Why the sky is blue?

All colors and shapes that the sky has to offer, have a common property: they can not be imitated with the human. Whenever you try to reproduce them on canvas, paper, wood or metal will inevitably fail. They are the work of a teacher who has truly means "heavenly." His brush is sunlight, and his canvas is the volatile ether with its clouds and the soft tissue fine atmospheric dust: no artist available.

The sea air that surrounds us, is an inexhaustible source of joy to our eyes. The blue of a clear spring morning, the red-orange of a sunset on a plain, have made men enjoy, poetic language and investigate again and again. No matter where on Earth we live, we all share the same sky. Overhead, the sky appears to us as soon as pink ultramarine blue, now white or a delicate blue, decorated with clouds in the form of flakes, shreds torn or broken in powerfully swollen. The variability of this image is so large that never reproduced exactly. And the colors come in a palette so rich that our painters go, again and again, his eyes to heaven, for inspiration in the colors of a sunset or the rainbow.
The beauty of heaven is not merely the result of the interaction of sunlight with the atmosphere. A number of moisture, relatively small, accompanied by dust and ash is sufficient to cause the sky color the many manifestations of .
When there are extreme weather conditions may occur chromatic atmospheric phenomena such as the Rainbow Circles Ulloa, the Crowns solar and lunar halos False False Suns and Moons and more "rare" (Mirages, the Green Ray, Sacred Light, Aurora Polar Fires of San Telmo ...), optical phenomena that are totally explainable. Here we shall consider just the most common optical phenomenon that is the color of the sky in its various possible manifestations.
The secret of blue sky is related to the composition of sunlight-composed of the various colors of the rainbow-and humidity of the atmosphere. (The Sun is in charge of procuring air humidity. With heat, water causes some of Earth's surface evaporates. In invisible but incessant stream, moisture is directed into the sky from the oceans, seas, lakes and rivers, from the soil, plants and the bodies of animals and man).
To explain the blue sky, imagine that we let a ray of sunlight through a prism of glass. The light is opened in a range of colors (scattered) by refraction and as a result of this dispersion, we see a range of colors: purple, blue, green, yellow and red. The violet ray is the one that has spread over the white beam direction and that is precisely the explanation of the color of the sky. The maximum deviation of the rays of short wavelength (violet and blue), and lowest for long-wavelength (yellow and red), which are hardly deflected. Violet and blue rays, once diverted, collide with other particles of air and vary their path again, and so on: do, then a zigzag dance within the air before reaching the ground floor. When, at last, come to our eyes, do not seem to come directly from the Sun, but we come from all regions of the sky, as in a fine rain. Hence the sky appears blue to us, as the sun appears yellow, as yellow and red rays are slightly deflected and go almost directly in line from the Sun to our eyes.
If we delve a little more, the explanation is more complex. Light is an electromagnetic wave and the fundamental pieces of matter in its most common on Earth, are atoms. If the particles in the atmosphere, have a size equal to or less than the wavelength of incident light (single atoms or small molecules), the wave transfers its energy from nuclear crust begins to oscillate, so a first effect of the interaction of light with small particles of air is that the incident radiation is weakened by ceding part of its energy, what happens when sunlight passes through the atmosphere. Clearly this energy is not stored in the air, as any atom or small particle whose bark is shaken, just radiating all their energy in the form of electromagnetic wave to the environment in any direction. The entire process of assignment and transfer of energy by atomic sized particles known as Rayleigh scattering (named after the English physicist Lord Rayleigh was the first to give explanation) being the intensity of the scattered light is inversely proportional to the fourth power of the wavelength. The spread is greater, therefore, to shorter waves: As a result, we reach the same conclusion, violet light is more diffused and less red. The net result is that some of the light coming from the Sun in a straight line to reach the atmosphere diffuses in all directions and filled the whole sky.
The color of the sky should be violet because this is the shortest wavelength, but not, for two main reasons: because sunlight contains more violet and blue light because the human eye (which ultimately is the captures the images, although the brain interprets-), is more sensitive to blue light than violet.
The blue sky is due both to increased short-wave broadcasting. The color of the sun is yellow-red and not white, because if the white light from the sun, which is the sum of all colors, they should take the blue light gives a yellow-red.
The diffusion of gases is produced by very weak, however, when the thickness of gas is very large, as in the atmosphere, is obvious from the scattered light.
The fact that the spread is greater for shorter waves, is the basis for the use of fog lights.
Regardless of all the possibilities that may arise, it can be argued that the greater the number of particles that cloud the air, the worse will be the visibility through the air.
