrounding the Earth and some other planets. The main gaseous components of the atmosphere are nitrogen (78. 09 %) and oxygen (20. 95 %).
It contains also small amounts of water, solid particles (like dust), and some gases like argon, helium, hydrogen and ozone (Why Is the Sky Blue? ). Liquid and solid particles suspended in the air are often called aerosols (Observing Visibility and Sky Color 2). The composition of the atmosphere varies depending on the location, the weather, and other factors. There may be more water in the air after a rainstorm or near the ocean. Volcanoes can put large quantities of dust particles high into the atmosphere.
Pollution can also contribute by some gases or dust and soot (Why Is the Sky Blue? ). What is light? It is a kind of energy which radiates (travels) in electromagnetic waves; light is one small part of the spectrum. Light of the sun may look white but it is a combination of colours.
They blend continuously into one another. At one end of the spectrum are the reds and oranges. They gradually shade into yellow, green, blue, indigo and violet. The colours have different wavelengths (frequencies). In the visible spectrum, where our eyes can only see the light, violet has the shortest wavelength (the highest frequency) while red has the longest one (the lowest frequency) (Why Is the Sky Blue? ).
What about light in the air? Light travels through space in a straight line as long as nothing disturbs it. As light moves through the atmosphere, it continues to go straight until it bumps into a gas molecule or a bit of dust. What happens then to the light depends on its wavelength and the size of the thing it hits.
Dust particles and water droplets are larger than the wavelength of visible light. When light hits such particles, it gets reflected in various directions. The different colours of light are all reflected by the particle in the same way. The reflected light appears white because it still contains all of the same colours. Gas molecules are smaller than the wavelength of visible light.
If light bumps into them, it acts differently. When light hits a gas molecule, some of it may get absorbed. After a while, the molecule radiates the light in a different direction. The colour radiated is the same as that absorbed (Gibbs). The different colours of light are differently affected. All of them can be absorbed.
However the higher frequencies (blues) are absorbed more often than the lower frequencies (reds). This process is called Rayleigh scattering in honour of Lord John Rayleigh who described this phenomenon in 1871 (Siddalingaiah; Why Is the Sky Blue? ), though the first steps towards explaining the sky colour were taken by John Tyndall in 1859 (Gibbs). Why is the sky blue on a clear day? This is due to Rayleigh scattering. As light moves through the air, most of the longer wavelengths pass straight through; little of the red, orange and yellow light is affected by the air.
However much of the shorter wavelength light is absorbed by the gas molecules. The absorbed blue light is then radiated in different directions. It gets scattered all around the sky. Whichever direction you look, some of this scattered blue light reaches you. Since you see the blue light from everywhere overhead, the sky looks blue.
As the observer looks closer to the horizon, the sky appears paler in colour. To reach such a watcher, the scattered blue light should pass through more air. Some of it gets scattered again away in other directions. As a result, less blue light reaches the man. The sky colour near the horizon appears paler or white (Why Is the Sky Blue? ). Why is the sunset red?
One of explanations is following. As the sun begins to set, the light travels farther through the atmosphere before it gets to the watcher. More of the light is reflected and scattered. As less reaches the observer directly, the sun appears less bright. The colour of the sun itself appears to change, first to orange and then to red. This is because even more of the short wavelength blues and greens are now scattered; only the longer wavelengths are left in the direct beam that reaches the watcher.
Of interest is also the fact that the sky around the setting sun may take on many colours. The most impressive shows occur when the atmosphere contains many small particles of water or dust. These particles reflect light in all directions. Then, as some of the light heads towards the observer, various quantities of the shorter wavelength colours are scattered out. The watcher sees the longer wavelengths, and the sky appears red, pink or orange (Why Is the Sky Blue? ). According to Gibbs, when the air is clear the sunset appears yellow because the light from the sun has passed a long distance through air and some of the blue light is scattered away.
If the air is polluted with small particles, the sunset is redder. Sunsets over the sea may be orange due to salt particles in the air that are effective scatterers. The sky around the sun is seen reddened like the light coming from the sun directly. This is because all light is scattered well through small angles but blue light is then more likely to be scattered twice or more over the greater distances, leaving the yellow, red and orange colours. Siddalingaiah supposes that in the morning and evening, when the sun is closer to the horizon, sunlight travels through more of the atmosphere before it gets to our eyes. More atmosphere means more scattering of blue light.
What happens is that most of the blue light is scattered, leaving mostly red light. The end result of this is that the sky colour changes depending on the suns angle. This angle changes during a day, so we get different colours from morning to evening. This angle changes also during a year causing seasons change. Depending on the latitude, the watcher sees a change in the sky colour for different seasons. Among other atmospheric factors affecting the sky colour, Siddalingaiah mentions humidity and various particles in the air; they can cause the sky to look green since the air is not so clear.
Temperature of air is also of importance. E. g. , warm air is capable of holding more water (more humidity), so hot and humid days result in more grey skies. Humidity is also lower in the winter; hence the sky is clearer in that season. Works Cited: Gibbs, Philip. Why Is the Sky Blue?
May 1997. 28 Mar. 2007 < web >. Observing Visibility and Sky Color. The GLOBE Program. 2007. The University Corporation for Atmospheric Research and Colorado State University. 8 Apr. 2007 < web >. Siddalingaiah, Madhu. "Re: Why does the sky change color with the seasons?" Online posting. 24 Oct. 1998. Mad Sci Network. 21 Mar. 2007 < web >.
Why Is the Sky Blue? Science Made Simple: Newsletter. Science Made Simple, Inc. 24 Mar. 2007 < web >.
Free research essays on topics related to: blue light, sky, visible light, dust particles, mar 2007
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