If the fog is "dry" due to the presence of smoke, dust or small water droplets, the yellow light - that part of the fog-just fade because of the filing of this fog, so it is visible through it. If the fog is "wet", the best lights against it fail almost completely, as the damp mist is formed by large droplets that spread almost equally, all colors of white light. The Sun itself, seen through the fog of large droplets, appears blurred and milky white, while observed when the fog is due to fine dust disk looks red, as often happens to be the star.
If light interacts with a large particle, not the mechanism of Rayleigh, occurs much simpler process: the particle simply absorbs some light and reflects the other side. Each particle behaves as a tiny mirror that reflects more or less light depending on their chemical composition and alter the color of the reflected light if the particle is composed of colored substances. If the light is with a distribution of large particles, some light is scattered and, moreover, may change color. This process is known as Mie diffusion, and the simplest example we have in the clouds, where water drops colorless, scatter light in all directions, but without altering their color. (The sky of Mars is another example of diffusion of Mie, caused by large-sized colored particles, so it is not blue, because the size of the particles does not allow the Rayleigh scattering).
If dissemination of Mie act en masse, if the diffusing particles are colored, the result is the attenuation of white light into increasingly darker gray. This is the reason why in the very cloudy days when the clouds are very thick, the sky appears more or less gray, and sometimes almost black.
The rising and setting sun give us a beautiful day shows, the most beautiful air can offer to our eyes.
If the horizon is large (as in the city of Badajoz), the effects are multiplied and the show is all a poem.
At sunset, the way that sunlight goes into the atmosphere is longer, the successive rebounds and other particles are likely to grow in the light ends colliding with a particle absorbing and disappear, so that even the yellow are affected and released and only the red rays, the most directional, follow a nearly straight line. Hence the red color of the sun.
The colors that offers us the sky in these cases also arise through the intervention of the molecules from the air and particles that it has suspended "the atmospheric aerosol, scattering sunlight and unfold in many ways .
Even before the sun sinks into the horizon, we see the color of the sky becomes more intense, more saturated. As the light that appears around the solar disc veers towards yellow-red and the horizon is yellowish-green, the blue becomes more intense at the zenith.
When the Sun is at an angular distance of the horizon of 1 or 2 degrees, pour the twilight sky on the edge of its magical light. Gradually, the yellow glow is transformed into a red-orange light, and finally, in a twinkling light tan, which sometimes tends to form the red color of blood. Day when the sun has disappeared below the horizon, is seen in the western sky a bright purple, which reaches its maximum when the sun has dropped about 5 degrees below the horizon. Above the place where the sun has been separated from the horizon by a narrow red-brown, is often a semicircle whose color varies from purple and pink. This coloration is due essentially to the refraction of sunlight on particles that cloud the air between 10 and 20 km. high, and disappears when the sun has come at 7 degrees below the horizon.
When there is an abnormally large quantity of aerosols (airborne dust), the light of sunrise and sunset is especially red. It usually happens when there are high atmospheric pressure (anticyclone) and the concentration of dust particles in the air is greater at high pressures. Very intense red color that we usually consider here in Extremadura, in the month of October and sometimes sporadic, may be due to increased aerosols from the burning of fallow crop.
If the earth had no atmosphere, sunlight would reach our eyes directly from the solar disk and would not receive scattered light and sky appear as black as night (the astronauts can see during the day the stars, moon and planets because they are outside the atmosphere).
In exceptional cases, special stains can occur due to the contribution of active volcanoes. At the time of the eruption of Krakatoa (26 and 27 August 1883, -36 000 killed by the eruption-) on the Earth witnessed a notable example. The eruption released to the air mass volume of the small rocky island of Krakatoa (located in the Sunda Strait, between Sumatra and Java) which is estimated at about 18 km. Chunks of rock the size of a human head were thrown upward with initial speeds of 600 to 1000 m / s, and the roar of the explosion was heard in Rodrigues (island of Madagascar) to 4774 miles away. The sky remained dark for several days. The finer particles of volcanic ash from the volcano spread to 80 km in height, were carried by air currents high and drove around the Earth twice. Were in the air phenomena fantastic color that continued even months after catastrophe, among others, there were amazing colors for the rising and setting sun and were soles of all colors, including red and green copper. Soles were also blue, as can also be seen on rare occasions in Europe when in Canada, for example, is a large forest fire and westerly winds dragged to our continent fine ash particles.
Because at dusk, the road that runs sunlight within the atmosphere is longer, as indicated above, is why the Sun looks more flat and wide as the effect of refraction through the atmosphere is very large.
Finally, the black of night, is because the atmosphere surrounding the viewer barely light and therefore can not provide enough circulation.


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